Perl6::Bible::A04(3) User Contributed Perl Documentation Perl6::Bible::A04(3)NAME
Apocalypse_04 - Syntax
AUTHOR
Larry Wall <larry@wall.org>
VERSION
Maintainer: Larry Wall <larry@wall.org>
Date: 18 Jan 2002
Last Modified: 6 Dec 2004
Number: 4
Version: 5
This Apocalypse is all about syntax in the large. The corresponding
chapter in the Camel book is entitled "Statements and Declarations",
but it could just as easily have been entitled, "All About Blocks". The
basic underlying question is "What exactly do those curlies mean?"
For Perl 5 and earlier, the answer to that question was, "Too many
things". Or rather, too many things with inconsistent rules. We'll
continue to use curlies for much of what we've used them for up till
now, but by making a few critical simplifications, the rules will be
much more consistent. In particular, built-ins will parse with the same
rules as user-defined constructs. It should be possible to make user-
extensible syntax look just like built-in syntax. Perl 5 started down
this road, but didn't get all the way there. In Perl 6, all blocks
operate under the same rules. Effectively, every block is a kind of
closure that can be run by user-defined constructs as well as built-
ins.
Associated with block structure are the various constructs that make
use of block structure. Compound constructs like loops and conditionals
use blocks explicitly, whereas declarations refer to their enclosing
block implicitly. This latter feature was also inconsistently applied
in Perl 5. In Perl 6, the rule is simple: A lexically scoped
declaration is in effect from the declaration to the end of its
enclosing block. Since blocks are delimited only by curlies or by the
ends of the current compilation unit (file or string), that implies
that we can't allow multi-block constructs in which lexically scoped
variables "leak" or "tunnel" from the end of one block to the beginning
of the next. A right curly (without an intervening left curly)
absolutely stops the current lexical scope. This has direct bearing on
some of these RFCs. For instance, RFC 88 proposes to let lexical scope
leak from a "try" block into its corresponding "finally" block. This
will not be allowed. (We'll find a different way to solve that
particular issue.)
While lexical declarations may not leak out of a block, control flow
must be able to leak out of blocks in a controlled fashion. Obviously,
falling off the end of a block is the most "normal" way, but we need to
exit blocks in other "abnormal" ways as well. Perl 5 has several
different ways of exiting a block: "return", "next", "last", "redo",
and "die", for instance. The problem is that these various keywords are
hard-wired to transfer control outward to a particular built-in
construct, such as a subroutine definition, a loop, or an "eval". That
works against our unifying concept that every block is a closure. In
Perl 6, all these abnormal means of block exit are unified under the
concept of exceptions. A "return" is a funny kind of exception that is
trapped by a "sub" block. A "next" is an exception that is trapped by a
loop block. And of course "die" creates a "normal" exception that is
trapped by any block that chooses to trap such exceptions. Perl 6 does
not require that this block be an "eval" or "try" block.
You may think that this generalization implies excessive overhead,
since generally exception handling must work its way up the call stack
looking for an appropriate handler. But any control flow exception can
be optimized away to a "goto" internally when its target is obvious and
there are no user-defined blocks to be exited in between. Most
subroutine return and loop control operators will know which subroutine
or loop they're exiting from because it'll be obvious from the
surrounding lexical scope. However, if the current subroutine contains
closures that are being interpreted elsewhere in user-defined
functions, it's good to have the general exception mechanism so that
all needed cleanup can be automatically accomplished and consistent
semantics maintained. That is, we want user-defined closure handlers to
stay out of the user's face in the same way that built-ins do. Control
flow should pretend to work like the user expects, even when it
doesn't.
Here are the RFCs covered in this Apocalypse. PSA stands for "problem,
solution, acceptance", my private rating of how this RFC will fit into
Perl 6. Interestingly, this time I've rejected more RFCs than I
accepted. I must be getting cruel and callous in my old age. ":-)"
RFC PSA Title
------ -----
006 acc Lexical variables made default
019 baa Rename the C<local> operator
022 abc Control flow: Builtin switch statement
063 rr Exception handling syntax
064 bdc New pragma 'scope' to change Perl's default scoping
083 aab Make constants look like variables
088 bbc Omnibus Structured Exception/Error Handling Mechanism
089 cdr Controllable Data Typing
106 dbr Yet another lexical variable proposal: lexical variables made default
113 rr Better constants and constant folding
119 bcr Object neutral error handling via exceptions
120 bcr Implicit counter in for statements, possibly $#
167 bcr Simplify do BLOCK Syntax
173 bcc Allow multiple loop variables in foreach statements
199 abb Short-circuiting built-in functions and user-defined subroutines
209 cdr Fuller integer support in Perl
262 cdr Index Attribute
279 cdr my() syntax extensions and attribute declarations
297 dcr Attributes for compiler hints
309 adr Allow keywords in sub prototypes
330 acc Global dynamic variables should remain the default
337 bcc Common attribute system to allow user-defined, extensible attributes
340 dcr with takes a context
342 bcr Pascal-like "with"
Accepted RFCs
Note that, although these RFCs are in the "accepted" category, most are
accepted with major caveats (a ""c"" acceptance rating), or at least
some "buts" (a ""b"" rating). I'll try to list all those caveats here,
but where there are systematic changes, I may indicate these generally
in this document without attempting to rewrite the RFC in every detail.
Those who implement these features must be sensitive to these
systematic changes and not just uncritically implement everything the
RFC says.
I'd like to talk about exceptions first, but before that I have to deal
with the switch statement, because I think it's silly not to unify
exception handlers with switch statements.
RFC 022: Control flow: Builtin switch statement
Some OO purists say that any time you want to use a switch statement,
you ought to make the discriminant of the switch statement into a type,
and use method dispatch instead. Fortunately, we are not OO purists
here, so forget that argument.
Another argument against having a switch statement in Perl 6 is that we
never had it in the first five versions of Perl. But it would be
incorrect to say that we didn't miss it. What actually happened was
that every time we started discussing how to add a switch statement, it
wasn't obvious how far to go. A switch statement in Perl ought to do
more than a switch statement in C (or in most any other language, for
that matter). So the fact that we haven't added a switch statement so
far says more about how hard it is to design a good one than about how
much we wanted a lousy one. Eventually the ever inventive Damian Conway
came up with his famous design, with a Perl 5 module as proof of
concept, and pretty much everyone agreed that he was on the right
track, for some definition of "right" (and "track"). This RFC is
essentially that design (not surprisingly, since Damian wrote it), so
it will be accepted, albeit with several tweaks.
In the first place, as a quasi-linguist, I loathe the keywords "switch"
and "case". I would prefer keywords that read better in English. Much
as I love verbing nouns, they don't work as well as real verbs or real
prepositions when topicalizers are called for. After thrashing over
several options with Damian and other folks, we've settled on using
"given" instead of "switch", and "when" instead of "case":
given EXPR {
when EXPR { ... }
when EXPR { ... }
...
}
The other great advantage of using different words is that people won't
expect it to work exactly like any other switch statement they may be
familiar with.
That being said, I should point out that it is still called "the switch
statement", and the individual components are still "cases". But you
don't have to put "switch" or "case" into constant-width font, because
they're not keywords.
Because curlies are so extremely overloaded in Perl 5, I was at first
convinced that we would need a separator of some sort between the
expression and the block, maybe a ":" or "=>" or some such. Otherwise
it would be too ambigous to come upon a left curly when expecting an
operator--it would be interpreted as a hash subscript instead. Damian's
RFC proposes to require parentheses in certain situations to
disambiguate the expression.
But I've come to the conclusion that I'd rather screw around (a little)
with the "insignificant whitespace" rule than to require an extra
unnatural delimiter. If we observe current practice, we note that 99%
of the time, when people write a hash subscript they do so without any
whitespace before it. And 99% of the time, when they write a block,
they do put some whitespace in front of it. So we'll just dwim it using
the whitespace. (No, we're not going all the way to whole-hog
whitespace dwimmery--Python will remain the best/worst example of that
approach.)
Subscripts are the only valid use of curlies when an operator is
expected. (That is, subscripts are essentially postfix operators.) In
contrast, hash composers and blocks are terms, not operators.
Therefore, we will make the rule that a left curly that has whitespace
in front of it will never be interpreted as a subscript in Perl 6. (If
you think this is totally bizarre thing to do, consider that this new
approach is actually consistent with how Perl 5 already parses
variables within interpolated strings.) If there is any space before
the curly, we force it to start a term, not an operator, which means
that the curlies in question must delimit either a hash composer or a
block. And it's a hash composer only if it contains a "=>" pair
constructor at the top level (or an explicit "hash" keyword on the
front.) Therefore it's possible to unambiguously terminate an
expression by following it with a block, as in the constructs above.
Interestingly, this one tweak to the whitespace rule also means that
we'll be able to simplify the parentheses out of other similar built-in
constructs:
if $foo { ... }
elsif $bar { ... }
else { ... }
while $more { ... }
for 1..10 { ... }
I think throwing out two required punctuation characters for one
required whitespace is an excellent trade in terms of readability,
particularly when it already matches common practice. (You can still
put in the parens if you want them, of course, just for old times'
sake.) This tweak also allows greater flexibility in how user-defined
constructs are parsed. If you want to define your own constructs, they
should be able to follow the same syntax rules as built-ins.
By a similar chain of logic (or illogic), I also want to tweak the
whitespace rules for the trailing curly. There are severe problems in
any C-derived language that allows user-defined constructs containing
curlies (as Perl does). Even C doesn't entirely escape the head-
scratching puzzle of "When do I put a semicolon after a curly?" A
"struct" definition requires a terminating semicolon, for instance,
while an "if" or a "while" doesn't.
In Perl, this problem comes up most often when people say "Why do I
have to put a semicolon after "do {}" or "eval {}" when it looks like a
complete statement?"
Well, in Perl 6, you don't, if the final curly is on a line by itself.
That is, if you use an expression block as if it were a statement
block, it behaves as one. The win is that these rules are consistent
across all expression blocks, whether user-defined or built-in. Any
expression block construct can be treated as either a statement or a
component of an expression. Here's a block that is being treated as a
term in an expression:
$x = do {
...
} + 1;
However, if you write
$x = do {
...
}
+ 1;
then the + will be taken erroneously as the start of a new statement.
(So don't do that.)
Note that this special rule only applies to constructs that take a
block (that is, a closure) as their last (or only) argument. Operators
like "sort" and "map" are unaffected. However, certain constructs that
used to be in the statement class may become expression constructs in
Perl 6. For instance, if we change "BEGIN" to an expression construct
we can now use a "BEGIN" block inside an expression to force compile-
time evaluation of a non-static expression:
$value = BEGIN { call_me_once() } + call_me_again();
On the other hand, a one-line "BEGIN" would then have to have a
semicolon.
[Update: We ended up solving this by distinguishing statement "BEGIN"
from term "BEGIN". Also, we can't really syntactically distinguish the
closures for constructs like "sort" or "map" if they are subject to
multiple dispatch, since we wouldn't know the signature. However, you
never have to end such a block on a line by itself, since you can
always put a comma, or even feed the list to the operator using the
"<==" pipe operator.]
Anyway, back to switch statements. Damian's RFC proposes various
specific kinds of dwimmery, and while some of those dwims are spot on,
others may need adjustment. In particular, there is an assumption that
the programmer will know when they're dealing with an object reference
and when they're not. But everything will be an object reference in
Perl 6, at some level or other. The underlying characteristics of any
object are most generally determined by the answer to the question,
"What methods does this object respond to?"
Unfortunately, that's a run-time question in general. But in specific,
we'd like to be able to optimize many of these switch statements at
compile time. So it may be necessary to supply typological hints in
some cases to do the dwimmery efficiently. Fortunately, most cases are
still fairly straightforward. A 1 is obviously a number, and a "foo" is
obviously a string. But unary "+" can force anything to a number, and
unary "_" can force anything to a string. Unary "?" can force a
boolean, and unary "." can force a method call. More complicated
thoughts can be represented with closure blocks.
[Update: String context is now forced by unary "~".]
Another thing that needs adjustment is that the concept of "isa"
matching seems to be missing, or at least difficult to express. We need
good "isa" matching to implement good exception handling in terms of
the switch mechanism. This means that we need to be able to say
something like:
given $! {
when Error::Overflow { ... }
when Error::Type { ... }
when Error::ENOTTY { ... }
when /divide by 0/ { ... }
...
}
and expect it to check "$!.isa(Error::Overflow)" and such, along with
more normal pattern matching. In the case of the actual exception
mechanism, we won't use the keyword "given", but rather "CATCH":
CATCH {
when Error::Overflow { ... }
when Error::Type { ... }
when Error::ENOTTY { ... }
when /divide by 0/ { ... }
...
}
"CATCH" is a "BEGIN"-like block that can turn any block into a "try"
block from the inside out. But the insides of the "CATCH" are an
ordinary switch statement, where the discriminant is simply the current
exception object, $!. More on that later--see RFC 88 below.
Some of you may recall that I've stated that Perl 6 will have no
barewords. That's still the case. A token like "Error::Overflow" is not
a bareword because it's a declared class. Perl 6 recognizes package
names as symbolic tokens. So when you call a class method as
"Class::Name.method()", the "Class::Name" is actually a class object
(that just happens to stringify to ""Class::Name""). But the class
method can be called without a symbolic lookup on the package name at
run time, unlike in Perl 5.
Since "Error::Overflow" is just such a class object, it can be
distinguished from other kinds of objects in a switch statement, and an
"isa" can be inferred. It would be nice if we could go as far as to say
that any object can be called with any class name as a method name to
determine whether it "isa" member of that class, but that could
interfere with use of class name methods to implement casting or
construction. So instead, since switch statements are into heavy
dwimmery anyway, I think the switch statement will have to recognize
any "Class::Name" known at compile time, and force it to call
"$!.isa(Class::Name)".
[Update: Actually, it just follows from how smart matching works. But
in fact, smart match calls the ".does" method, which is more general
than the ".isa" method, insofar as ".does" can check both roles and
inheritance. See A12 for more.]
Another possible adjustment will involve the use of switch statements
as a means of parallelizing regular expression evaluation.
Specifically, we want to be able to write parsers easily in Perl, which
means that we need some way of matching a token stream against
something like a set of regular expressions. You can think of a token
stream as a funny kind of string. So if the "given" of a switch
statement is a token stream, the regular expressions matched against it
may have special abilities relating to the current parse's data
structure. All the regular expressions of such a switch statement will
likely be implicitly anchored to the current parse location, for
instance. There may be special tokens referring to terminals and non-
terminals. Basically, think of something like a yacc grammar, where
alternative pattern/action grammar rules are most naturally expressed
via switch statement cases. More on that in the next Apocalypse.
[Update: Parsing rules tend to use the ":p" modifier, which anchors the
next token automatically to the start of the rule. That's all that
needs to be user visible. Everything else is the domain of the
optimizer.]
Another possible adjustment is that the proposed "else" block could be
considered unnecessary. The code following the final "when" is
automatically an "else". Here's a duodecimal digit converter:
$result = given $digit {
when "T" { 10 }
when "E" { 11 }
$digit;
}
Nevertheless, it's probably good documentation to line up all the
blocks, which means it would be good to have a keyword. However, for
reasons that will become clearer when we talk about exception handlers,
I don't want to use "else". Also, because of the identification of
"when" and "if", it would not be clear whether an "else" should
automatically supply a "break" at the end of its block as the ordinary
"when" case does.
So instead of "else", I'd like to borrow a bit more from C and use
"default":
$result = given $digit {
when "T" { 10 }
when "E" { 11 }
default { $digit }
}
Unlike in C, the "default" case must come last, since Perl's cases are
evaluated (or at least pretend to be evaluated) in order. The optimizer
can often determine which cases can be jumped to directly, but in cases
where that can't be determined, the cases are evaluated in order much
like cascaded "if"/"elsif"/"else" conditions. Also, it's allowed to
intersperse ordinary code between the cases, in which case the code
must be executed only if the cases above it fail to match. For example,
this should work as indicated by the print statements:
given $given {
print "about to check $first";
when $first { ... }
print "didn't match $first; let's try $next";
when $next { ... }
print "giving up";
default { ... }
die "panic: shouldn't see this";
}
We can still define "when" as a variant of "if", which makes it
possible to intermix the two constructs when (or if) that is desirable.
So we'll leave that identity in--it always helps people think about it
when you can define a less familiar construct in terms of a more
familiar one. However, the "default" isn't quite the same as an "else",
since "else" can't stand on its own. A "default" is more like an "if"
that's always true. So the above code is equivalent to:
given $given {
print "about to check $first";
if $given =~ $first { ...; break }
print "didn't match $first; let's try $next";
if $given =~ $next { ...; break }
print "giving up";
if 1 { ...; break; }
die "panic: shouldn't see this";
}
We do need to rewrite the relationship table in the RFC to handle some
of the tweaks and simplifications we've mentioned. The comparison of
bare refs goes away. It wasn't terribly useful in the first place,
since it only worked for scalar refs. (To match identities we'll need
an explicit ".id" method in any event. We won't be relying on the
default numify or stringify methods to produce unique representations.)
[Update: Instead of an ".id" method, which would be unclear how to
compare, there's a "=:=" operator for comparing identities.]
I've rearranged the table to be applied in order, so that default
interpretations come later. Also, the "Matching Code" column in the RFC
gave alternatives that aren't resolved. In these cases I've chosen the
"true" definition rather than the "exists" or "defined" definition.
(Except for certain set manipulations with hashes, people really
shouldn't be using the defined/undefined distinction to represent true
and false, since both true and false are considered defined concepts in
Perl.)
Some of the table entries distinguish an array from a list. Arrays look
like this:
when [1, 3, 5, 7, 9] { "odd digit intersection" }
when @array { "array intersection" }
while a list looks like this:
when 1, 3, 5, 7, 9 { "odd digit" }
when @foo, @bar, @baz { "intersection with at least one array" }
Ordinarily lists and arrays would mean the same thing in scalar
context, but "when" is special in differentiating explicit arrays from
lists. Within a "when", a list is a recursive disjunction. That is, the
comma-separated values are treated as individual cases OR-ed together.
We could use some other explicit notation for disjunction such as:
when any(1, 3, 5, 7, 9) { "odd" }
But that seems a lot of trouble for a very common case of case, as it
were. We could use vertical bars as some languages do, but I think the
comma reads better.
[Update: since we ended up adding junctions, you do use vertical bars
now. If you like commas you can always use "any()".]
Anyway, here's another simplification. The following table will also
define how the Perl 6 "=~" operator works! That allows us to use a
recursive definition to handle matching against a disjunctive list of
cases. (See the first entry in the table below.) Of course, for
precedence reasons, to match a list of things using "=~" you'll have to
use parens:
$digit =~ (1, 3, 5, 7, 9) and print "That's odd!";
Alternatively, you can look at this table as the definition of the "=~"
operator, and then say that the switch statement is defined in terms of
"=~". That is, for any switch statement of the form
given EXPR1 {
when EXPR2 { ... }
}
it's equivalent to saying this:
for (scalar(EXPR1)) {
if ($_ =~ (EXPR2)) { ... }
}
[Update: the "=~" operator has been renamed "~~".]
Table 1: Matching a switch value against a case value
$a $b Type of Match Implied Matching Code
====== ===== ===================== =============
expr list recursive disjunction match if $a =~ any($b)
list list recursive disjunction* match if any($a) =~ any($b)
hash sub(%) hash sub truth match if $b(%$a)
array sub(@) array sub truth match if $b(@$a)
expr sub($) scalar sub truth match if $b($a)
expr sub() simple closure truth* match if $b()
hash hash hash key intersection* match if grep exists $a{$_}, $b.keys
hash array hash value slice truth match if grep {$a{$_}} @$b
hash regex hash key grep match if grep /$b/, keys %$a
hash scalar hash entry truth match if $a{$b}
array array array intersection* match if any(@$a) =~ any(@$b)
array regex array grep match if grep /$b/, @$a
array number array entry truth match if $a[$b]
array expr array as list match if any($a) =~ $b
object class class membership match if $a.isa($b)
object method method truth match if $a.$b()
expr regex pattern match match if $a =~ /$b/
expr subst substitution match match if $a =~ subst
expr number numeric equality match if $a == $b
expr string string equality match if $a eq $b
expr boolean simple expression truth* match if $b
expr undef undefined match unless defined $a
expr expr run-time guessing match if ($a =~ $b) at runtime
[Update: This is inaccurate in several ways; see the most recent table
in S4.]
In order to facilitate optimizations, these distinctions are made
syntactically at compile time whenever possible. For each comparison,
the reverse comparison is also implied, so $a/$b can be thought of as
either given/when or when/given. (We don't reverse the matches marked
with * are because it doesn't make sense in those casees.)
If type of match cannot be determined at compile time, the default is
to try to apply the very same rules in the very same order at run time,
using the actual types of the arguments, not their compile-time type
appearance. Note that there are no run-time types corresponding to
"method" or "boolean". Either of those notions can be expressed at
runtime as a closure, of course.
In fact, whenever the default behavior is not what you intend, there
are ways to force the arguments to be treated as you intend:
Intent Natural Forced
====== ======= ======
array @foo [list] or @{expr}
hash %bar {pairlist} or %{expr}
sub(%) { %^foo.aaa } sub (%foo) { ... }
sub(@) { @^bar.bbb } sub (@bar) { ... }
sub($) { $^baz.ccc } sub ($baz) { ... }
number numeric literal +expr int(expr)num(expr)
string string literal _expr str(expr)
regex //, m//, qr// /$(expr)/
method .foo(args) { $_.$method(args) }
boolean $a == $b ?expr or true expr or { expr }
[Update: "qr//" is now spelled "rx//". String context is forced "~".
A regex is "interpolated" with "/<$expr"/>, but it's not interpolated
as a separate pass as in Perl 5.]
A method must be written with a unary dot to distinguish it from other
forms. The method may have arguments. In essence, when you write
.foo(1,2,3)
it is treated as if you wrote
{ $_.foo(1,2,3) }
and then the closure is evaluated for its truth.
A class match works only if the class name is known at compile time.
Use ".isa("Class")" for more complicated situations.
[Update: You can specify a run-time class name via "::($expr)" without
explicitly using ".does" (or ".isa", for that matter).]
Boolean expressions are recognized at compile time by the presence of a
top-level operator that is a comparison or logical operator. As the
table shows, an argumentless closure (a "sub ()", that is) also
functions as a boolean. However, it's probably better documentation to
use the "true" function, which does the opposite of "not". (Or the
unary "?" operator, which does the opposite of unary "!".)
It might be argued that boolean expressions have no place here at all,
and that you should use "if" if that's what you mean. (Or use a "sub()"
closure to force it to ignore the given.) However, the "comb" structure
of a switch is an extremely readable way to write even ordinary boolean
expressions, and rather than forcing people to write:
anyblock {
when { $a == 1 } { ... }
when { $b == 2 } { ... }
when { $c == 3 } { ... }
default { ... }
}
I'd rather they be able to write:
anyblock {
when $a == 1 { ... }
when $b == 2 { ... }
when $c == 3 { ... }
default { ... }
}
This also fits better into the use of "when" within "CATCH" blocks:
CATCH {
when $!.tag eq "foo" { ... }
when $!.tag eq "bar" { ... }
default { die }
}
To force all the "when" clauses to be interpreted as booleans without
using a boolean operator on every case, simply provide an empty given,
to be read as "given nothing...":
given () {
when $a.isa(Ant) { ... }
when $b.isa(Bat) { ... }
when $c.isa(Cat) { ... }
default { ... }
}
A "when" can be used by other topicalizers than just "given". Just as
"CATCH" will imply a given of $!, a "for" loop (the "foreach" variety)
will also imply a given of the loop variable:
for @foo {
when 1 { ... }
when 2 { ... }
when "x" { ... }
default { ... }
}
By symmetry, a "given" will by default alias $_ to the "given".
Basically, the only difference between a "given" and a "for" is that a
"given" takes a scalar expression, while a "for" takes a pre-flattened
list and iterates over it.
[Update: "for" takes a lazily flattened list, not a pre-flattened list.
Use unary "**" to pre-flatten a list.]
Suppose you want to preserve $_ and alias $g to the value instead. You
can say that like this:
given $value -> $g {
when 1 { /foo/ }
when 2 { /bar/ }
when 3 { /baz/ }
}
In the same way, a loop's values can be aliased to one or more loop
variables.
for @foo -> $a, $b { # two at a time
...
}
That works a lot like the definition of a subroutine call with two
formal parameters, $a and $b. (In fact, that's precisely what it is.)
You can use modifiers on the formal paramaters just as you would in a
subroutine type signature. This implies that the aliases are
automatically declared as "my" variables. It also implies that you can
modify the formal parameter with an "rw" property, which allows you to
modify the original elements of the array through the variable. The
default loop:
for @foo { ... }
is really compiled down to this:
for @foo -> $_ is rw { ... }
Since "for" and "given" work by passing arguments to a closure, it's a
small step to generalize that in the other direction. Any method
definition is a topicalizer within the body of the method, and will
assume a "given" of its $self object (or whatever you have named it).
Bare closures topicalize their first argument, implicitly aliasing it
to $_ unless $^a or some such is used. That is, if you say this:
grep { $_ eq 3 } @list
it's equivalent to this more explicit use of a curried function:
grep { $^a eq 3 } @list
But even a "grep" can use the aliasing syntax above:
grep -> $x { $x eq 3 } @list
Outside the scope of any topicalizer, a "when" will assume that its
given was stored in $_ and will test implicitly against that variable.
This allows you to use "when" in your main loop, for instance, even if
that main loop was supplied by Perl's "-n" or "-p" switch. Whenever a
loop is functioning as a switch, the break implied by finishing a case
functions as a "next", not a "last". Use "last" if that's what you
mean.
A "when" is the only defaulting construct that pays attention to the
current topicalizer regardless of which variable it is associated with.
All other defaulting constructs pay attention to a fixed variable,
typically $_. So be careful what you're matching against if the given
is aliased to something other than $_:
$_ = "foo";
given "bar" -> $f {
if /foo/ { ... } # true, matches against $_
when /bar/ { ... } # true, matches against $f
}
[Update: The first argument is always aliased to $_ now.]
Oh, one other tweak. The RFC proposes to overload "next" to mean "fall
through to the next case". I don't think this is wise, since we'll
often want to use loop controls within a switch statement. Instead, I
think we should use "skip" to do that. (To be read as "Skip to the next
statement.")
[Update: That's now spelled "continue".]
Similarly, if we make a word to mean to explicitly break out of a
topicalizer, it should not be "last". I'd suggest "break"! It will, of
course, be unnecessary to break out of the end of a "when" case because
the "break" is implied. However, there are times when you might want to
break out of a "when" block early. Also, since we're allowing "when"
modifiers that do not implicitly break, we could use an explicit break
for that situation. You might see cases like this:
given $x {
warn("Odd value") when !/xxx/;
warn("No value"), break when undef;
when /aaa/ { break when 1; ... }
when /bbb/ { break when 2; ... }
when /ccc/ { break when 3; ... }
}
So it looks to me like we need a break.
RFC 088: Omnibus Structured Exception/Error Handling Mechanism
This RFC posits some requirements for exception handling (all of which
I agree with), but I do have some additional requirements of my own:
· The exception-catching syntax must be considered a form of switch
statement.
· It should be easy to turn any kind of block into a "try" block,
especially a subroutine.
· Even "try"-less try blocks must also be able to specify mandatory
cleanup on exit.
· It should be relatively easy to determine how much cleanup is
necessary regardless of how a block was exited.
· It must be possible to base the operation of "return", "next", and
"last" on exception handling.
· The cleanup mechanism should mesh nicely with the notions of post
condition processing under design-by-contract.
· The exception-trapping syntax must not violate encapsulation of
lexical scopes.
· At the same time, the exception-trapping syntax should not force
declarations out of their natural scope.
· Non-linear control flow must stand out visually, making good use of
block structure, indentation and even keyword case. "BEGIN" and
"END" blocks are to be considered prior art.
· Non-yet-thrown exceptions must be a useful concept.
· Compatibility with the syntax of any other language is specifically
NOT a goal.
RFC 88 is massive, weighing in at more than 2400 lines. Annotating the
entire RFC would make this Apocalypse far too big. ("Too late!" says
Damian.) Nonetheless, I will take the approach of quoting various bits
of the RFC and recasting those bits to work with my additional
requirements. Hopefully this will convey my tweaks most succinctly.
Here's what the RFC gives as its first example:
exception 'Alarm';
try {
throw Alarm "a message", tag => "ABC.1234", ... ;
}
catch Alarm => { ... }
catch Error::DB, Error::IO => { ... }
catch $@ =~ /divide by 0/ => { ... }
catch { ... }
finally { ... }
Here's how I see that being written in Perl 6:
my class X::Alarm is Exception { } # inner class syntax?
try {
throw X::Alarm "a message", tag => "ABC.1234", ... ;
CATCH {
when X::Alarm { ... }
when Error::DB, Error::IO { ... }
when /divide by 0/ { ... }
default { ... }
}
POST { ... }
}
[Update: the comma in the second case should be a "|".]
The outer block does not have to be a "try" block. It could be a
subroutine, a loop, or any other kind of block, including an "eval"
string or an entire file. We will call such an outer block a try block,
whether or not there is an explicit "try" keyword.
The biggest change is that the various handlers are moved inside of the
try block. In fact, the "try" keyword itself is mere documentation in
our example, since the presence of a "CATCH" or "POST" block is
sufficient to signal the need for trapping. Note that the "POST" block
is completely independent of the "CATCH" block. (The "POST" block has a
corresponding "PRE" block for design-by-contract programmers.) Any of
these blocks may be placed anywhere in the surrounding block--they are
independent of the surrounding control flow. (They do have to follow
any declarations they refer to, of course.) Only one "CATCH" is
allowed, but any number of "PRE" and "POST" blocks. (In fact, we may
well encourage ourselves to place "POST" blocks near the constructors
to be cleaned up after.) "PRE" blocks within a particular try block are
evaluated in order before anything else in the block. "POST" blocks
will be evaluated in reverse order, though order dependencies between
"POST" blocks are discouraged. "POST" blocks are evaluated after
everything else in the block, including any "CATCH".
[Update: "PRE" and "POST" are now spelled "ENTER" and "LEAVE". ("PRE"
and "POST" are reserved for Design By Contract assertions).]
A "try {}" without a "CATCH" is equivalent to Perl 5's "eval {}". (In
fact, "eval" will go back to evaluating only strings in Perl 6, and
"try" will evaluate only blocks.)
The "CATCH" and "POST" blocks are naturally in the lexical scope of the
try block. They may safely refer to lexically scoped variables declared
earlier in the try block, even if the exception is thrown during the
elaboration sequence. (The run-time system will guarantee that
individual variables test as undefined (and hence false) before they
are elaborated.)
The inside of the "CATCH" block is precisely the syntax of a switch
statement. The discriminant of the switch statement is the exception
object, $!. Since the exception object stringifies to the error
message, the "when /divide by 0/" case need not be explicitly compared
against $!. Likewise, explicit mention of a declared class implies an
"isa" lookup, another built-in feature of the new switch statement.
In fact, a "CATCH" of the form:
CATCH {
when xxx { ... } # 1st case
when yyy { ... } # 2nd case
... # other cases, maybe a default
}
means something vaguely like:
BEGIN {
%MY.catcher = {
given current_exception() -> $! {
when xxx { ... } # 1st case from above
when yyy { ... } # 2nd case from above
... # other cases, maybe a default
die; # rethrow $! as implicit default
}
$!.markclean; # handled cleanly, in theory
}
}
The unified "current exception" is $!. Everywhere this RFC uses $@, it
should be read as $! instead. (And the too-precious "@@" goes away
entirely in favor of an array stored internally to the $! object that
can be accessed as "@$!" or $![-1].) (For the legacy Perl 5 parser, $@
and $? will be emulated, but that will not be available to the Perl 6
parser.)
Also note that the "CATCH" block implicitly supplies a rethrow (the
"die" above) after the cases of the switch statement. This will not be
reached if the user has supplied an explicit "default" case, since the
"break" of that default case will always bypass the implicit "die".
And if the switch rethrows the exception (either explicitly or
implicitly), $! is not marked as clean, since the "die" will bypass the
code that marks the exception as "cleanly caught". It should be
considered an invariant that any $! in the normal control flow outside
of a "CATCH" is considered "cleanly caught", according to the
definition in the RFC. Unclean exceptions should only be seen inside
"CATCH" blocks, or inside any "POST" blocks that have to execute while
an exception is propagating to an outer block because the current try
block didn't handle it. (If the current try block does successfully
handle the exception in its "CATCH", any "POST" blocks at the same
level see a $! that is already marked clean.)
RFC:
"eval {die "Can't foo."}; print $@;" continues to work as before.
That will instead look like
try { die "Can't foo" }; print $!;
in Perl 6. A "try" with no "CATCH":
try { ... }
is equivalent to:
try { ... CATCH { default { } } }
(And that's another reason I didn't want to use "else" for the default
case of a switch statement--an "else" without an "if" looks really
bizarre...)
Just as an aside, what I'm trying to do here is untangle the exception
trapping semantics of "eval" from its code parsing and running
semantics. In Perl 6, there is no "eval {}". And "eval $string" really
means something like this:
try { $string.parse.run }
RFC:
This RFC does not require core Perl functions to use exceptions for
signalling errors.
However, Perl core functions will by default signal failure using
unthrown proto-exceptions (that is, interesting values of undef) that
can easily be turned into thrown exceptions via "die". By "interesting
values of undef", I don't mean undef with properties. I mean full-
fledged exception objects that just happen to return false from their
".defined" and ".true" methods. However, the ".str" method successfully
returns the error message, and the ".int" method returns the error code
(if any). That is, they do stringify and numify like $! ought to. An
exception becomes defined and true when it is thrown. (Control
exceptions become false when cleanly caught, to avoid spoofing old-
style exception handlers.)
RFC:
This means that all exceptions propagate unless they are cleanly
caught, just as in Perl 5. To prevent this, use:
try { fragile(); } catch { } # Go on no matter what.
This will simply be:
try { fragile; }
But it means the same thing, and it's still the case that all
exceptions propagate unless they are cleanly caught. In this case, the
caught exception lives on in $! as a new proto-exception that could be
rethrown by a new "die", much as we used to use $@. Whether an
exception is currently considered "cleanly caught" can be reflected in
the state of the $! object itself. When $! passes through the end of a
"CATCH", it is marked as clean, so that subsequent attempts to
establish a new $! know that they can clear out the old "@$!" stack.
(If the current $! is not clean, it should just add its information
without deleting the old information--otherwise an error in a "CATCH"
could delete the exception information you will soon be wanting to
print out.)
RFC:
try { ... } catch <test> => { ... } finally { ... }
Now:
{ ... CATCH { when <test> { ... } } POST { ... } }
(The angle brackets aren't really there--I'm just copying the RFC's
metasyntax here.)
Note that we're assuming a test that matches the "boolean" entry from
the switch dwimmery matrix. If not, you can always wrap closure curlies
around the test:
{ ... CATCH { when { <test> } { ... } } POST { ... } }
That will force the test to be called as a subroutine that ignores its
argument, which happens to be $!, the exception object. (Recall that
the implied "given" of a "CATCH" statement sets $! as the given value.
That given value is automatically passed to any "when" cases that look
like subroutines or closures, which are free either to ignore the
passed value, or access it as $_ or $^a.)
Or you might just prefer to use the unary "true" operator:
{ ... CATCH { when true <test> { ... } } POST { ... } }
I personally find that more readable than the closure.
RFC:
The test argument of the catch clause is optional, and is described
below.
The test argument of a "when" clause is NOT optional, since it would be
impossible to distinguish a conditional closure from the following
block. Use "default" for the default case.
RFC:
"try", "catch", and "finally" blocks should share the same lexical
scope, in the way that "while" and "continue" do.
Actually, this is not so--the "while" and "continue" blocks don't share
the same lexical scope even in Perl 5. But we'll solve this issue
without "tunneling" in any case. (And we'll change the "continue" block
into a "NEXT" block that goes inside, so we can refer to lexical
variables from within it.)
RFC:
Note that "try" is a keyword, not a function. This is so that a ";" is
not needed at the end of the last block. This is because a
"try"/"catch"/"finally" now looks more like an "if"/"elsif"/"else",
which does not require such a ";", than like an eval, which does).
Again, this entire distinction goes away in Perl 6. Any expression
block that terminates with a right curly on its own line will be
interpreted as a statement block. And "try" is such an expression
block.
RFC:
$@ contains the current exception, and "@@" contains the current
exception stack, as defined above under "die". The "unshift" rule
guarantees that "$@ == $@[0]".
Why an "unshift?" A stack is most naturally represented in the other
direction, and I can easily imagine some kinds of handlers that might
well treat it like a stack, stripping off some entries and pushing
others.
Also, "@@" is a non-starter because everything about the current
exception should all be in a single data structure. Keeping the info
all in one place makes it easy to rethrow an exception without losing
data, even if the exception was marked as cleanly caught. Furthermore I
don't think that the exception stack needs to be Huffman coded that
badly.
So $! contains the current exception, and "$!.stack" accesses the
current exception stack. Through the magic of overloading, the $!
object can likely be used as an array even though it isn't one, in
which case "@$!" refers to that stack member. The "push" rule
guarantees that "$!.id == $![-1].id".
RFC (speaking of the "exception" declaration):
If the given name matches "/::/", something like this happens:
@MyError::App::DB::Foo::ISA = 'MyError::App::DB';
and all non-existent parent classes are automatically created as
inheriting from their parent, or "Exception" in the tail case. If a
parent class is found to exist and not inherit from "Exception", a
run-time error exception is raised.
If I understand this, I think I disagree. A package ought to able to
contain exceptions without being an exception class itself. There
certainly ought to be a shorthand for exceptions within the current
package. I suspect they're inner classes of some sort, or inner classes
of an inner package, or some such.
RFC:
If the given name does not match "/::/" (say it's just "Alarm"), this
happens instead:
@Alarm::ISA = 'Exception';
This means that every exception class isa "Exception", even if
"Exception::" is not used at the beginning of the class name.
Ack! This could be really bad. What if two different modules declare an
"Alarm" exception with different derivations?
I think we need to say that unqualified exceptions are created within
the current package, or maybe within the X subpackage of the current
package. If we have inner classes, they could even be lexically scoped
(and hence anonymous exceptions outside the current module). That might
or might not be a feature.
I also happen to think that "Exception" is too long a name to prefix
most common exceptions, even though they're derived from that class. I
think exceptions will be better accepted if they have pithier names
like X::Errno that are derived from "Exception":
our class X::Control is Exception;
our class X::Errno is Exception;
our class X::NumericError is Exception;
our class C::NEXT is X::Control;
our class E::NOSPC is X::Errno;
our class X::FloatingUnderflow is X::NumericError;
Or maybe those could be:
c::NEXT
e::NOSPC
x::FloatingUnderflow
if we decide uppercase names are too much like user-defined package
names. But that looks strange. Maybe we just reserve single letter top-
level package names for Perl. Heck, let's just reserve all top-level
package names for Perl. Er, no, wait... ":-)"
RFC 80 suggests that exception objects numerify to the system's errno
number when those are available. That's a possibility, though by the
current switch rules we might have to write
CATCH {
when +$ENOSPC { ... }
}
to force $ENOSPC to do a numeric comparison. It may well be better to
go ahead and make the errno numbers into exception classes, even if we
have to write something like this:
CATCH {
when X::ENOSPC { ... }
}
That's longer, but I think it's clearer. Possibly that's "E::NOSPC"
instead. But in any event, I can't imagine getting people to prefix
every exception with ""Exception::"". That's just gonna discourage
people from using exceptions. I'm quite willing to at least reserve the
"X" top-level class for exceptions. I think "X::" is quite sufficiently
distinctive.
RFC:
try { my $f = open "foo"; ... } finally { $f and close $f; }
Now:
{
my $f = open "foo"; ...
POST { $f and close $f }
}
Note that $f is naturally in scope and guaranteed to have a boolean
value, even if the exception is thrown before the declaration statement
is elaborated! (An implementation need not allocate an actual variable
before the "my". The code of the "POST" block could always be compiled
to know that $f is to be assumed undefined if the allocating code has
not yet been reached.)
We could go as far as to make
POST { close $f }
do something reasonable even without the guard. Maybe an undefined
object could "emulate" any method for you within a "POST". Maybe "try"
is really a unary operator:
POST { try close $f }
Or some such. I dunno. This needs more thought along transactional
lines...
Time passes...
Actually, now that I've thought on it, it would be pretty easy to put
wrappers around "POST" blocks that could do commit or rollback
depending on whether the block exits normally. I'd like to call them
"KEEP" and "UNDO". "KEEP" blocks would only be executed if the block
succeeded. "UNDO" blocks would only be executed if the block failed.
One could even envision a syntax that ties the block to particular
variable:
UNDO $f { close $f }
After all, like the "CATCH" block, all of these blocks are just fancy
"BEGIN" blocks that attach some meaning to some predefined property of
the block.
It's tempting to make the execution of "UNDO" contingent upon whether
the block itself was passed during execution, but I'm afraid that might
leave a window in which a variable could already be set, but subsequent
processing might raise an exception before enabling the rollback in
question. So it's probably better to tie it to a particular variable's
state more directly than just by placing the block at some point after
the declaration. In fact, it could be associated directly with the
variable in question at declaration time via a property:
my $f is undo { close $f } = open $file or die;
[Update: All of these should use "will" rather than "is", or the
closure has to be an explicit argument to "undo" with no intervening
space.]
Note that the block is truly a closure because it relies on the lexical
scoping of $f. (This form of lexical scoping works in Perl 6 because
the name $f is introduced immediately within the statement. This
differs from the Perl 5 approach where the name is not introduced till
the end of the current statement.)
Actually, if the "close" function defaults to $_, we can say
my $f is undo { close } = open $file;
presuming the managing code is smart enough to pass $f as a parameter
to the closure. Likewise one could attach a "POST" block to a variable
with:
my $f is post { close } = open $file;
Since properties can be combined, you can set multiple handlers on a
variable:
my $f is post { close } is undo { unlink $file } = open ">$file" or die;
There is, however, no "catch" property to go with the "CATCH" block.
I suppose we could allow a "pre" property to set a "PRE" block on a
variable.
RFC:
sub attempt_closure_after_successful_candidate_file_open
{
my ($closure, @fileList) = @_; local (*F);
foreach my $file (@fileList) {
try { open F, $file; } catch { next; }
try { &$closure(*F); } finally { close F; }
return;
}
throw Exception "Can't open any file.",
debug => @fileList . " tried.";
}
Now:
sub attempt_closure_after_successful_candidate_file_open
(&closure, @fileList)
{
foreach my $file (@fileList) {
my $f is post { close }
= try { open $file or die; CATCH { next } }
&closure($f);
return;
}
throw Exception "Can't open any file.",
debug => @fileList . " tried.";
}
[Update: That "foreach" should now be "for @fileList -" $file>. And
the "is post" should be "is leave". Also these days I'd leave the
ampersand off the call to "closure()" (though it's still necessary in
the declaration of the parameter).]
Note that the "next" within the "CATCH" refers to the loop, not the
"CATCH" block. It is legal to "next" out of "CATCH" blocks, since we
won't use "next" to fall through switch cases.
However, "X::Control" exceptions (such as "X::NEXT") are a subset of
"Exceptions", so
CATCH {
when Exception { ... } # catch any exception
}
will stop returns and loop exits. This could be construed as a feature.
When it's considered a bug, you could maybe say something like
CATCH {
when X::Control { die } # propagate control exceptions
when Exception { ... } # catch all others
}
to force such control exceptions to propagate outward. Actually, it
would be nice to have a name for non-control exceptions. Then we could
say (with a tip of the hat to Maxwell Smart):
CATCH {
when X::Chaos { ... } # catch non-control exceptions
}
And any control exceptions will then pass unimpeded (since by default
uncaught exceptions are rethrown implicitly by the "CATCH").
Fortunately or unfortunately, an explicit "default" case will not
automatically rethrow control exceptions.
[Update: We now distinguish a "CONTROL" block from a "CATCH" block.
Control exceptions are invisible to the CATCH block.]
Following are some more examples of how the expression evaluation of
"when" can be used. The RFC versions sometimes look more concise, but
recall that the "try" is any block in Perl 6, whereas in the RFC form
there would have to be an extra, explicit "try" block inside many
subroutines, for instance. I'd rather establish a culture in which it
is expected that subroutines handle their own exceptions.
RFC:
try { ... } catch $@->{message} =~ /.../ => { ... }
Now:
try {
...
CATCH {
when $!.message =~ /.../ { ... }
}
}
This works because =~ is considered a boolean operator.
[Update: The operator is "~~" now.]
RFC:
catch not &TooSevere => { ... }
Now:
when not &TooSevere { ... }
The unary "not" is also a boolean operator.
RFC:
try { ... } catch ref $@ =~ /.../ => { ... }
Now:
try { ... CATCH { when $!.ref =~ /.../ { ... } } }
RFC:
try { ... } catch grep { $_->isa("Foo") } @@ => { ... }
Now:
try {
...
CATCH {
when grep { $_.isa(Foo) } @$! { ... }
}
}
I suppose we could also assume grep to be a boolean operator in a
scalar context. But that's kind of klunky. If we accept Damian's
superposition RFC, it could be written this way:
try {
...
CATCH {
when true any(@$!).isa(Foo) { ... }
}
}
Actually, by the "any" rules of the "=~" table, we can just say:
try {
...
CATCH {
when @$! =~ Foo { ... }
}
}
[Update: Lists must be explicitly made into junctions, so now we'd
write that as "any($![]) ~~ Foo".]
The RFC proposes the following syntax for finalization:
try { my $p = P->new; my $q = Q->new; ... }
finally { $p and $p->Done; }
finally { $q and $q->Done; }
A world of hurt is covered over by that ""..."", which could move the
"finally" clauses far, far away from what they're trying to clean up
after. I think the intent is much clearer with "POST". And note also
that we avoid the "lexical tunneling" perpetrated by "finally":
{
my $p = P.new; POST { $p and $p.Done; }
my $q = Q.new; POST { $q and $q.Done; }
...
}
More concisely, we can say:
{
my $p is post { .Done } = P.new;
my $q is post { .Done } = Q.new;
...
}
RFC:
try { TryToFoo; }
catch { TryToHandle; }
finally { TryToCleanUp; }
catch { throw Exception "Can't cleanly Foo."; }
How I'd write that:
try {
try {
TryToFoo;
POST { TryToCleanUp; }
CATCH { TryToHandle; }
}
CATCH { throw Exception "Can't cleanly Foo."; }
}
That also more clearly indicates to the reader that the final "CATCH"
governs the inner try completely, rather than just relying on ordering.
RFC:
Instances of the actual (non-subclassed) "Exception" class itself are
used for simple exceptions, for those cases in which one more or less
just wants to say "throw Exception "My message."", without a lot of
extra tokens, and without getting into higher levels of the taxonomy of
exceptions.
"die "My message."" has much the same effect. I think "fail "My
message."" will also default similarly, though with return-or-throw
semantics that depend on the caller's "use fatal" settings.
RFC (regarding "on_raise"):
Derived classes may override this method to attempt to "handle" an
exception or otherwise manipulate it, just before it is raised. If
"on_raise" throws or returns true the exception is raised, otherwise it
is not. An exception can be manipulated or replaced and then propagated
in modified form simply by re-raising it in "on_raise".
Offhand, I don't see this one. Not only does it seem to be making the
$SIG{__DIE__} mistake all over again, it also makes little sense to me
to use "throw" to do something that doesn't throw. A throw should
guarantee termination of control, or you're just going to run user code
that wasn't expected to be run. It'd be like "return" suddenly not
returning! Let's please use a different method to generate an unthrown
exception. I think a "fail" method is the right approach--it terminates
the control flow one way or another, even if just returning the
exception as a funny-looking undef.
The "on_catch" might be a bit more useful.
RFC:
...because the authors are of the opinion that overloading "else" and
"continue" with unwind semantics not traditionally associated with
"else" and "continue" can be confusing, especially when intermixed with
local flow-control forms of "else" and "continue" (which may be present
in any "{ ... }" block), or when an "else die $@" is forgotten on a
"switch" that needs to re-throw.
"CATCH" will rethrow by default (unless there is a user-specified
default).
RFC:
Some perl6-language-error discussions have suggested leaving out the
try altogether, as in simply writing "{ } else { }" to indicate non-
local flow-control at work. Yikes!
The "try" is not for Perl's sake. It's for the developer's sake. It
says, watch out, some sort of non-local flow control is going on
here. It signals intent to deal with action at a distance
(unwinding semantics). It satisfies the first requirement listed
under MOTIVATION.
"try {}" is the new spelling of "eval {}", so it can still be used when
self-documentation is desired. It's often redundant, however, since I
think the all-caps "CATCH" and "POST" also serve the purpose of telling
the developer to "watch out". I expect that developers will get used to
the notion that many subroutines will end with a "CATCH" block. And I'm
always in favor of reducing the bracket count of ordinary code where
practical. (That's why the "package" declaration has always had a
bracketless syntax. I hope to do the same for classes and modules in
Perl 6.)
RFC:
The comma or "=" in a conditional catch clause is required so the
expression can be parsed from the block, in the fashion of Perl 5's
parsing of: "map <expression>, <list>"; Without the comma, the form
"catch $foo { ... }" could be a test for $foo or a test for $foo{...}
(the hash element).
We now require whitespace before a non-subscript block, so this is not
much of a problem.
RFC:
How can we subclass "Exception" and control the class namespace? For
example, if the core can use any "Exception::Foo", where does one
connect non-core "Exception"s into the taxonomy? Possibly the core
exceptions can derive from "Exception::CORE", and everyone else can use
the "Exception::MyPackage" convention.
I don't think defining things as core vs non-core is very
useful--"core" is not a fundamental type of exception. I do think the
standard exception taxonomy should be extensible, so that non-standard
exceptions can migrate toward being standard over time. I also think
that modules and classes should have their own subpackage in which to
store exceptions.
RFC:
How can we add new instance variables and methods to classes derived
from "Exception" and control those namespaces? Perhaps this will be
covered by some new Perl 6 object technology. Otherwise, we will need
yet another naming scheme convention.
Instance variables and methods in a derived class will not interfere
with base classes (except by normal hiding of duplicate method names).
RFC:
What should the default values be for "Exception" object instance
variables not specified to the constructor? For example, "tag" could
default to file + line number.
Depends on the constructor, I suspect.
RFC:
What assertions should be placed on the instance variables, if any?
Probably depends on the class.
RFC:
What should stringification return?
I lean towards just the message, with a different method for more info.
But this is somewhat dependent on which representational methods we
define for all Objects. And that has not been entirely thunk through.
RFC:
Mixed Flow Control
Some of the reference texts, when discussing exception handling,
refer to the matter that it may be difficult to implement a "go to"
across an unwinding semantics block, as in:
try { open F, $f } catch { next; }
This matter will have to be referred to the internals experts. It's
ok if this functionality is not possible, it can always be
simulated with lexical state variables instead.
However, the authors would very much prefer that "goto"s across
unwinding boundaries would dwim. If that is not possible, hopefully
some sort of compile-time warning could be produced.
We can do this with special control exceptions that aren't caught until
it makes sense to catch them. (Where exactly control exceptions fit in
the class hierarchy is still open to debate.) In any event, there's no
problem throwing a control exception from a "CATCH", since any
exception thrown in a "CATCH" or "POST" would propagate outside the
current try block in any event.
Ordinary "goto" should work as long as it's leaving the current try
scope. Reentering the try somewhere in the middle via "goto" is likely
not possible, or even desirable. A failed try should be re-entered from
the top, once things have been cleared up. (If the try is a loop block,
going to the next iteration out of its "CATCH" will probably be
considered safe, just as if there had been an explicit "try" block
within the loop. But I could be wrong on that.)
RFC:
Use "%@" for Errors from Builtins
RFC 151 proposes a mechanism for consolidating the information
provided by of $@, $!, $?, and $^E. In the opinion of the author of
RFC 88, merging $@ and $! should not be undertaken, because $@
should only be set if an exception is raised.
The RFC appears to give no justification for this last assertion. If we
unify the error variables, "die" with no arguments can simply raise the
current value of $!, and we stay object oriented all the way down. Then
$! indicates the current error whether or not it's being thrown. It
keeps track of its own state, as to whether it is currently in an
"unclean" state, and refuses to throw away information unless it's
clean.
"%@" should be used to hold this fault-hash, based on the following
arguments for symmetry.
$@ current exception
@@ current exception stack
%@ current core fault information
$@[0] same as $@
$@{type} "IO::File::NotFound"
$@{message} "can't find file"
$@{param} "/foo/bar/baz.dat"
$@{child} $?
$@{errno} $!
$@{os_err} $^E
$@{chunk} That chunk thingy in some msgs.
$@{file} Source file name of caller.
$@{line} Source line number of caller.
"%@" should not contain a severity or fatality classification.
Every call to a core API function should clear "%@" if it returns
successfully.
Internally, Perl can use a simple structured data type to hold the
whole canonical "%@". The code that handles reading from "%@" will
construct it out of the internal data on the fly.
If "use fatal;" is in scope, then just before returning, each core
API function should do something like: "%@ and internal_die %@;"
The "internal_die" becomes the one place where a canonical
"Exception" can be generated to encapsulate "%@" just before
raising an exception, whether or not the use of such canonical
"Exception"s is controlled by a pragma such as "use exceptions;".
This "%@" proposal just looks like a bunch of unnecessary complication
to me. A proto-exception object with methods can be just as easily (and
lazily) constructed, and will map straight into a real exception,
unlike this hash. And an object can always be used as a hash to access
parameterless methods such as instance variable accessors.
RFC:
eval
The semantics of "eval" are, "clear $@ and don't unwind unless the
user re-dies after the "eval"". The semantics of "try" are "unwind
after "try", unless any raised exception was cleanly and completely
handled, in which case clear $@".
In the author's opinion, both "eval" and "try" should exist in Perl
6. This would also mean that the legacy of examples of how to use
"eval" in Perl will still work.
And, of course, we still need "eval $string".
Discussions on perl6-language-errors have shown that some would
prefer the "eval { ... }" form to be removed from Perl 6, because
having two exception handling methods in Perl could be confusing to
developers. This would in fact be possible, since the same effect
can be achieved with:
try { } catch { } # Clears $@.
my $e;
try { ... } catch { $e = $@; }
# now process $e instead of $@
On the other hand, "eval" is a convenient synonym for all that,
given that it already works that way.
I don't think the exact semantics of "eval {...}" are worth preserving.
I think having bare "try {...}" assume a "CATCH { default {} }" will be
close enough. Very few Perl 5 programs actually care whether $@ is set
within the eval. Given that and the way we've defined $!, the
translation from Perl 5 to Perl 6 involves simply changing "eval {...}"
to "try {...}" and $@ to $! (which lives on as a "clean" exception
after being caught by the "try"). Perhaps some attempt can be made to
pull an external handler into an internal "CATCH" block.
RFC:
"catch v/s else + switch"
Some participants in discussions on perl6-language-errors have
expressed the opinion that not only should "eval" be used instead
of "try", but "else" should be used instead of multiple "catch"
blocks. They are of the opinion that an "else { switch ... }"
should be used to handle multiple catch clauses, as in:
eval { ... }
else {
switch ($@) {
case $@->isa("Exception::IO") { ... }
case $@->my_method { ... }
}
}
This problem with "else { switch ... }" is: how should the code
implicitly rethrow uncaught exceptions? Many proponents of this
model think that uncaught exceptions should not be implicitly
rethrown; one suggests that the programmer should "undef $@" at the
end of *every* successful case block, so that Perl re-raises any $@
still extant at the end of the "else".
This RFC allows a "switch" to be used in a "catch { ... }" clause,
for cases where that approach would minimize redundant code in
"catch <expr> { ... }" clauses, but with the mechanism proposed in
this RFC, the switch functionality shown above can be written like
this, while still maintaining the automatic exception propagation
when no cases match:
try { ... }
catch Exception::IO => { ... }
catch $@->my_method => { ... }
The switch construct works fine, because the implied "break" of each
handled case jumps over the default rethrow supplied by the "CATCH".
There's no reason to invent a parallel mechanism, and lots of reason
not to.
RFC:
Mechanism Hooks
In the name of extensibility and debugging, there should be hooks
for callbacks to be invoked when a "try", "catch", or "finally"
block is entered or exited, and when a conditional "catch" is
evaluated. The callbacks would be passed information about what is
happening in the context they are being called from.
In order to scope the effect of the callbacks (rather than making
them global), it is proposed that the callbacks be specified as
options to the try statement, something like this:
try on_catch_enter => sub { ... },
on_catch_exit => sub { ... },
{
...
}
The (dynamic, not lexical) scope of these callbacks is from their
try down through all trys nested under it (until overridden at a
lower level). Nested callbacks should have a way of chaining to
callbacks that were in scope when they come into scope, perhaps by
including a reference to the outer-scope callback as a parameter to
the callback. Basically, they could be kept in "global" variables
overridden with "local".
Yuck. I dislike cluttering up the "try" syntax with what are
essentially "temp" assignments to dynamically scoped globals. It should
be sufficient to say something like:
{
temp &*on_catch_enter = sub { ... };
temp &*on_catch_exit = sub { ... };
...
}
provided, of course, the implementation is smart enough to look for
those hooks when it needs them.
RFC:
Mixed-Mode Modules
Authors of modules who wish to provide a public API that respects
the current state of "use fatal;" if such a mechanism is available,
can do so as follows.
Internal to their modules, authors can use lexically scoped "use
fatal;" to explicitly control whether or not they want builtins to
raise exceptions to signal errors.
Then, if and only if they want to support the other style, and only
for public API subroutines, they do something like one of these:
· Use return internally, now add support for throw at API:
sub Foo
{
my $err_code = ... ; # real code goes here
# Replace the old return $err_code with this:
return $err_code unless $FATAL_MODE && $error_code != $ok;
throw Error::Code "Couldn't Foo.", code => $err_code;
}
· Use throw internally, add support for return at API:
sub Foo
{
try {
# real code goes here, may execute:
throw Exception "Couldn't foo.", code => $err_code;
}
catch !$FATAL_MODE => { return $@->{code}; }
return $ok;
}
Yow. Too much mechanism. Why not just:
return proto Exception "Couldn't foo.", code => $err_code;
The "proto" method can implement the standard "use fatal" semantics
when that is desired by the calling module, and otherwise set things up
so that
Foo() or die;
ends up throwing the proto-exception. (The current proto-exception can
be kept in $! for use in messages, provided it's in thread-local
storage.)
Actually, this is really important to make simple. I'd be in favor of a
built-in that clearly says what's going on, regardless of whether it
ends in a throw or a return of undef:
fail "Couldn't foo", errno => 2;
Just as an aside, it could be argued that all such "built-ins" are
really methods on an implicit class or object. In this case, the
"Exception" class...
RFC:
$SIG{__DIE__}
The try, catch, and finally clauses localize and undef
$SIG{__DIE__} before entering their blocks. This behavior can be
removed if $SIG{__DIE__} is removed.
$SIG{__DIE__} must die. At least, that name must die--we may install a
similar global hook for debugging purposes.
RFC:
Legacy
The only changes in respect of Perl 5 behaviour implied by this RFC
are that (1) $@ is now always an "Exception" object (which
stringifies reasonably), it is now read-only, and it can only be
set via "die", and (2) the "@@" array is now special, and it is now
read-only too.
Perhaps $! could be implicitly declared to have a type of "Exception".
But I see little reason to make $! readonly by default. All that does
is prevent clever people from doing clever things that we haven't
thought of yet. And it won't stop stupid people from doing stupid
things. In any event, $! is just a reference to an object, and access
to the object will controlled by the class, not by Perl.
RFC 199: Short-circuiting built-in functions and user-defined subroutines
First I should note in passing that it is likely that
my ($found) = grep { $_ == 1 } (1..1_000_000);
will be smart enough to stop on the first one without additional hints,
since the left side will only demand one value of the right side.
However, we do need to unify the behaviors of built-ins with user-
defined control structures. From an internal point of view, all of
these various ways of exiting a block will be unified as exceptions.
It will be easy enough for a user-defined subroutine to catch the
appropriate exceptions and do the right thing. For instance, to
implement a loop wrapper (ignoring parser issues), you might write
something like this:
sub mywhile ($keyword, &condition, &block) {
my $l = $keyword.label;
while (&condition()) {
&block();
CATCH {
my $t = $!.tag;
when X::Control::next { die if $t && $t ne $l); next }
when X::Control::last { die if $t && $t ne $l); last }
when X::Control::redo { die if $t && $t ne $l); redo }
}
}
}
Remember that those "die" calls are just rethrows of the current
exception to get past the current try scope (the "while" in this case).
How a block gets a label in general is an interesting question. It's
all very well to say that the keyword is the label, but that doesn't
help if you have two nested constructs with the same name. In Perl 5,
labels are restricted to being at the beginning of the statement, but
then how do you label a "grep"? Should there be some way of specifying
a label on a keyword rather than on a statement? We could end up with
something like this:
my $found = grep:NUM { $_ == 1 and last NUM: $_ } (1..1_000_000);
On the other hand, considering how often this feature is (not) going to
used, I think we can stick with the tried-and-true statement label:
my $found = do { NUM: grep { $_ == 1 and last NUM: $_ } (1..1_000_000) };
This has the advantage of matching the label syntax with a colon on the
end in both places. I like that.
I don't think every block should implicitly have a way to return, or
we'll have difficulty optimizing away blocks that don't do anything
blockish. That's because setting up a try environment is always a bit
blockish, and does in fact impose some overhead that we'd just as soon
avoid when it's unnecessary.
However, it's probably okay if certain constructs that would know how
to deal with a label are implicitly labelled by their keyword name when
they don't happen to have an explicit label. So I think we can allow
something like:
last grep: $_
Despite its appearance, that is not a method call, because "grep" is
not a predefined class. What we have is a unary operator "last" that is
taking an adverbial modifier specifying what to return from the loop.
[Update: there's now a general "leave" verb that can exit from an inner
block.]
The interesting policy question as we go on will be whether a given
construct responds to a given exception or not. Some exceptions will
have to be restricted in their use. For instance, we should probably
say that only explicit "sub" declarations may respond to a "return".
People will expect "return" to exit the subroutine they think they're
in, even if there are blocks floating around that are actually closures
being interpreted elsewhere. It might be considered antisocial for
closure interpreters like "grep" or "map" or "sort" to trap
X::Control::return sooner than the user expects.
As for using numbers instead of labels to indicate how many levels to
break out of, that would be fine, except that I don't believe in
breaking out by levels. If the problem is complex enough that you need
to break out more than one level, you need a name, not a number. Then
it doesn't matter if you refactor your code to have more block levels
or less. I find I frequently have to refactor my code that way.
It's possible to get carried away and retrofit "grep" and "map" with
every conceivable variety of abort, retry, accept, reject, reduce,
reuse, recycle, or whatever exception. I don't think that's necessary.
There has to be some reason for writing your own code occasionally. If
we get rid of all the reasons for writing user-defined subroutines, we
might as well pack our bags and go home. But it's okay at minimum to
treat a looping construct like a loop.
RFC 006: Lexical variables made default
This RFC proposes that "strict vars" should be on by default. This is
motivated by the desire that Perl better support (or cajole, in this
case) the disciplines that enable successful programming in the large.
This goal is laudable.
However, the programming-in-the-small advocates also have a valid
point: they don't want to have to go to all the trouble of turning off
strictures merely to write a succinct one-liner, since keystrokes are
at a premium in such programming, and in fact the very strictures that
increase clarity in large programs tend to decrease clarity in small
programs.
So this is one of those areas where we desire to have it both ways, and
in fact, we pretty much can. The only question is where to draw the
line. Some discussion suggested that only programs specified on the
command line via the "-e" switch should be exempt from stricture. But I
don't want to force every little file-based script into the large model
of programming. And we don't need to.
Large programming requires the definition of modules and classes. The
typical large program will (or should) consist mostly of modules and
classes. So modules and classes will assume "strict vars". Small
programming does not generally require the definition of modules and
classes, though it may depend on existing modules and classes. But even
small programs that use a lot of external modules and classes may be
considered throw-away code. The very fact that the main code of a
program is not typically reused (in the sense that modules and classes
are reused) means that there is where we should draw the line. So in
Perl 6, the main program will not assume "strict vars", unless you
explicitly do something to turn it on, such as to declare "class Main".
RFC 330: Global dynamic variables should remain the default
This is fine for the main program, but modules and classes should be
held to the higher standard of "use strict".
RFC 083: Make constants look like variables
It's important to keep in mind the distinction between variables and
values. In a pure OO environment, variables are merely references to
values, and have no properties of their own--only the value itself
would be able to say whether it is constant. Some values are naturally
constant, such as a literal string, while other values could be marked
constant, or created without methods that can modify the object, or
some such mechanism. In such an environment, there is little use for
properties on variables. Any time you put a property on a variable,
it's potentially lying about its value.
However, Perl does not aspire to be a pure OO environment. In Perl-
think, a variable is not merely a container for a value. Rather, a
variable provides a "view" of a value. Sometimes that view could even
be construed as a lie. That's okay. Lying to yourself is a useful
survival skill (except when it's not). We find it necessary to repeat
"I think I can" to ourselves precisely when we think we can't.
Conversely, it's often valuable psychologically to treat possible
activities as forbidden. Abstinence is easier to practice if you don't
have to decide anew every time there's a possible assignation, er, I
mean, assignment.
Constant declarations on variables fall into this category. The value
itself may or may not naturally be constant, but we will pretend that
it is. We could in theory go farther than that. We could check the
associated object to make sure that it is constant, and blow up if it's
not, but that's not necessary in this case for consistent semantics.
Other properties may be stricter about this. If you have a variable
property that asserts a particular shape of multidimensional array, for
instance, the object in question had better be able to supply semantics
consistent with that view, and it's probably a good idea to blow up
sooner rather than later if it can't. This is something like strong
typing, except that it's optional, because the variable property itself
is optional.
Nevertheless, the purpose of these variable properties is to allow the
compiler to deduce things about the program that it could not otherwise
deduce, and based on those deductions, produce both a more robust and
more efficient compile-time interpretation of the semantics of the
program. That is to say, you can do more optimizations without
compromising safety. This is obviously true in the case of inlining
constants, but the principle extends to other variable properties as
well.
The proposed syntax is fine, except that we'll be using "is" instead of
":" for properties, as discussed in Apocalypse 2. (And it's "constant",
not "const".)
[Update: compile-time properties are now called "traits".]
RFC 337: Common attribute system to allow user-defined, extensible
attributes
As already revealed in Apocalypse 2, attributes will be known as
"properties" in Perl 6, to avoid confusion with existing OO
nomenclature for instance variables. Also, we'll use the "is" keyword
instead of the colon.
[Update: run-time properties are set with "but", while traits are set
with "is".]
Setting properties on array and hash elements bothers me, particularly
when those properties have names like "public" and "private". This
seems to me to be an attempt to paper over the gap of some missing OO
functionality. So instead, I'd rather keep arrays and hashes mostly for
homogenous data structures, and encourage people to use objects to
store data of differing types. Then public and private can be
properties of object attributes, which will look more like real
variables in how they are declared. And we won't have to worry about
the meaning of "my @foo[2]", because that still won't be allowed.
Again, we need to be very clear that the object representing the
variable is different than any objects contained by the variable. When
we say
my Dog @dogpound is loud;
we mean that the individual elements of @dogpound are of type "Dog",
not that the array variable is of type "Dog". But the "loud" property
applies to the array, not to the dogs in the array. If the array
variable needs to have a type, it can be supplied as if it were a
property:
my Dog @dogpound is DogPound is loud;
That is, if a property is the name of a known package/class, it is
taken to be a kind of "tie". Given the declaration above, the following
is always true:
@dogpound.is.loud
[Update: The ".is" is no longer needed, since properties are really
mixin methods now.]
since the "loud" is a property of the array object, even if it contains
no dogs. It turns out that
@dogpound.is.DogPound
is also true. This does not do an isa lookup. For that, say:
@dogpound.isa(Pound)
Note that you can use:
@dogpound =~ Dog
to test the individual elements for Doghood.
[Update: We use "~~" instead of "=~" now. And "@dogpound.DogPound"
throws an exception if @dogpound doesn't have a "DogPound" method.]
RFC 173: Allow multiple loop variables in foreach statements
Unfortunately, the proposed syntax could also be interpreted as
parallel traversal:
foreach ($a, $b) (@a, @b)
Also the RFC assumes pairs will be passed as two elements, which is no
longer necessarily the case. A hash by itself in list context will
return a list of pair objects. We'll need to say something like:
%hash.kv
to get a flattened list of keys alternating with values. (The same
method on arrays produces alternating indices and values.)
I like the idea of this RFC, but the proposed syntax is not what I'd
like. There are various possible syntaxes that could also potentially
fulfill the intent of RFC 120:
for [$i => $elem] (@array) { }
for {$i => $elem} (@array) { }
for ($i, $elem) = (@array.kv) { }
But I like the idea of something that feels like repeated binding. We
could use the ":=" binding operator, but since binding is actually the
operation performed by formal parameters of subroutines, and since we'd
like to keep the list near the "for" and the formals near the closure,
we'll use a variant of subroutine declaration to declare "for" loops:
for @list -> $x { ... } # one value at a time
for @list -> $a, $b { ... } # two values at a time
You can un-interleave an array by saying:
for @xyxyxy -> $x, $y { ... }
Iterating over multiple lists in parallel needs a syntax much like a
multi-dimensional slice. That is, something like a comma that binds
looser than a comma. Since we'll be using semicolon for that purpose to
delimit the dimensions of multi-dimensional slices, we'll use similar
semicolons to delimit a parallel traversal of multiple lists: So
parallel arrays could be stepped through like this:
for @xxx; @yyy; @zzz -> $x; $y; $z { ... }
If there are semicolons on the right, there must be the same number as
on the left.
Each "stream" is considered separately, so you can traverse two arrays
each two elements at a time like this:
for @ababab; @cdcdcd -> $a, $b; $c, $d { ... }
If there are no semicolons on the right, the values are taken
sequentially across the streams. So you can say
for @aaaa; @bbbb -> $a, $b { ... }
and it ends up meaning the same thing as if the comma were a semicolon,
but only because the number of variables on the right happens to be the
same as the number of streams on the right. That doesn't have to be the
case. To get values one at a time across three streams, you can say
for @a; @b; @c -> $x { ... }
Each semicolon delimited expression on the left is considered to be a
list of generated values, so it's perfectly legal to use commas or
"infinite" ranges on the left. The following prints "a0", "b2", "c3",
and so on forever (or at least for a very long time):
for 0 .. Inf; "a" .. "z" x 1000 -> $i; $a {
print "$a$i";
}
[Update: This has been simplified. The arguments to a closure are
always separated by commas, and you can use the "each" function to
interleave two or more arrays on the left.]
RFC 019: Rename the local operator
We'll go with "temp" for the temporizing operator.
In addition, we're going to be storing more global state in objects
(such as file objects). So it ought to be possible to temporize (that
is, checkpoint/restore) an attribute of an object, or at least any
attributes that can be treated as an lvalue.
RFC 064: New pragma 'scope' to change Perl's default scoping
I can't stop people from experimenting, but I'm not terribly interested
in performing this experiment myself. I made "my" short for a reason.
So I'm accepting this RFC in principle, but only in principle. Standard
Perl declarations will be plainly marked with "my" or "our".
Rejected RFCs
Just because I've rejected these RFCs doesn't mean that they weren't
addressing at a valid need. Usually an RFC gets rejected simply because
I think there's a better way to do it. Often there's little difference
between a rejected RFC that I've borrowed ideas from and an RFC
accepted with major caveats.
We're already running long, so these descriptions will be terse. Please
read the RFC if you don't understand the commentary.
RFC 089: Controllable Data Typing
This is pretty close to what we've been planning for Perl for a long
time. However, a number of the specifics are suboptimal.
If you declare a constant, it's a constant. There's no point in
allowing warnings on that by default. It should be fatal to modify a
constant. Otherwise you lose all your optimization possibilities.
For historical reasons, the assignment in
my ($a, $b) = new Foo;
will not distribute automatically over $a and $b. If you want that, use
the "^=" hyperassignment instead, maybe.
Constraint lists are vaguely interesting, but seem to be too much
mechanism for the possible benefits. If you really want a data type
that can be polymorphic, why not just define a polymorphic type?
In general, there seems to be a lot of confusion in this RFC between
constraints on variables and constraints on values. For constraints to
be useful to the compiler, they have to be on the variable, and you
can't be "pushing" constraints at runtime.
On aliasing via subroutine calls, note that declared parameters will be
constant by default.
So anyway, although I'm rejecting this RFC, we'll certainly have a
declaration syntax resembling some of the tables in the RFC.
RFC 106: Yet another lexical variable proposal: lexical variables made
default
Yes, it's true that other widely-admired languages like Ruby do
implicit declaration of lexicals, but I think it's a mistake, the
results of which don't show up until things start getting complicated.
(It's a sign of this weakness that in Ruby you see the workaround of
faking up an assignment to force declaration of a variable.)
I dislike the implicit declaration of lexicals because it tends to
defeat the primary use of them, namely, catching typos. It's just too
easy to declare additional variable names by accident. It's also too
easy to broaden the scope of a variable by accident. You might have a
bunch of separate subroutines each with their own lexical, and suddenly
find that they're all the same variable because you accidentally used
the same variable name in the module initialization code.
When you think about it, requiring "my" on declaration is a form of
orthogonality. Otherwise you find your default scoping rules
arbitrarily tied to an inner scope, or an outer scope, or a subroutine
scope. All of these are suboptimal choices. And I don't buy the notion
of using "my" optionally to disambiguate when you feel like it. Perl
gives you a lot of rope to hang yourself with, but this is the wrong
kind of rope, because it obscures a needful visual distinction.
Declarations should look like declarations, not just to the programmer,
but also to whoever has to read the program after them, whether carbon-
based or silicon-based.
And when it comes down to it, I believe that declarations with "my" are
properly Huffman encoded. Declaring a lexical ought to be harder than
assigning to one. And declaring a global ought to be harder than
declaring a lexical (at least within classes and modules).
RFC 119: Object neutral error handling via exceptions
Good goals, but I don't want yet another independent system of
exception handling. Simplicity comes through unification. Also, the
proposed syntax is all just a little too intertwingled for my tastes.
Let's see, how can I explain what I mean?
The out-of-band stuff doesn't stand out visually enough to me, and I
don't like thinking about it as control flow. Nevertheless, I think
that what we've ended up with solves a number of the problems pointed
out in this RFC. The RFC essentially asks for the functionality of
"POST", "KEEP" and "UNDO" at a statement level. Although "POST",
"KEEP", and "UNDO" blocks cannot be attached to any statement, I
believe that allowing "post", "keep", and "undo" properties in scoped
declarations is powerful enough, and gives the compiler something
tangible to attach the actions to. There is a kind of precision in
attaching these actions to a specific variable--the state is bound to
the variable in a transactionally instantaneous way. I'm afraid if we
attach transactional actions to statements as the RFC proposes, it
won't be clear exactly when the statement's state change is to be
considered successful, since the transaction can't "know" which
operation is the crucial one.
Nonetheless, some ideas from this RFC will live on in the "post",
"keep", and "undo" property blocks.
RFC 120: Implicit counter in for statements, possibly $#.
I am prejudiced against this one, simply because I've been burned too
many times by implicit variables that mandate implicit overhead. I
think if you need an index, you should declare one, so that if you
don't declare one, the compiler knows not to bother setting up for it.
Another problem is that people will keep asking what
for (@foo,@bar) { print $# }
is supposed to mean.
I expect that we'll end up with something more like what we discussed
earlier:
for @array.kv -> $i, $elem { ... }
RFC 262: Index Attribute
Everyone has a use for ":" these days...
This one seems not to be of very high utility, suffering from similar
problems as the RFC 120 proposal. I don't think it's possible to
efficiently track the container of a value within each contained object
unless we know at compile time what a looping construct is, which is
problematic with user-defined control structures.
And what if an item is a member of more than one list?
Again, I'd rather have something declared so we know whether to take
the overhead. Then we don't have to pessimize whenever we can't do a
complete static analysis.
RFC 167: Simplify do BLOCK Syntax
I think the "do" on a "do" block is useful to emphasize that the
closure in the braces is to be executed immediately. Otherwise Perl (or
the user (or both)) might be confused as to whether someone was trying
to write a closure that is to be executed later, particularly if the
block is the last item in a subroutine that might be wanting to return
a closure. In fact, we'll probably outlaw bare blocks at the statement
level as too ambiguous. Use "for 1 {}" or some such when you want a
one-time loop, and use "return" or "sub" when you want to return a
closure.
We'll solve the ";" problem by jiggering the definition of "{...}", not
by fiddling with "do".
[Update: Bare blocks are still legal, and always execute immediately as
expected. To return a closure you should use an explicit "return".]
RFC 209: Fuller integer support in Perl.
The old "use integer" pragma was a hack. I think I'd rather use types
and representation specs on individual declarations for compile-time
selection, or alternate object constructors for run-time selection,
particularly when infinite precision is desired. I'm not against using
pragmas to alter the defaults, but I think it's generally better to be
more specific when you have the capability. You can force your programs
to be lexically scoped with pragmas, but data wants to flow wherever it
likes to go, so your lexically scoped module had better be able to deal
rationally with any data thrown at it, even if it isn't in the exact
form that you prefer.
By the way, the RFC is misleading when it asserts that 32-bit integer
precision is lost when represented in floating point. That's only true
if you use 32-bit floats. Perl has always used 64-bit doubles, which
give approximately 15 digits of integer precision. (The issue does
arise with 64-bit integers, of course.)
All that being said, Perl 6 will certainly have better support for
integer types of various sorts. I just don't think that a pragma
redefining what an "integer" is will provide good documentation to
whoever is trying to understand the program. Better to declare things
of type MagicNum, or whatever.
I could be wrong, of course. If so, write your pragma, and have the
appropriate amount of fun.
RFC 279: "my()" syntax extensions and attribute declarations
We already treated this in Apocalypse 2.
The RFC assumes that the type always distributes over a "my" list.
This is not what is necessary for function signatures, which need
individual types for each formal argument.
And again, it doesn't make much sense to me to put properties on a
variable at run-time.
It makes even less sense to me to be able to declare the type of an
array element lexically. This is the province of objects, not arrays
pretending to be structs.
RFC 297: Attributes for compiler hints
Sorry, we can't have the semantics suddenly varying drastically merely
because the user decided to run the program through a different
translator. I think there's a happy medium in there somewhere where we
can have the same semantics for both interpreter and compiler.
RFC 309: Allow keywords in sub prototypes
This RFC is rejected only because it doesn't go far enough. What we'll
eventually need is to allow a regex-ish syntax notation for parsing
that may be separate from the argument declarations. (Then again, maybe
not.) In any event, I think some kind of explicit regex notation is
called for, not the promotion of identifiers to token matchers. We may
want identifiers in signatures for something else later, so we'll hold
them in reserve.
[Update: This functionality is provided by macros using the "is parsed"
trait. See A6.]
RFC 340: with takes a context
This seems like a solution in search of a problem. Even if we end up
with a context stack as explicit as Perl 5's, I don't think the amount
we'll deal with it warrants a keyword. (And I dislike ""return with;""
as a needlessly opaque linguistic construct.)
That being said, if someone implements (as user-defined code) the
Pascalish "with" as proposed in RFC 342 (and rejected), and if the
"caller" function (or something similar) returns sufficient information
to build references to the lexical scope associated with the call frame
in question, then something like this could also be implemented as user
code. I can't decide whether it's not clear that this is a good idea,
or it's clear that this is not a good idea. In any event, I would warn
anyone doing this that it's likely to be extremely confusing, akin to
goto-considered-harmful, and for similar reasons, though in this case
by displacing scopes rather than control flow.
Note that some mechanism resembling this will be necessary for modules
to do exportation to a lexical scope (see %MY in Apocalypse 2).
However, lexical scope modification will be allowed only during the
compile time of the lexical scope in question, since we need to be
careful to preserve the encapsulation that lexical scoping provides.
Turning lexical variables back into dynamic variables will tend to
destroy that security.
So I think we'll stick with closures and continuations that don't
transport lexical scopes at runtime.
RFC 342: Pascal-like "with"
I expect Perl's parsing to be powerful enough that you could write a
"with" if you wanted one.
Withdrawn RFCs
RFC 063: Exception handling syntax
RFC 113: Better constants and constant folding
Other decisions
C-style for loop
Due to syntactic ambiguities with the new "for" syntax of Perl 6, the
generalized C-style "for" loop is going to get its keyword changed to
"loop". And "for" will now always mean "foreach". The expression "pill"
is now optional, so instead of writing an infinite loop like this:
for (;;) {
...
}
you can now write it like this:
loop {
...
}
C-style do {} while EXPR no longer supported
In Perl 5, when you used a "while" statement modifier on a statement
consisting of nothing but a "do {}", something magical happened, and
the block would be evaluated once before the condition was evaluated.
This special-cased construct, seldom used and often misunderstood, will
no longer be in Perl 6, and in fact will produce a compile-time error
to prevent people from trying to use it. Where Perl 5 code has this:
do {
...
} while CONDITION;
Perl 6 code will use a construct in which the control flow is more
explicit:
loop {
...
last unless CONDITION;
}
[Update: We'll also allow the "<loop {...} while CONDITION;"> form to
mean execute once first. Loop "until" also works, of course.]
Bare blocks
In Perl 5, bare blocks (blocks used as statements) are once-through
loops. In Perl 6, blocks are closures. It would be possible to
automatically execute any closure in void context, but unfortunately,
when a closure is used as the final statement in an outer block, it's
ambiguous as to whether you wanted to return or execute the closure.
Therefore the use of a closure at the statement level will be
considered an error, whether or not it's in a void context. Use "do {}"
for a "once" block, and an explicit "return" or "sub" when you want to
return a reference to the closure.
[Update: Bare blocks are legal again and work as expected.]
continue block
The "continue" block changes its name to "NEXT" and moves inside the
block it modifies, to work like "POST" blocks. Among other things, this
allows "NEXT" blocks to refer to lexical variables declared within the
loop, provided the "NEXT" block is placed after them. The generalized
loop:
loop (EXPR1; EXPR2; EXPR3) { ... }
can now be defined as equivalent to:
EXPR1;
while EXPR2 {
NEXT { EXPR3 }
...
}
(except that any variable declared in "EXPR3" would have different
lexical scope). The "NEXT" block is called only before attempting the
next iteration of the loop. It is not called when the loop is done and
about to exit. Use a "POST" for that.
[Update: s/POST/LEAVE/]
Well, that about wraps it up for now. You might be interesting to know
that I'm posting this from the second sesquiannual Perl Whirl cruise,
on board the Veendam, somewhere in the Carribean. If the ship
disappears in the Bermuda Triangle, you won't have to worry about the
upcoming Exegesis, since Damian is also on board. But for now, Perl 6
is cruising along, the weather's wonderful, wish you were here.
perl v5.14.0 2006-02-28 Perl6::Bible::A04(3)
