Perl6::Bible::S04(3) User Contributed Perl Documentation Perl6::Bible::S04(3)NAME
Synopsis_04 - Blocks and Statements
AUTHOR
Larry Wall <larry@wall.org>
VERSION
Maintainer: Larry Wall <larry@wall.org>
Date: 19 Aug 2004
Last Modified: 25 Feb 2006
Number: 4
Version: 10
This document summarizes Apocalypse 4, which covers the block and
statement syntax of Perl.
The Relationship of Blocks and Declarations
Every block is a closure. (That is, in the abstract, they're all
anonymous subroutines that take a snapshot of their lexical scope.)
How any block is invoked and how its results are used is a matter of
context, but closures all work the same on the inside.
Blocks are delimited by curlies, or by the beginning and end of the
current compilation unit (either the current file or the current "eval"
string). Unlike in Perl 5, there are (by policy) no implicit blocks
around standard control structures. (You could write a macro that
violates this, but resist the urge.) Variables that mediate between an
outer statement and an inner block (such as loop variables) should
generally be declared as formal parameters to that block. There are
three ways to declare formal parameters to a closure.
$func = sub ($a, $b) { print if $a eq $b }; # standard sub declaration
$func = -> $a, $b { print if $a eq $b }; # a "pointy" sub
$func = { print if $^a eq $^b } # placeholder arguments
A bare closure without placeholder arguments that uses $_ (either
explicitly or implicitly) is treated as though $_ were a a formal
parameter:
$func = { print if $_ }; # Same as: $func = -> $_ { print if $_ };
$func("printme");
In any case, all formal parameters are the equivalent of "my" variables
within the block. See S06 for more on function parameters.
Except for such formal parameter declarations, all lexically scoped
declarations are visible from the point of declaration to the end of
the enclosing block. Period. Lexicals may not "leak" from a block to
any other external scope (at least, not without some explicit aliasing
action on the part of the block, such as exportation of a symbol from a
module). The "point of declaration" is the moment the compiler sees
""my $foo"", not the end of the statement as in Perl 5, so
my $x = $x;
will no longer see the value of the outer $x; you'll need to say
my $x = $OUTER::x;
instead. (It's illegal to declare $x twice in the same scope.)
As in Perl 5, ""our $foo"" introduces a lexically scoped alias for a
variable in the current package.
The new "constant" declarator introduces a lexically scoped name for a
compile-time constant, either a variable or a 0-ary sub, which may be
initialized with either a pseudo-assignment or a block:
constant Num $pi = 3;
constant Num PI { 3 }
constant Num X = atan(2,2) * 4;
In any case the initializing value is evaluated at BEGIN time.
There is a new "state" declarator that introduces a lexically scoped
variable like "my" does, but with a lifetime that persists for the life
of the closure, so that it keeps its value from the end of one call to
the beginning of the next. Separate clones of the closure get separate
state variables.
Perl 5's ""local"" function has been renamed to "temp" to better
reflect what it does. There is also a "let" function that sets a
hypothetical value. It works exactly like "temp", except that the
value will be restored only if the current block exits unsuccessfully.
(See Definition of Success below for more.) "temp" and "let" temporize
or hypotheticalize the value or the variable depending on whether you
do assignment or binding.
Statement-ending blocks
A line ending with a closing brace ""}"", followed by nothing but
whitespace or comments, will terminates statement if an end of
statement can occur there. That is, these two statements are
equivalent:
my $x = sub { 3 }
my $x = sub { 3 };
End-of-statement cannot occur within a bracketed expression, so this
still works:
my $x = [
sub { 3 }, # this comma is not optional
sub { 3 } # the statement won't terminate here
];
Because subroutine declarations are expressions, not statements, this
is now invalid:
sub f { 3 } sub g { 3 } # two terms occur in a row
But these two are valid:
sub f { 3 }; sub g { 3 };
sub f { 3 }; sub g { 3 } # the trailing semicolon is optional
Conditional statements
The "if" and "unless" statements work almost exactly as they do in Perl
5, except that you may omit the parentheses on the conditional:
if $foo == 123 {
...
}
elsif $foo == 321 {
...
}
else {
...
}
Conditional statement modifiers also work as in Perl 5. So do the
implicit conditionals implied by short-circuit operators. And there's
a new "elsunless" in Perl 6--except that you have to spell it "elsif
not". ":-)"
Loop statements
The "while" and "until" statements work as in Perl 5, except that you
may leave out the parentheses around the conditional:
while $bar < 100 {
...
}
Looping statement modifiers are the same as in Perl 5, except that to
avoid confusion applying one to a "do" block is specifically
disallowed. Instead of
do {
...
} while $x;
you should write
loop {
...
} while $x;
Loop modifiers "next", "last", and "redo" work as in Perl 5.
There is no longer a "continue" block. Instead, use a "NEXT" block
within the loop. See below.
The general loop statement
The "loop" statement is the C-style "for" loop in disguise:
loop ($i = 0; $i < 10; $i++) {
...
}
As seen in the previous section, the 3-part loop spec may be entirely
omitted to write an infinite loop. If you omit the 3-part loop spec
you may add a "while" or "until" statement modifier at the end to make
it a "repeat at least once" loop. Unlike "do" in Perl 5, it's a real
loop block, so you may use loop modifiers.
The "for" statement
There is no "foreach" statement any more. It's always spelled "for" in
Perl 6, so it always takes a list as an argument:
for @foo { print }
As mentioned earlier, the loop variable is named by passing a parameter
to the closure:
for @foo -> $item { print $item }
Multiple parameters may be passed, in which case the list is traversed
more than one element at a time:
for %hash.kv -> $key, $value { print "$key => $value\n" }
To process two arrays in parallel, use the "each" function:
for each(@a;@b) -> $a, $b { print "[$a, $b]\n" }
or use the "zip" function to generate a list of tuples that each can be
bound to multiple arguments enclosed in square brackets:
for zip(@a;@b) -> [$a, $b] { print "[$a, $b]\n" }
The list is evaluated lazily by default, so instead of using a "while"
to read a file a line at a time as you would in Perl 5:
while (my $line = <STDIN>) {...}
in Perl 6 you should use a "for" (plus a unary "=" "iterate the
iterator" operator) instead:
for =$*IN -> $line {...}
This has the added benefit of limiting the scope of the $line parameter
to the block it's bound to. (The "while"'s declaration of $line
continues to be visible past the end of the block. Remember, no
implicit block scopes.) It is also possible to write
while =$*IN -> $line {...}
Note also that Perl 5's special rule causing
while (<>) {...}
to automatically assign to $_ is not carried over to Perl 6. That
should now be written:
for =<> {...}
which is short for
for =$*ARGS {...}
Parameters are by default readonly within the block. You can declare a
parameter read/write by including the ""is rw"" trait. If you rely on
$_ as the implicit parameter to a block, then $_ is considered
read/write by default. That is, the construct:
for @foo {...}
is actually short for:
for @foo -> $_ is rw {...}
so you can modify the current list element in that case. However, any
time you specify the arguments, they default to read only.
When used as statement modifers, "for" and "given" use a private
instance of $_ for the left side of the statement. The outer $_ can be
referred to as $OUTER::_. (And yes, this implies that the compiler may
have to retroactively change the binding of <$_> on the left side. But
it's what people expect of a pronoun like "it".)
The do-once loop
In Perl 5, a bare block is deemed to be a do-once loop. In Perl 6, the
bare block is not a do-once. Instead "do {...}" is the do-once loop
(which is another reason you can't put a "while" or "until" modifier on
it).
For any statement, prefixing with a "do" allows you to return the value
of that statement and use it in an expression:
$x = do if $a { $b } else { $c };
This construct only allows you to prefix a statement. If you want to
continue the expression after the statement you must use the curly
form.
Since "do" is defined as going in front of a statement, it follows that
it can always be followed by a statement label. This is particularly
useful for the do-once block, since it is offically a loop and can take
therefore loop control statements.
Switch statements
A switch statement is a means of topicalizing, so the switch keyword is
the English topicalizer, "given". The keyword for individual cases is
"when":
given EXPR {
when EXPR { ... }
when EXPR { ... }
default { ... }
}
The current topic is always aliased to the special variable $_. The
"given" block is just one way to set the current topic, but a switch
statement can be any block that sets $_, including a "for" loop (in
which the first loop parameter is the topic) or the body of a method
(if you have declared the invocant as $_). So switching behavior is
actually caused by the "when" statements in the block, not by the
nature of the block itself. A "when" statement implicitly does a
"smart match" between the current topic ($_) and the argument of the
"when". If the smart match succeeds, the associated closure is
executed, and the surrounding block is automatically broken out of. If
the smart match fails, control passes to the next statement normally,
which may or may not be a "when" statement. Since "when" statements
are presumed to be executed in order like normal statements, it's not
required that all the statements in a switch block be "when" statements
(though it helps the optimizer to have a sequence of contiguous "when"
statements, because then it can arrange to jump directly to the first
appropriate test that might possibly match.)
The default case:
default {...}
is exactly equivalent to
when true {...}
Because "when" statements are executed in order, the default must come
last. You don't have to use an explicit default--you can just fall off
the last "when" into ordinary code. But use of a "default" block is
good documentation.
If you use a "for" loop with a named parameter, the parameter is also
aliased to $_ so that it can function as the topic of any "when"
statements within the loop. If you use a "for" statement with multiple
parameters, only the first parameter is aliased to $_ as the topic.
You can explicitly break out of a "when" block (and its surrounding
switch) early using the "break" verb. You can explicitly break out of
a "when" block and go to the next statement by using "continue". (Note
that, unlike C's idea of falling through, subsequent "when" conditions
are evaluated. To jump into the next "when" block you must use a
"goto".)
If you have a switch that is the main block of a "for" loop, and you
break out of the switch either implicitly or explicitly, it merely goes
to the next iteration of the loop. You must use "last" to break out of
the entire loop early. Of course, an explicit "next" would be clearer
than a "break" if you really want to go to the next iteration.
Possibly we'll outlaw "break" in a loop topicalizer.
Exception handlers
Unlike many other languages, Perl 6 specifies exception handlers by
placing a "CATCH" block within that block that is having its exceptions
handled.
The Perl 6 equivalent to Perl 5's "eval {...}" is "try {...}". (Perl
6's "eval" function only evaluates strings, not blocks.) A "try" block
by default has a "CATCH" block that handles all exceptions by ignoring
them. If you define a "CATCH" block within the "try", it replaces the
default "CATCH". It also makes the "try" keyword redundant, because
any block can function as a "try" block if you put a "CATCH" block
within it.
An exception handler is just a switch statement on an implicit topic
supplied within the "CATCH" block. That implicit topic is the current
exception object, also known as $!. Inside the "CATCH" block, it's
also bound to $_, since it's the topic. Because of smart matching,
ordinary "when" statements are sufficiently powerful to pattern match
the current exception against classes or patterns or numbers without
any special syntax for exception handlers. If none of the cases in the
"CATCH" handles the exception, the exception is rethrown. To ignore
all unhandled exceptions, use an empty "default" case. (In other
words, there is an implicit "die $!" just inside the end of the "CATCH"
block. Handled exceptions break out past this implicit rethrow.)
Control Exceptions
All abnormal control flow is, in the general case, handled by the
exception mechanism (which is likely to be optimized away in specific
cases.) Here "abnormal" means any transfer of control outward that is
not just falling off the end of a block. A "return", for example, is
considered a form of abnormal control flow, since it can jump out of
multiple levels of closure to the end of the scope of the current
subroutine definition. Loop commands like "next" are abnormal, but
looping because you hit the end of the block is not. The implicit
break of a "when" block is abnormal.
A "CATCH" block handles only "bad" exceptions, and lets control
exceptions pass unhindered. Control exceptions may be caught with a
"CONTROL" block. Generally you don't need to worry about this unless
you're defining a control construct. You may have one "CATCH" block
and one "CONTROL" block, since some user-defined constructs may wish to
supply an implicit "CONTROL" block to your closure, but let you define
your own "CATCH" block.
A "return" always exits from the lexically surrounding sub or method
definition (that is, from a function officially declared with the
"sub", "method", or "submethod" keywords). Pointy subs and bare
closures are transparent to "return". If you pass a reference to a
closure outside of its official "sub" scope, it is illegal to return
from it. You may only leave the closure block itself with "leave" or
by falling off the end of it.
To return a value from a pointy sub or bare closure, you either just
let the block return the value of its final expression, or you can use
"leave". A "leave" by default exits from the innermost block. But you
may change the behavior of "leave" with selector adverbs:
leave :from(Loop) :label<LINE> <== 1,2,3; # XXX "with"?
The innermost block matching the selection criteria will be exited.
The return value, if any, must be passed as a list. To return pairs as
part of the value, you can use a pipe:
leave <== :foo:bar:baz(1) if $leaving;
or going the other way::
$leaving and :foo:bar:baz(1) ==> leave;
In theory, any user-defined control construct can catch any control
exception it likes. However, there have to be some culturally enforced
standards on which constructs capture which exceptions. Much like
"return" may only return from an "official" subroutine or method, a
loop exit like "next" should be caught by the construct the user
expects it to be caught by. In particular, if the user labels a loop
with a specific label, and calls a loop control from within the lexical
scope of that loop, and if that call mentions the outer loop's label,
then that outer loop is the one that must be controlled. (This search
of lexical scopes is limited to the current "official" subroutine.) If
there is no such lexically scoped outer loop in current subroutine.
Then a fallback search is made outward through the dynamic scopes in
the same way Perl 5 does. (The difference between Perl 5 and Perl 6 in
this respect arises only because Perl 5 didn't have user-defined
control structures, hence the sub's lexical scope was always the
innermost dynamic scope, so the preference to the lexical scope in the
current sub was implicit. For Perl 6 we have to make this preference
explicit.)
The goto statement
In addition to "next", "last", and "redo", Perl 6 also supports "goto".
As with ordinary loop controls, the label is searched for first
lexically within the current subroutine, then dynamically outside of
it. Unlike with loop controls, however, scanning a scope includes a
scan of any lexical scopes included within the current candidate scope.
As in Perl 5, it is possible to "goto" into a lexical scope, but only
for lexical scopes that require no special initialization of
parameters. (Initialization of ordinary variables does not
count--presumably the presence of a label will prevent code-movement
optimizations past the label.) So, for instance, it's always possible
to goto into the next case of a "when" or into either the "then" or
"else" branch of a conditional. You may not go into a "given" or a
"for", though, because that would bypass a formal parameter binding
(not to mention list generation in the case of "for"). (Note: the
implicit default binding of an outer $_ to an inner $_ can be emulated
for a bare block, so that doesn't fall under the prohibition on
bypassing formal binding.)
Exceptions
As in Perl 5, many built-in functions simply return undef when you ask
for a value out of range, or the function fails somehow. Unlike in
Perl 5, these may be "interesting" values of undef that contain
information about the error. If you try to use an undefined value,
that information can then be conveyed to the user. In essence, undef
can be an unthrown exception object that just happens to return 0 when
you ask it whether it's defined or it's true. Since $! contains the
current error code, saying "die $!" will turn an unthrown exception
into a thrown exception. (A bare "die" does the same.)
You can cause built-ins to automatically throw exceptions on failure
using
use fatal;
The "fail" function responds to the caller's "use fatal" state. It
either returns an unthrown exception, or throws the exception.
If an exception is raised while $! already contains an exception that
is active and "unhandled", no information is discarded. The old
exception is pushed onto the exception stack within the new exception,
which is then bound to $! and, hopefully, propagated. The default
printout for the new exception should include the old exception
information so that the user can trace back to the original error.
(Likewise, rethrown exceptions add information about how the exception
is propagated.) The exception stack within $! is available as $![].
Exception objects are born "unhandled". The $! object keeps track of
whether it's currently "handled" or "unhandled". The exception in $!
still exists after it has been caught, but catching it marks it as
handled if any of the cases in the switch matched. Handled exceptions
don't require their information to be preserved if another exception
occurs.
Closure traits
A "CATCH" block is just a trait of the closure containing it. Other
blocks can be installed as traits as well. These other blocks are
called at various times, and some of them respond to various control
exceptions and exit values:
BEGIN {...}* at compile time, ASAP
CHECK {...}* at compile time, ALAP
INIT {...}* at run time, ASAP
END {...} at run time, ALAP
FIRST {...}* at first block entry time
ENTER {...}* at every block entry time
LEAVE {...} at every block exit time
KEEP {...} at every successful block exit
UNDO {...} at every unsuccessful block exit
NEXT {...} at loop continuation time
LAST {...} at loop termination time
PRE {...} assert precondition at every block entry
POST {...} assert postcondition at every block exit
CATCH {...} catch exceptions
CONTROL {...} catch control exceptions
Those marked with a "*" can also be used within an expression:
my $compiletime = BEGIN { localtime };
our $temphandle = FIRST { maketemp() };
Code that is generated at run time can still fire off "CHECK" and
"INIT" blocks, though of course those blocks can't do things that would
require travel back in time.
Some of these also have corresponding traits that can be set on
variables. These have the advantage of passing the variable in
question into the closure as its topic:
my $r will first { .set_random_seed() };
our $h will enter { .rememberit() } will undo { .forgetit() };
Apart from "CATCH" and "CONTROL", which can only occur once, most of
these can occur multiple times within the block. So they aren't really
traits, exactly--they add themselves onto a list stored in the actual
trait. So if you examine the "ENTER" trait of a block, you'll find
that it's really a list of closures rather than a single closure.
The semantics of "INIT" and "FIRST" are not equivalent to each other in
the case of cloned closures. An "INIT" only runs once for all copies
of a cloned closure. A "FIRST" runs separately for each clone, so
separate clones can keep separate state variables:
our $i = 0;
...
$func = { state $x will first{$i++}; dostuff($i) };
But "state" automatically applies "first" semantics to any initializer,
so this also works:
$func = { state $x = $i++; dostuff($i) }
Each subsequent clone gets an initial state that is one higher than the
previous, and each clone maintains its own state of $x, because that's
what "state" variables do.
All of these trait blocks can see any previously declared lexical
variables, even if those variables have not been elaborated yet when
the closure is invoked. (In which case the variables evaluate to an
undefined value.)
Note: Apocalypse 4 confused the notions of "PRE"/"POST" with
"ENTER"/"LEAVE". These are now separate notions. "ENTER" and "LEAVE"
are used only for their side effects. "PRE" and "POST" must return
boolean values that are evaluated according to the usual Design by
Contract rules. (Plus, if you use "ENTER"/"LEAVE" in a class block,
they only execute when the class block is executed, but "PRE"/"POST" in
a class block are evaluated around every method in the class.)
"LEAVE" blocks are evaluated after "CATCH" and "CONTROL" blocks,
including the "LEAVE" variants, "KEEP" and "UNDO". "POST" blocks are
evaluated after everything else, to guarantee that even "LEAVE" blocks
can't violate DBC. Likewise "PRE" blocks fire off before any "ENTER"
or "FIRST" (though not before "BEGIN", "CHECK", or "INIT", since those
are done at compile or process initialization time).
Statement parsing
In this statement:
given EXPR {
when EXPR { ... }
when EXPR { ... }
...
}
parentheses aren't necessary around "EXPR" because the whitespace
between "EXPR" and the block forces the block to be considered a block
rather than a subscript. This works for all control structures, not
just the new ones in Perl 6. A bare block where an operator is
expected is always considered a statement block if there's space before
it:
if $foo { ... }
elsif $bar { ... }
else { ... }
while $more { ... }
for 1..10 { ... }
(You can still parenthesize the expression argument for old times'
sake, as long as there's a space between the closing paren and the
opening brace.)
On the other hand, anywhere a term is expected, a block is taken to be
a closure definition (an anonymous subroutine). If the closure appears
to delimit nothing but a comma-separated list starting with a pair
(counting a single pair as a list of one element), the closure will be
immediately executed as a hash composer.
$hashref = { "a" => 1 };
$hashref = { "a" => 1, $b, $c, %stuff, @nonsense };
$coderef = { "a", 1 };
$coderef = { "a" => 1, $b, $c ==> print };
If you wish to be less ambiguous, the "hash" list operator will
explicitly evaluate a list and compose a hash of the returned value,
while "sub" introduces an anonymous subroutine:
$coderef = sub { "a" => 1 };
$hashref = hash("a" => 1);
$hashref = hash("a", 1);
If a closure is the right argument of the dot operator, the closure is
interpreted as a hash subscript, even if there is space before the dot.
$ref = {$x}; # closure because term expected
if $term{$x} # subscript because operator expected
if $term {$x} # expression followed by statement block
if $term .{$x} # valid subscript (term expected after dot)
Similar rules apply to array subscripts:
$ref = [$x]; # array composer because term expected
if $term[$x] # subscript because operator expected
if $term [$x] # syntax error (two terms in a row)
if $term .[$x] # valid subscript (term expected after dot)
And to the parentheses delimiting function arguments:
$ref = ($x); # grouping parens because term expected
if $term($x) # function call because operator expected
if $term ($x) # syntax error (two terms in a row)
if $term .($x) # valid function call (term expected after dot)
Outside of any kind of expression brackets, a trailing curly on a line
by itself (not counting whitespace or comments) always reverts to the
precedence of semicolon whether or not you put a semicolon after it.
(In the absence of an explicit semicolon, the current statement may
continue on a subsequent line, but only with valid statement
continuators such as "else". A modifier on a "loop" statement must
continue on the same line, however.)
Final blocks on statement-level constructs always imply semicolon
precedence afterwards regardless of the position of the closing curly.
Statement-level constructs are distinguished in the grammar by being
declared in the statement syntactic group:
macro statement_control:<if> ($expr, &ifblock) {...}
macro statement_control:<while> ($expr, &whileblock) {...}
macro statement_control:<BEGIN> (&beginblock) {...}
Statement-level constructs may start only where the parser is expecting
the start of a statement. To embed a statement in an expression you
must use something like "do {...}" or "try {...}".
$x = do { given $foo { when 1 {2} when 3 {4} } + $bar;
$x = try { given $foo { when 1 {2} when 3 {4} } + $bar;
Just because there's a "statement_control:<BEGIN>" does not preclude us
from also defining a "prefix:<BEGIN>" that can be used within an
expression:
macro prefix:<BEGIN> (&beginblock) { beginblock().repr }
Then you can say things like:
$recompile_by = BEGIN { time } + $expiration_time;
But "statement_control:<BEGIN>" hides "prefix:<BEGIN>" at the start of
a statement. You could also conceivably define a "prefix:<if>", but
then you would get a syntax error when you say:
print if $foo
since "prefix:<if>" would hide "statement_modifier:<if>".
Smart matching
Here is the current table of smart matches (which probably belongs in
S03). The list is intended to reflect forms that can be recognized at
compile time. If none of these forms is recognized at compile time, it
falls through to do MMD to "infix:<~~>()", which presumably reflects
similar semantics, but can finesse things that aren't exact type
matches. Note that all types are scalarized here. Both "~~" and
"given"/"when" provide scalar contexts to their arguments. (You can
always hyperize "~~" explicitly, though.) So both $_ and $x here are
potentially references to container objects. And since lists promote
to arrays in scalar context, there need be no separate entries for
lists.
$_ $x Type of Match Implied Matching Code
====== ===== ===================== =============
Any Code<$> scalar sub truth match if $x($_)
Hash Hash hash keys identical match if $_.keys.sort XeqX $x.keys.sort
Hash any(Hash) hash key intersection match if $_{any(Hash.keys)}
Hash Array hash value slice truth match if $_{any(@$x)}
Hash any(list) hash key slice existence match if exists $_{any(list)}
Hash all(list) hash key slice existence match if exists $_{all(list)}
Hash Rule hash key grep match if any($_.keys) ~~ /$x/
Hash Any hash entry existence match if exists $_{$x}
Hash .{Any} hash element truth* match if $_{Any}
Hash .<string> hash element truth* match if $_<string>
Array Array arrays are identical match if $_ X~~X $x
Array any(list) list intersection match if any(@$_) ~~ any(list)
Array Rule array grep match if any(@$_) ~~ /$x/
Array Num array contains number match if any($_) == $x
Array Str array contains string match if any($_) eq $x
Array .[number] array element truth* match if $_[number]
Num NumRange in numeric range match if $min <= $_ <= $max
Str StrRange in string range match if $min le $_ le $max
Any Code<> simple closure truth* match if $x() (ignoring $_)
Any Class class membership match if $_.does($x)
Any Role role playing match if $_.does($x)
Any Num numeric equality match if $_ == $x
Any Str string equality match if $_ eq $x
Any .method method truth* match if $_.method
Any Rule pattern match match if $_ ~~ /$x/
Any subst substitution match* match if $_ ~~ subst
Any boolean simple expression truth* match if true given $_
Any undef undefined match unless defined $_
Any Any run-time dispatch match if infix:<~~>($_, $x)
Matches marked with * are non-reversible, typically because "~~" takes
its left side as the topic for the right side, and sets the topic to a
private instance of $_ for its right side, so $_ means something
different on either side. Such non-reversible constructs can be made
reversible by putting the leading term into a closure to defer the
binding of $_. For example:
$x ~~ .does(Storeable) # okay
.does(Storeable) ~~ $x # not okay--gets wrong $_ on left
{ .does(Storeable) } ~~ $x # okay--closure binds its $_ to $x
Exactly the same consideration applies to "given" and "when":
given $x { when .does(Storeable) {...} } # okay
given .does(Storeable) { when $x {...} } # not okay
given { .does(Storeable) } { when $x {...} } # okay
Boolean expressions are those known to return a boolean value, such as
comparisons, or the unary "?" operator. They may reference $_
explicitly or implicitly. If they don't reference $_ at all, that's
okay too--in that case you're just using the switch structure as a more
readable alternative to a string of elsifs.
The primary use of the "~~" operator is to return a boolean value in a
boolean context. However, for certain operands such as regular
expressions, use of the operator within scalar or list context
transfers the context to that operand, so that, for instance, a regular
expression can return a list of matched substrings, as in Perl 5. The
complete list of such operands is TBD.
It has not yet been determined if run-time dispatch of "~~" will
attempt to emulate the compile-time precedence table before reverting
to MMD, or just go directly to MMD. There are good arguments for both
sides, and we can decide when we see more examples of how it'll work
out.
Definition of Success
Hypothetical variables are somewhat transactional--they keep their new
values only on successful exit of the current block, and otherwise are
rolled back to their original value.
It is, of course, a failure to leave the block by propagating an error
exception, though returning a defined value after catching an exception
is okay.
In the absence of exception propagation, a successful exit is one that
returns a defined value in scalar context, or any number of values in
list context as long as the length is defined. (A length of +Inf is
considered a defined length. A length of 0 is also a defined length,
which means it's a "successful" return even though the list would
evaluate to false in a boolean context.) A list can have a defined
length even if it contains undefined scalar values. A list is of
undefined length only if it contains an undefined generator, which,
happily, is what is returned by the "undef" function when used in list
context. So any Perl 6 function can say
return undef;
and not care about whether the function is being called in scalar or
list context. To return an explicit scalar undef, you can always say
return scalar(undef);
Then in list context, you're returning a list of length 1, which is
defined (much like in Perl 5). But generally you should be using
"fail" in such a case to return an exception object. Exception objects
also behave like undefined generators in list context. In any case,
returning an unthrown exception is considered failure from the
standpoint of "let". Backtracking over a closure in a rule is also
considered failure of the closure, which is how hypothetical variables
are managed by rules. (And on the flip side, use of "fail" within a
rule closure initiates backtracking of the rule.)
When is a closure not a closure
Everything is conceptually a closure in Perl 6, but the optimizer is
free to turn unreferenced closures into mere blocks of code. It is
also free to turn referenced closures into mere anonymous subroutines
if the block does not refer to any external lexicals that could
themselves be cloned. In particular, named subroutines in any scope do
not consider themselves closures unless you take a reference to them.
So
sub foo {
my $x = 1;
my sub bar { print $x } # not cloned yet
my &baz = { bar(); print $x }; # cloned immediately
my $barref = &bar; # now bar is cloned
return &baz;
}
When we say "clone", we mean the way the system takes a snapshot of the
routine's lexical scope and binds it to the current instance of the
routine so that if you ever use the current reference to the routine,
it gets the current snapshot of its world, lexically speaking.
Some closures produce references at compile time that cannot be cloned,
because they're not attached to any runtime code that can actively
clone them. "BEGIN", "CHECK", "INIT", and "END" blocks probably fall
into this category. Therefore you can't reliably refer to run-time
variables from them even if they appear to be in scope. (The compile-
time closure may, in fact, see a some kind of permanent copy of the
variable for some storage classes, but the variable is likely to be
undefined when the closure is run in any case.) It's only safe to
refer to package variables and file-scoped lexicals from such a
routine.
On the other hand, it is required that "CATCH" and "LEAVE" blocks be
able to see transient variables in their current lexical scope, so
their cloning status depends at least on the cloning status of the
block they're in.
perl v5.14.0 2006-02-28 Perl6::Bible::S04(3)
