Perl6::Bible::E04(3) User Contributed Perl Documentation Perl6::Bible::E04(3)NAME
Exegesis_04 - Syntax
AUTHOR
Damian Conway <damian@conway.org>
VERSION
Maintainer: Allison Randal <al@shadowed.net>
Date: 2 Apr 2002
Last Modified: unknown
Number: 4
Version: 1
And I'd se-ell my-y so-oul for flow of con-tro-ol ... over Perl
-- The Motels, "Total Control" (Perl 6 remix)
In Apocalypse 4, Larry explains the fundamental changes to flow and
block control in Perl 6. The changes bring fully integrated exceptions;
a powerful new switch statement; a coherent mechanism for polymorphic
matching; a greatly enhanced "for" loop; and unification of blocks,
subroutines and closures.
Let's dive right in.
"Now, Witness the Power of This Fully Operational Control Structure"
We'll consider a simple interactive RPN calculator. The real thing
would have many more operators and values, but that's not important
right now.
class Err::BadData is Exception {...}
module Calc;
my class NoData is Exception {
method warn(*@args) { die @args }
}
my %var;
my sub get_data ($data) {
given $data {
when /^\d+$/ { return %var{""} = $_ }
when 'previous' { return %var{""} // fail NoData }
when %var { return %var{""} = %var{$_} }
default { die Err::BadData : msg=>"Don't understand $_" }
}
}
sub calc (str $expr, int $i) {
our %operator is private //= (
'*' => { $^a * $^b },
'/' => { $^a / $^b },
'~' => { ($^a + $^b) / 2 },
);
my @stack;
my $toknum = 1;
for split /\s+/, $expr -> $token {
try {
when %operator {
my @args = splice @stack, -2;
push @stack, %operator{$token}(*@args)
}
when '.', ';', '=' {
last
}
use fatal;
push @stack, get_data($token);
CATCH {
when Err::Reportable { warn $!; continue }
when Err::BadData { $!.fail(at=>$toknum) }
when NoData { push @stack, 0 }
when /division by zero/ { push @stack, Inf }
}
}
NEXT { $toknum++ }
}
fail Err::BadData: msg=>"Too many operands" if @stack > 1;
return %var{'$' _ $i} = pop(@stack) but true;
}
module main;
for 1..Inf -> $i {
print "$i> ";
my $expr = <> err last;
print "$i> $( Calc::calc(i=>$i, expr=>$expr) )\n";
}
An Exceptionally Promising Beginning
The calculator is going to handle internal and external errors using
Perl 6's OO exception mechanism. This means that we're going to need
some classes for those OO exceptions to belong to.
To create those classes, the "class" keyword is used. For example:
class Err::BadData is Exception {...}
After this declaration, "Err::BadData" is a class name (or rather, by
analogy to "filehandle," it's a "classname"). Either way, it can then
be used as a type specifier wherever Perl 6 expects one. Unlike Perl 5,
that classname is not a bareword string: It's a genuine first-class
symbol in the program. In object-oriented terms, we could think of a
classname as a meta-object -- an object that describes the attributes
and behavior of other objects.
Modules and packages are also first class in Perl 6, so we can also
refer to their names directly, or take references to them, or look them
up in the appropriate symbol table.
Classes can take properties, just like variables and values. Generally,
those properties will specify variations in the behavior of the class.
For example:
class B::Like::Me is interface;
specifies that the B::Like::Me class defines a (Java-like) interface
that any subclass must implement.
The "is Exception" is not, however, a standard property. Indeed,
"Exception" is the name of another (standard, built-in) class. When a
classname like this is used as if it were a property, the property it
confers is inheritance. Specifically, "Err::BadData" is defined as
inheriting from the "Exception" base class. In Perl 5, that would have
been:
# Perl 5 code
package Err::BadData;
use base 'Exception';
So now class "Err::BadData" will have all the exceptionally useful
properties of the "Exception" class.
Having classnames as "first class" symbols of the program means that
it's also important to be able to pre-declare them (to avoid compile-
time "no such class or module" errors). So we need a new syntax for
declaring the existence of classes/modules/packages, without actually
defining their behavior.
To do that we write:
class MyClass {...}
That right. That's real, executable, Perl 6 code.
We're defining the class, but using the new Perl 6 "yada-yada-yada"
operator in a block immediately after the classname. By using the "I'm-
eventually-going-to-put-something-here-but-not-just-yet" marker, we
indicate that this definition is only a stub or placeholder. In this
way, we introduce the classname into the current scope without needing
to provide the complete description of the class.
By the way, this is also the way we can declare other types of symbols
in Perl 6 without actually defining them:
module Alpha {...}
package Beta {...}
method Gamma::delta(Gamma $self: $d1, $d2) {...}
sub epsilon() {...}
In our example, the "Err::BadData" classname is introduced in precisely
that way:
class Err::BadData is Exception {...}
which means that we can refer to the class by name, even though it has
not yet been completely defined.
In fact, in this example, "Err::BadData" is never completely defined.
So we'd get a fatal compile-time error: "Missing definition for class
Err::BadData." Then we'd realize we either forgot to eventually define
the class, or that we had really meant to write:
class Err::BadData is Exception {} # Define new exception class with
# no methods or attributes
# except those it inherits
# See below.
Lexical Exceptions
Most of the implementation of the calculator is contained in the "Calc"
module. In Perl 6, modules are specified using the "module" keyword:
module Calc;
which is similar in effect to a Perl 5:
# Perl 5 code
package Calc;
Modules are not quite the same as packages in Perl 6. Most
significantly, they have a different export mechanism: They export via
a new, built-in, declarative mechanism (which will be described in a
future Apocalypse) and the symbols they export are exported lexically
by default.
The first thing to appear in the module is a class declaration:
my class NoData is Exception {
method warn(*@args) { die @args }
}
This is another class derived from "Exception", but one that has two
significant differences from the declaration of "class Err::BadData":
· The leading "my" makes it lexical in scope, and
· the trailing braces give it an associated block in which its
attributes and methods can be specified.
Let's look at each of those.
"NoData" exceptions are only going to be used within the "Calc" module
itself. So it's good software engineering to make them visible only
within the module itself.
Why? Because if we ever attempt to refer to the exception class outside
"Calc" (e.g. if we tried to catch such an exception in "main"), then
we'll get a compile-time "No such class: NoData" error. Any such errors
would indicate a flaw in our class design or implementation.
In Perl 6, classes are first-class constructs. That is, like variables
and subroutines, they are "tangible" components of a program, denizens
of a symbol table, able to be referred to both symbolically and by
explicit reference:
$class = \Some::Previously::Defined::Class;
# and later
$obj = $class.new();
Note that the back slash is actually optional in that first line, just
as it would be for an array or hash in the same position.
"First class" also means that classnames live in a symbol table. So it
follows that they can be defined to live in the current lexical symbol
table (i.e. %MY::), by placing a "my" before them.
A lexical class or module is only accessible in the lexical scope in
which it's declared. Of course, like Perl 5 packages, Perl 6 classes
and modules don't usually have an explicit lexical scope associated
with their declaration. They are implicitly associated with the
surrounding lexical scope (which is normally a file scope).
But we can give them their own lexical scope to preside over by adding
a block at the end of their declaration:
class Whatever {
# definition here
}
This turns out to be important. Without the ability to specify a
lexical scope over which the class has effect, we would be stuck with
no way to embed a "nested" lexical class:
class Outer;
# class Outer's namespace
my class Inner;
# From this line to the end of the file
# is now in class Inner's namespace
In Perl 6, we avoid this problem by writing:
class Outer;
# class Outer's namespace
my class Inner {
# class Inner's namespace
}
# class Outer's namespace again
In our example, we use this new feature to redefine "NoData"'s "warn"
method (upgrading it to a call to "die"). Of course, we could also have
done that with just:
my class NoData is Exception; # Open NoData's namespace
method warn(*@args) { die @args } # Defined in NoData's namespace
but then we would have needed to "reopen" the "Calc" module's namespace
afterward:
module Calc; # Open Calc's namespace
my class NoData is Exception; # Open NoData's (nested) namespace
method warn(*@args) { die @args } # Defined in NoData's namespace
module Calc; # Back to Calc's namespace
Being able to "nest" the "NoData" namespace:
module Calc; # Open Calc's namespace
my class NoData is Exception { # Open NoData's (nested) namespace
method warn(*@args) { die @args } # Defined in NoData's namespace
}
# The rest of module Calc defined here.
is much cleaner.
By the way, because classes can now have an associated block, they can
even be anonymous:
$anon_class = class {
# definition here
};
# and later
$obj = $anon_class.new();
which is a handy way of implementing "singleton" objects:
my $allocator = class {
my $.count = "ID_000001";
method next_ID { $.count++ }
}.new;
# and later...
for @objects {
$_.set_id( $allocator.next_ID );
}
Maintaining Your State
To store the values of any variables used by the calculator, we'll use
a single hash, with each key being a variable name:
my %var;
Nothing more to see here. Let's move along.
It's a Given
The "get_data" subroutine may be given a number (i.e. a literal value),
a numerical variable name (i.e. '$1', '$2', etc.) , or the keyword
'previous'.
It then looks up the information in the %var hash, using a switch
statement to determine the appropriate look-up:
my sub get_data ($data) {
given $data {
The "given $data" evaluates its first argument (in this case, $data) in
a scalar context, and makes the result the "topic" of each subsequent
"when" inside the block associated with the "given". (Though, just
between us, that block is merely an anonymous closure acting as the
"given"'s second argument -- in Perl 6 all blocks are merely closures
that are slumming it.)
Note that the "given $data" statement also makes $_ an alias for $data.
So, for example, if the "when" specifies a pattern:
when /^\d+$/ { return %var{""} = $_ }
then that pattern is matched against the contents of $data (i.e.
against the current topic). Likewise, caching and returning $_ when the
pattern matches is the same as caching and returning $data.
After a "when"'s block has been selected and executed, control
automatically passes to the end of the surrounding "given" (or, more
generally, to the end of whatever block provided the "when"'s topic).
That means that "when" blocks don't "fall through" in the way that
"case" statements do in C.
You can also explicitly send control to the end of a "when"'s
surrounding "given", using a "break" statement. For example:
given $number {
when /[02468]$/ {
if ($_ == 2) {
warn "$_ is even and prime\n";
break;
}
warn "$_ is even and composite\n";
}
when &is_prime {
warn "$_ is odd and prime\n";
}
warn "$_ is odd and composite\n";
}
Alternatively, you can explicitly tell Perl not to automatically
"break" at the end of the "when" block. That is, tell it to "fall
through" to the statement immediately after the "when". That's done
with a "continue" statement (which is the new name for The Statement
Formerly Known As "skip"):
given $number {
when &is_prime { warn "$_ is prime\n"; continue; }
when /[13579]$/ { warn "$_ is odd"; }
when /[02468]$/ { warn "$_ is even"; }
}
In Perl 6, a "continue" means: "continue executing from the next
statement after the current "when", rather than jumping out of the
surrounding "given"." It has nothing to do with the old Perl 5
"continue" block, which in Perl 6 becomes "NEXT".
The "topic" that "given" creates can also be aliased to a name of our
own choosing (though it's always aliased to $_ no matter what else we
may do). To give the topic a more meaningful name, we just need to use
the "topical arrow:"
given check_online().{active}{names}[0] -> $name {
when /^\w+$/ { print "$name's on first\n" }
when /\?\?\?/ { print "Who's on first\n" }
}
Having been replaced by the dot, the old Perl 5 arrow operator is given
a new role in Perl 6. When placed after the topic specifier of a
control structure (i.e. the scalar argument of a "given", or the list
of a "for"), it allows us to give an extra name (apart from $_) to the
topic associated with that control structure.
In the above version, the "given" statement declares a lexical variable
$name and makes it yet another way of referring to the current topic.
That is, it aliases both $name and $_ to the value specified by
"check_online().{active}{names}[0]".
This is a fundamental change from Perl 5, where $_ was only aliased to
the current topic in a "for" loop. In Perl 6, the current topic --
whatever its name and however you make it the topic -- is always
aliased to $_.
That implies that everywhere that Perl 5 used $_ as a default (i.e.
"print", "chomp", "split", "length", "eval", etc.), Perl 6 uses the
current topic:
for @list -> $next { # iterate @list, aliasing each element to
# $next (and to $_)
print if length > 10; # same as: print $next if length $next > 10
%count{$next}++;
}
This is subtly different from the "equivalent" Perl 5 code:
# Perl 5 code
for my $next (@list) { # iterate @list, aliasing each element to
# $next (but not to $_)
print if length > 10; # same as: print $_ if length $_ > 10
# using the $_ value from *outside* the loop
%count{$next}++;
}
If you had wanted this Perl 5 behavior in Perl 6, then you'd have to
say explicitly what you meant:
my $outer_underscore := $_;
for @list -> $next {
print $outer_underscore
if length $outer_underscore > 10;
%count{$next}++;
}
which is probably a good thing in code that subtle.
Oh, and yes: the "p52p6" translator program will take that new behavior
into account and correctly convert something pathological like:
# Perl 5 code
while (<>) {
for my $elem (@list) {
print if $elem % 2;
}
}
to:
# Perl 6 code
for <> {
my $some_magic_temporary_variable := $_;
for @list -> $elem {
print $some_magic_temporary_variable if $elem % 2;
}
}
Note that this works because, in Perl 6, a call to "<>" is lazily
evaluated in list contexts, including the list of a "for" loop.
Other whens
The remaining cases of the data look-up are handled by subsequent
"when" statements. The first:
when 'previous' { return %var{""} // fail NoData }
handles the special keyword "previous". The previous value is always
stored in the element of %var whose key is the empty string.
If, however, that previous value is undefined, then the defaulting
operator -- "//" -- causes the right-hand side of the expression to be
evaluated instead. That right-hand side is a call to the "fail" method
of class "NoData" (and could equally have been written
"NoData.fail()").
The standard "fail" method inherited from the "Exception" class
constructs an instance of the appropriate class (i.e. an exception
object) and then either throws that exception (if the "use fatal"
pragma is in effect) or else returns an "undef" value from the scope in
which the "fail" was invoked. That is, the "fail" acts like a "die
SomeExceptionClass" or a "return undef", depending on the state of the
"use fatal" pragma.
This is possible because, in Perl 6, all flow-of-control -- including
the normal subroutine "return" -- is exception-based. So, when it is
supposed to act like a "return", the "Exception::fail" method simply
throws the special "Ctl::Return" exception, which "get_data"'s caller
will (automagically) catch and treat as a normal return.
So then why not just write the usual:
return undef;
instead?
The advantage of using "fail" is that it allows the callers of
"get_data" to decide how that subroutine should signal failure. As
explained above, normally "fail" fails by returning "undef". But if a
"use fatal" pragma is in effect, any invocation of "fail" instead
throws the corresponding exception.
What's the advantage in that? Well, some people feel that certain types
of failures ought to be taken deadly seriously (i.e. they should kill
you unless you explicitly catch and handle them). Others feel that the
same errors really aren't all that serious and you should be allowed
to, like, chill man and just groove with the heavy consequences, dude.
The "fail" method allows you, the coder, to stay well out of that kind
of fruitless religious debate.
When you use "fail" to signal failure, not only is the code nicely
documented at that point, but the mode of failure becomes caller-
selectable. Fanatics can "use fatal" and make each failure punishable
by death; hippies can say "no fatal" and make each failure just return
"undef".
You no longer have to get caught up in endless debate as to whether the
exception-catching:
try { $data = get_data($str) }
// warn "Couldn't get data" }
is inherently better or worse than the "undef"-sensing:
do { $data = get_data($str) }
// warn "Couldn't get data";
Instead, you can just write "get_data" such that There's More Than One
Way To Fail It.
By the way, "fail" can fail in other ways, too: in different contexts
or under different pragmas. The most obvious example would be inside a
regex, where it would initiate back-tracking. More on that in
Apocalypse 5.
Still Other Whens
Meanwhile, if $data isn't a number or the "previous" keyword, then
maybe it's the name of one of the calculator's variables. The third
"when" statement of the switch tests for that:
when %var { return %var{""} = %var{$_} }
If a "when" is given a hash, then it uses the current topic as a key in
the hash and looks up the corresponding entry. If that value is true,
then it executes its block. In this case, that block caches the value
that was looked up (i.e. %var{$_}) in the "previous" slot and returns
it.
"Aha!" you say, "that's a bug! What if the value of %var{$_} is
false?!" Well, if it were possible for that to ever happen, then it
certainly would be a bug, and we'd have to write something ugly:
when defined %var{$_} { return %var{""} = %var{$_} }
But, of course, it's much easier just to redefine Truth, so that any
literal zero value stored in %var is no longer false. See below.
Finally, if the $data isn't a literal, then a "previous", or a variable
name, it must be an invalid token, so the default alternative in the
switch statement throws an "Err::BadData" exception:
default { die Err::BadData : msg=>"Don't understand $_" }
Note that, here again, we are actually executing a method call to:
Err::BadData.die(msg=>"Don't understand $_");
as indicated by the use of the colon after the classname.
Of course, by using "die" instead of "fail" here, we're giving clients
of the "get_data" subroutine no choice but to deal with "Err::BadData"
exceptions.
An Aside: the "Smart Match" Operator
The rules governing how the argument of a "when" is matched against the
current topic are designed to be as DWIMish as possible. Which means
that they are actually quite complex. They're listed in Apocalypse 4,
so we won't review them here.
Collectively, the rules are designed to provide a generic "best attempt
at matching" behavior. That is, given two values (the current topic and
the "when"'s first argument), they try to determine whether those
values can be combined to produce a "smart match" -- for some
reasonable definitions of "smart" and "match."
That means that one possible use of a Perl 6 switch statement is simply
to test whether two values match without worrying about how those two
values match:
sub hey_just_see_if_dey_match_willya ($val1, $val2) {
given $val1 {
when $val2 { return 1 }
default { return 0 }
}
}
That behavior is sufficiently useful that Larry wanted to make it much
easier to use. Specifically, he wanted to provide a generic "smart
match" operator.
So he did. It's called "=~".
Yes, the humble Perl 5 "match a string against a regex" operator is
promoted in Perl 6 to a "smart-match an anything against an anything"
operator. So now:
if ($val1 =~ $val2) {...}
works out the most appropriate way to compare its two scalar operands.
The result might be a numeric comparison ("$val1 == $val2") or a string
comparison ("$val1 eq $val2") or a subroutine call ("$val1.($val2)") or
a pattern match ("$val1 =~ /$val2/") or whatever else makes the most
sense for the actual run-time types of the two operands.
This new turbo-charged "smart match" operator will also work on arrays,
hashes and lists:
if @array =~ $elem {...} # true if @array contains $elem
if $key =~ %hash {...} # true if %hash{$key}
if $value =~ (1..10) {...} # true if $value is in the list
if $value =~ ('a',/\s/,7) {...} # true if $value is eq to 'a'
# or if $value contains whitespace
# or if $value is == to 7
That final example illustrates some of the extra intelligence that Perl
6's "=~" has: When one of its arguments is a list (not an array), the
"smart match" operator recursively "smart matches" each element and ORs
the results together, short-circuiting if possible.
Being Calculating
The next component of the program is the subroutine that computes the
actual results of each expression that the user enters. It takes a
string to be evaluated and an integer indicating the current iteration
number of the main input loop (for debugging purposes):
sub calc (str $expr, int $count) {
Give us a little privacy, please
Perl 5 has a really ugly idiom for creating "durable" lexical
variables: variables that are lexically scoped but stick around from
call to call.
If you write:
sub whatever {
my $count if 0;
$count++;
print "whatever called $count times\n";
}
then the compile-time aspect of a "my $count" declaration causes $count
to be declared as a lexical in the subroutine block. However, at run-
time -- when the variable would normally be (re-)allocated -- the "if
0" prevents that process. So the original lexical variable is not
replaced on each invocation, and is instead shared by them all.
This awful "if 0" idiom works under most versions of Perl 5, but it's
really just a freakish accident of Perl's evolution, not a carefully
designed and lovingly crafted feature. So just say "No!".
Perl 6 allows us to do the same thing, but without feeling the need to
wash afterward.
To understand how Perl 6 cleans up this idiom, notice that the durable
variable is really much more; like a package variable that just happens
to be accessible only in a particular lexical scope. That kind of
restricted-access package variable is going to be quite common in Perl
6 -- as an attribute of a class.
So the way we create such a variable is to declare it as a package
variable, but with the "is private" property:
module Wherever;
sub whatever {
our $count is private;
$count++;
print "whatever called $count times\n";
}
Adding "is private" causes Perl to recognize the existence of the
variable $count within the "Wherever" module, but then to restrict its
accessibility to the lexical scope in which it is first declared. In
the above example, any attempt to refer to $Wherever::count outside the
&Wherever::whatever subroutine produces a compile-time error. It's
still a package variable, but now you can't use it anywhere but in the
nominated lexical scope.
Apart from the benefit of replacing an ugly hack with a clean explicit
marker on the variable, the real advantage is that Perl 6 private
variables can be also be initialized:
sub whatever {
our $count is private //= 1;
print "whatever called $count times\n";
$count++;
}
That initialization is performed the first time the variable
declaration is encountered during execution (because that's the only
time its value is "undef", so that's the only time the "//=" operator
has any effect).
In our example program we use that facility to do a one-time-only
initialization of a private package hash. That hash will then be used
as a (lexically restricted) look-up table to provide the
implementations for a set of operator symbols:
our %operator is private //= (
'*' => { $^a * $^b },
'/' => { $^a / $^b },
'~' => { ($^a + $^b) / 2 },
);
Each key of the hash is an operator symbol and the corresponding value
is an anonymous subroutine that implements the appropriate operation.
Note the use of the "place-holder" variables ($^a and $^b) to
implicitly specify the parameters of the closures.
Since all the data for the %operator hash is constant, we could have
achieved a similar effect with:
my %operator is constant = (
'*' => { $^a * $^b },
'/' => { $^a / $^b },
'~' => { ($^a + $^b) / 2 },
);
Notionally this is quite different from the "is private" version, in
that -- theoretically -- the lexical constant would be reconstructed
and reinitialized on each invocation of the "calc" subroutine.
Although, in practice, we would expect the compiler to notice the
constant initializer and optimize the initialization out to compile-
time.
If the initializer had been a run-time expression, then the "is
private" and "is constant" versions would behave very differently:
our %operator is private //= todays_ops(); # Initialize once, the first
# time statement is reached.
# Thereafter may be changed
# at will within subroutine.
my %operator is constant = todays_ops(); # Re-initialize every time
# statement is reached.
# Thereafter constant
# within subroutine
Let's Split!
We then have to split the input expression into (whitespace-delimited)
tokens, in order to parse and execute it. Since the calculator language
we're implementing is RPN, we need a stack to store data and interim
calculations:
my @stack;
We also need a counter to track the current token number (for error
messages):
my $toknum = 1;
Then we just use the standard "split" built-in to break up the
expression string, and iterate through each of the resulting tokens
using a "for" loop:
for split /\s+/, $expr -> $token {
There are several important features to note in this "for" loop. To
begin with, there are no parentheses around the list. In Perl 6, they
are not required (they're not needed for any control structure), though
they are certainly still permissible:
for (split /\s+/, $expr) -> $token {
More importantly, the declaration of the iterator variable ($token) is
no longer to the left of the list:
# Perl 5 code
for my $token (split /\s+/, $expr) {
Instead, it is specified via a topical arrow to the right of the list.
By the way, somewhat surprisingly, the Perl 6 arrow operator isn't a
binary operator. (Actually, neither is the Perl 5 arrow operator, but
that's not important right now.)
Even more surprisingly, what the Perl 6 arrow operator is, is a synonym
for the declarator "sub". That's right, in Perl 6 you can declare an
anonymous subroutine like so:
$product_plus_one = -> $x, $y { $x*$y + 1 };
The arrow behaves like an anonymous "sub" declarator:
$product_plus_one = sub($x, $y) { $x*$y + 1 };
except that its parameter list doesn't require parentheses. That
implies:
· The Perl 6 "for", "while", "if", and "given" statements each take
two arguments: an expression that controls them and a
subroutine/closure that they execute. Normally, that closure is
just a block (in Perl6 all blocks are really closures):
for 1..10 { # no comma needed before opening brace
print
}
but you can also be explicit:
for 1..10, sub { # needs comma if a regular anonymous sub
print
}
or you can be pointed:
for 1..10 -> { # no comma needed with arrow notation
print
}
or referential:
for 1..10, # needs comma if a regular sub reference
&some_sub;
· The variable after the arrow is effectively a lexical variable
confined to the scope of the following block (just as a subroutine
parameter is a lexical variable confined to the scope of the
subroutine block). Within the block, that lexical becomes an alias
for the topic (just as a subroutine parameter becomes an alias for
the corresponding argument).
· Topic variables created with the arrow notation are, by default,
read-only aliases (because Perl 6 subroutine parameters are, by
default, read-only aliases):
for @list -> $i {
if ($cmd =~ 'incr') {
$i++; # Error: $i is read-only
}
}
Note that the rule doesn't apply to the default topic ($_), which
is given special dispensation to be a modifiable alias (as in Perl
5).
· If you want a named topic to be modifiable through its alias, then
you have to say so explicitly:
for @list -> $i is rw {
if ($cmd =~ 'incr') {
$i++; # Okay: $i is read-write
}
}
· Just as a subroutine can have more than one parameter, so too we
can specify more than one named iterator variable at a time:
for %phonebook.kv -> $name, $number {
print "$name: $number\n"
}
Note that in Perl 6, a hash in a list context returns a list of
pairs, not the Perl 5-ish "key, value, key, value, ..." sequence.
To get the hash contents in that format, we have to call the hash's
"kv" method explicitly.
What actually happens in this iteration (and, in fact, in all such
instances) is that the "for" loop looks at the number of arguments
its closure takes and iterates that many elements at a time.
Note that "map" and "reduce" can do that too in Perl 6:
# process @xs_and_ys two-at-a-time...
@list_of_powers = map { $^x ** $^y } @xs_and_ys;
# reduce list three-at-a-time
$sum_of_powers = reduce { $^partial_sum + $^x ** $^y } 0, @xs_and_ys;
And, of course, since "map" and "reduce" take a subroutine
reference as their first argument -- instead of using the higher-
order placeholder notation -- we could use the arrow notation here
too:
@list_of_powers = map -> $x, $y { $x ** $y } @xs_and_ys;
or even an old-fashioned anonymous subroutine:
@list_of_powers = map sub($x,$y){ $x ** $y }, @xs_and_ys;
Phew. If that all makes your head hurt, then don't worry. All you
really need to remember is this: If you don't want to use $_ as the
name of the current topic, then you can change it by putting an arrow
and a variable name before the block of most control statements.
A Trying Situation
Once the calculator's input has been split into tokens, the "for" loop
processes each one in turn, by applying them (if they represent an
operator), or jumping out of the loop (if they represent an end-of-
expression marker: '.', ';', or '='), or pushing them onto the stack
(since anything else must be an operand):
try {
when %operator { # apply operator
my @args = splice @stack, -2;
push @stack, %operator{$token}(*@args);
}
when '.', ';', '=' { # or jump out of loop
last;
}
use fatal;
push @stack, get_data($token); # or push operand
The first two possibilities are tested for using "when" statements.
Recall that a "when" tests its first argument against the current
topic. In this case, however, the token was made the topic by the
surrounding "for". This is a significant feature of Perl 6: "when"
blocks can implement a switch statement anywhere there is a valid
topic, not just inside a "given".
The block associated with "when %operator" will be selected if
%operator{$token} is true (i.e. if there is an operator implementation
in %operator corresponding to the current topic). In that case, the top
two arguments are spliced from the stack and passed to the closure
implementing that operation ("%operator{$token}(*@args)"). Note that
there would normally be a dot (".") operator between the hash entry
(i.e. a subroutine reference) and the subroutine call, like so:
%operator{$token}.(*@args)
but in Perl 6 it may be omitted since it can be inferred (just as an
inferrable "->" can be omitted in Perl 5).
Note too that we used the flattening operator ("*") on @args, because
the closure returned by %operator{$token} expects two scalar arguments,
not one array.
The second "when" simply exits the loop if it finds an "end-of-
expression" token. In this example, the argument of the "when" is a
list of strings, so the "when" succeeds if any of them matches the
token.
Of course, since the entire body of the "when" block is a single
statement, we could also have written the "when" as a statement
modifier:
last when '.', ';', '=';
The fact that "when" has a postfix version like this should come as no
surprise, since "when" is simply another control structure like "if",
"for", "while", etc.
The postfix version of "when" does have one interesting feature. Since
it governs a statement, rather than a block, it does not provide the
block-"when"'s automatic ""break" to the end of my topicalizing block"
behavior. In this instance, it makes no difference since the "last"
would do that anyway.
The final alternative -- pushing the token onto the stack -- is simply
a regular Perl "push" command. The only interesting feature is that it
calls the "get_data" subroutine to pre-translate the token if
necessary. It also specifies a "use fatal" so that "get_data" will fail
by an throwing exception, rather than returning "undef".
The loop tries each of these possibilities in turn. And "tries" is the
operative word here, because either the application of operations or
the pushing of data onto the stack may fail, resulting in an exception.
To prevent that exception from propagating all the way back to the main
program and terminating it, the various alternatives are placed in a
"try" block.
A "try" block is the Perl 6 successor to Perl 5's "eval" block. Unless
it includes some explicit error handling code (see "Where's the
Catch???"), it acts exactly like a Perl 5 "eval {...}", intercepting a
propagating exception and converting it to an "undef" return value:
try { $quotient = $numerator / $denominator } // warn "couldn't divide";
Where's the Catch???
In Perl 6, we aren't limited to just blindly catching a propagating
exception and then coping with an "undef". It is also possible to set
up an explicit handler to catch, identify and deal with various types
of exceptions. That's done in a "CATCH" block:
CATCH {
when Err::Reportable { warn $!; continue }
when Err::BadData { $!.fail(at=>$toknum) }
when NoData { push @stack, 0 }
when /division by zero/ { push @stack, Inf }
}
A "CATCH" block is like a "BEGIN" block (hence the capitalization).
Its one argument is a closure that is executed if an exception ever
propagates as far as the block in which the "CATCH" was declared. If
the block eventually executes, then the current topic is aliased to the
error variable $!. So the typical thing to do is to populate the
exception handler's closure with a series of "when" statements that
identify the exception contained in $! and handle the error
appropriately. More on that in a moment.
The "CATCH" block has one additional property. When its closure has
executed, it transfers control to the end of the block in which it was
defined. This means that exception handling in Perl 6 is non-
resumptive: once an exception is handled, control passes outward, and
the code that threw the exception is not automatically re-executed.
If we did want "try, try, try again" exception handling instead, then
we'd need to explicitly code a loop around the code we're trying:
# generate exceptions (sometimes)
sub getnum_or_die {
given <> { # readline and make it the topic
die "$_ is not a number"
unless defined && /^\d+$/;
return $_;
}
}
# non-resumptive exception handling
sub readnum_or_cry {
return getnum_or_die; # maybe generate an exception
CATCH { warn $! } # if so, warn and fall out of sub
}
# pseudo-resumptive
sub readnum_or_retry {
loop { # loop endlessly...
return getnum_or_die; # maybe generate an exception
CATCH { warn $! } # if so, warn and fall out of loop
} # (i.e. loop back and try again)
}
Note that this isn't true resumptive exception handling. Control still
passes outward -- to the end of the "loop" block. But then the "loop"
reiterates, sending control back into "getnum_or_die" for another
attempt.
Catch as Catch Can
Within the "CATCH" block, the example uses the standard Perl 6
exception handling technique: a series of "when" statements. Those
"when" statements compare their arguments against the current topic.
In a "CATCH" block, that topic is always aliased to the error variable
$!, which contains a reference to the propagating exception object.
The first three "when" statements use a classname as their argument.
When matching a classname against an object, the "=~" operator (and
therefore any "when" statement) will call the object's "isa" method,
passing it the classname. So the first three cases of the handler:
when Err::Reportable { warn $!; continue }
when Err::BadData { $!.fail(at=>$toknum) }
when NoData { push @stack, 0 }
are (almost) equivalent to:
if $!.isa(Err::Reportable) { warn $! }
elsif $!.isa(Err::BadData) { $!.fail(at=>$toknum) }
elsif $!.isa(NoData) { push @stack, 0 }
except far more readable.
The first "when" statement simply passes the exception object to
"warn". Since "warn" takes a string as its argument, the exception
object's stringification operator (inherited from the standard
"Exception" class) is invoked and returns an appropriate diagnostic
string, which is printed. The "when" block then executes a "continue"
statement, which circumvents the default ""break" out of the
surrounding topicalizer block" semantics of the "when".
The second "when" statement calls the propagating exception's "fail"
method to cause "calc" either to return or rethrow the exception,
depending on whether "use fatal" was set. In addition, it passes some
extra information to the exception, namely the number of the token that
caused the problem.
The third "when" statement handles the case where there is no cached
data corresponding to the calculator's "previous" keyword, by simply
pushing a zero onto the stack.
The final case that the handler tests for:
when /division by zero/ { push @stack, Inf }
uses a regex, rather than a classname. This causes the topic (i.e. the
exception) to be stringified and pattern-matched against the regex. As
mentioned above, by default, all exceptions stringify to their own
diagnostic string. So this part of the handler simply tests whether
that string includes the words "division by zero," in which case it
pushes the Perl 6 infinity value onto the stack.
One Dot Only
The "CATCH" block handled bad data by calling the "fail" method of the
current exception:
when Err::BadData { $!.fail(at=>$toknum) }
That's a particular instance of a far more general activity: calling a
method on the current topic. Perl 6 provides a shortcut for that -- the
prefix unary dot operator. Unary dot calls the method that is its
single operand, using the current topic as the implicit invocant. So
the "Err::BadData" handler could have been written:
when Err::BadData { .fail(at=>$toknum) }
One of the main uses of unary dot is to allow "when" statements to
select behavior on the basis of method calls. For example:
given $some_object {
when .has_data('new') { print "New data available\n" }
when .has_data('old') { print "Old data still available\n" }
when .is_updating { sleep 1 }
when .can('die') { .die("bad state") } # $some_object.die(...)
default { die "internal error" } # global die
}
Unary dot is also useful within the definition of methods themselves.
In a Perl 6 method, the invocant (i.e. the first argument of the
method, which is a reference to the object on which the method was
invoked) is always the topic, so instead of writing:
method dogtag (Soldier $self) {
print $self.rank, " ", $self.name, "\n"
unless $self.status('covert');
}
we can just write:
method dogtag (Soldier $self) { # $self is automagically the topic
print .rank, " ", .name, "\n"
unless .status('covert');
}
or even just:
method dogtag { # @_[0] is automagically the topic
print .rank, " ", .name, "\n"
unless .status('covert');
}
Yet another use of unary dot is as a way of abbreviating multiple
accesses to hash or array elements. That is, "given" also implements
the oft-coveted "with" statement. If many elements of a hash or array
are to be accessed in a set of statements, then we can avoid the
tedious repetition of the container name:
# initialize from %options...
$name = %options{name} // %options{default_name};
$age = %options{age};
$limit = max(%options{limit}, %options{rate} * %options{count});
$count = $limit / %options{max_per_count};
by making it the topic and using unary dot:
# initialize from %options...
given %options {
$name = .{name} // .{default_name};
$age = .{age};
$limit = max(.{limit}, .{rate} * .{count});
$count = $limit / .{max_per_count};
}
Onward and Backward
Back in our example, after each token has been dealt with in its loop
iteration, the iteration is finished. All that remains to do is
increment the token number.
In Perl 5, that would be done in a "continue" block at the end of the
loop block. In Perl 6, it's done in a "NEXT" statement within the loop
block:
NEXT { $toknum++ }
Like a "CATCH", a "NEXT" is a special-purpose "BEGIN" block that takes
a closure as its single argument. The "NEXT" pushes that closure onto
the end of a queue of "next-iteration" handlers, all of which are
executed each time a loop reaches the end of an iteration. That is,
when the loop reaches the end of its block or when it executes an
explicit "next" or "last".
The advantage of moving from Perl 5's external "continue" to Perl 6's
internal "NEXT" is that it gives the "next-iteration" handler access to
any lexical variables declared within the loop block. In addition, it
allows the "next-iteration" handler to be placed anywhere in the loop
that's convenient (e.g. close to the initialization it's later supposed
to clean up).
For example, instead of having to write:
# Perl 5 code
my $in_file, $out_file;
while (<>) {
open $in_file, $_ or die;
open $out_file, "> $_.out" or die;
# process files here (maybe next'ing out early)
}
continue {
close $in_file or die;
close $out_file or die;
}
we can just write:
while (<>) {
my $in_file = open $_ or die;
my $out_file = open "> $_.out" or die;
NEXT {
close $in_file or die;
close $out_file or die;
}
# process files here (maybe next'ing out early)
}
There's no need to declare $in_file and $out_file outside the loop,
because they don't have to be accessible outside the loop (i.e. in an
external "continue").
This ability to declare, access and clean up lexicals within a given
scope is especially important because, in Perl 6, there is no reference
counting to ensure that the filehandles close themselves automatically
immediately at the end of the block. Perl 6's full incremental garbage
collector does guarantee to eventually call the filehandle's
destructors, but makes no promises about when that will happen.
Note that there is also a "LAST" statement, which sets up a handler
that is called automatically when a block is left for the last time.
For example, this:
for reverse 1..10 {
print "$_..." and flush;
NEXT { sleep 1 }
LAST { ignition() && print "lift-off!\n" }
}
prints:
10...9...8...7...6...5...4...3...2...1...lift-off!
sleeping one second after each iteration (including the last one), and
then calling &ignition at the end of the countdown.
"LAST" statements are also extremely useful in nonlooping blocks, as a
way of giving the block a "destructor" with which it can clean up its
state regardless of how it is exited:
sub handler ($value, $was_handled is rw) {
given $value {
LAST { $was_handled = 1 }
when &odd { return "$value is odd" }
when /0$/ { print "decimal compatible" }
when /2$/ { print "binary compatible"; break }
$value %= 7;
when 1,3,5 { die "odd residual" }
}
}
In the above example, no matter how the "given" block exits -- i.e.
via the "return" of the first "when" block, or via the (implicit)
"break" of the second "when", or via the (explicit and redundant)
"break" of the third "when", or via the "odd residual" exception, or by
falling off the end of the "given" block -- the $was_handled parameter
is always correctly set.
Note that the "LAST" is essential here. It wouldn't suffice to write:
sub handler ($value, $was_handled is rw) {
given $value {
when &odd { return '$value is odd" }
when /3$/ { print "ternary compatible" }
when /2$/ { print "binary compatible"; break }
$value %= 7;
when 1,3,5 { die "odd residual" }
}
$was_handled = 1;
}
because then $handled wouldn't be set if an exception was thrown. Of
course, if that's actually the semantics you wanted, then you don't
want "LAST" in that case.
WHY ARE YOU SHOUTING???
You may be wondering why "try" is in lower case but "CATCH" is in
upper. Or why "NEXT" and "LAST" blocks have those "loud" keywords.
The reason is simple: "CATCH", "NEXT" and "LAST" blocks are just
specialized "BEGIN" blocks that install particular types of handlers
into the block in which they appear.
They install those handlers at compile-time so, unlike a "try" or a
"next" or a "last", they don't actually do anything when the run-time
flow of execution reaches them. The blocks associated with them are
only executed if the appropriate condition or exception is encountered
within their scope. And, if that happens, then they are executed
automatically, just like "AUTOLOAD", or "DESTROY", or "TIEHASH", or
"FETCH", etc.
So Perl 6 is merely continuing the long Perl tradition of using a
capitalized keyword to highlight code that is executed automatically.
In other words: I'M SHOUTING BECAUSE I WANT YOU TO BE AWARE THAT
SOMETHING SUBTLE IS HAPPENING AT THIS POINT.
Cache and Return
Meanwhile, back in "calc"...
Once the loop is complete and all the tokens have been processed, the
result of the calculation should be the top item on the stack. If the
stack of items has more than one element left, then it's likely that
the expression was wrong somehow (most probably, because there were too
many original operands). So we report that:
fail Err::BadData : msg=>"Too many operands"
if @stack > 1;
If everything is OK, then we simply pop the one remaining value off the
stack and make sure it will evaluate true (even if its value is zero or
"undef") by setting its "true" property. This avoids the potential bug
discussed earlier.
Finally, we record it in %var under the key '$n' (i.e. as the n-th
result), and return it:
return %var{'$' _ $i} = pop(@stack) but true;
"But, but, but...", I hear you expostulate, "...shouldn't that be
"pop(@stack) is true"???"
Once upon a time, yes. But Larry has recently decided that compile-time
and run-time properties should have different keywords. Compile-time
properties (i.e. those ascribed to declarations) will still be
specified with the "is" keyword:
class Child is interface;
my $heart is constant = "true";
our $meeting is private;
whereas run-time properties (i.e. those ascribed to values) will now be
specified with the "but" keyword:
$str = <$trusted_fh> but tainted(0);
$fh = open($filename) but chomped;
return 0 but true;
The choice of "but" is meant to convey the fact that run-time
properties will generally contradict some standard property of a value,
such as its normal truth, chompedness or tainting.
It's also meant to keep people from writing the very natural, but very
misguided:
if ($x is true) {...}
which now generates a (compile-time) error:
Can't ascribe a compile-time property to the run-time value of $x.
(Did you mean "$x but true" or "$x =~ true"?)
The Forever Loop
Once the "Calc" module has all its functionality defined, all that's
required is to write the main input-process-output loop. We'll cheat a
little and write it as an infinite loop, and then (in solemn Unix
tradition) we'll require an EOF signal to exit.
The infinite loop needs to keep track of its iteration count. In Perl 5
that would be:
# Perl 5 code
for (my $i=0; 1; $i++) {
which would translate into Perl 6 as:
loop (my $i=0; 1; $i++) {
since Perl 5's C-like "for" loop has been renamed "loop" in Perl 6 --
to distinguish it from the Perl-like "for" loop.
However, Perl 6 also allows us to create semi-infinite, lazily
evaluated lists, so we can write the same loop much more cleanly as:
for 0..Inf -> $i {
When "Inf" is used as the right-hand operand to "..", it signifies that
the resulting list must be lazily built, and endlessly iterable. This
type of loop will probably be common in Perl 6 as an easy way of
providing a loop counter.
If we need to iterate some list of values, as well as tracking a loop
counter, then we can take advantage of another new feature of Perl 6:
iteration streams.
A regular Perl 6 "for" loop iterates a single stream of values,
aliasing the current topic to each in turn:
for @stream -> $topic_from_stream {
...
}
But it's also possible to specify two (or more) streams of values that
the one "for" loop will step through in parallel:
for @stream1 ; @stream2 -> $topic_from_stream1 ; $topic_from_stream2 {
...
}
Each stream of values is separated by a semicolon, and each topic
variable is similarly separated. The "for" loop iterates both streams
in parallel, aliasing the next element of the first stream (@stream1)
to the first topic ($topic_from_stream1) and the next element of the
second stream (@stream2) to the second topic ($topic_from_stream2).
The commonest application of this will probably be to iterate a list
and simultaneously provide an iteration counter:
for @list; 0..@list.last -> $next; $index {
print "Element $index is $next\n";
}
It may be useful to set that out slightly differently, to show the
parallel nature of the iteration:
for @list ; 0..@list.last
-> $next ; $index {
print "Element $index is $next\n";
}
It's important to note that writing:
for @a; @b -> $x; $y {...}
# in parallel, iterate @a one-at-a-time as $x, and @b one-at-a-time as $y
is not the same as writing:
for @a, @b -> $x, $y {...}
# sequentially iterate @a then @b, two-at-a-time as $x and $y
The difference is that semicolons separate streams, while commas
separate elements within a single stream.
If we were brave enough, then we could even combine the two:
for @a1, @a2; @b -> $x; $y1, $y2 {...}
# sequentially iterate @a1 then @a2, one-at-a-time as $x
# and, in parallel, iterate @b two-at-a-time as $y1 and $y2
This is definitely a case where a different layout would help make the
various iterations and topic bindings clearer:
for @a1, @a2 ; @b
-> $x ; $y1, $y2 {...}
Note, however, that the normal way in Perl 6 to step through an array's
values while tracking its indices will almost certainly be to use the
array's "kv" method. That method returns a list of interleaved indices
and values (much like the hash's "kv" method returns alternating keys
and values):
for @list.kv -> $index, $next {
print "Element $index is $next\n";
}
Read or Die
Having prompted for the next expression that the calculator will
evaluate:
print "$i> ";
we read in the expression and check for an EOF (which will cause the
"<>" operator to return "undef", in which case we escape the infinite
loop):
my $expr = <> err last;
Err..."err"???
In Apocalypse 3, Larry introduced the "//" operator, which is like a
"||" that tests its left operand for definedness rather than truth.
What he didn't mention (but which you probably guessed) was that there
is also the low-precedence version of "//". Its name is "err":
Operation High Precedence Low Precedence
INCLUSIVE OR || or
EXCLUSIVE OR ~~ xor
DEFINED OR // err
But why call it "err"?
Well, the "//" operator looks like a skewed version of "||", so the
low-precedence version should probably be a skewed version of "or". We
can't skew it visually (even Larry thought that using italics would be
going a bit far), so we skew it phonetically instead: "or" -> "err".
"err" also has the two handy mnemonic connotations:
· That we're handling an error marker (which a returned "undef"
usually is)
· That we're voicing a surprised double-take after something
unexpected (which a returned "undef" often is).
Besides all that, it just seems to work well. That is, something like
this:
my $value = compute_value(@args)
err die "Was expecting a defined value";
reads quite naturally in English (whether you think of "err" as an
abbreviation of "on error...", or as a synonym for "oops...").
Note that "err" is a binary operator, just like "or", and "xor", so
there's no particular need to start it on a new line:
my $value = compute_value(@args) err die "Was expecting a defined value";
In our example program, the "undef" returned by the "<>" operator at
end-of-file is our signal to jump out of the main loop. To accomplish
that we simply append "err last" to the input statement:
my $expr = <> err last;
Note that an "or last" wouldn't work here, as both the empty string and
the string "0" are valid (i.e. non-terminating) inputs to the
calculator.
Just Do It
Then it's just a matter of calling "Calc::calc", passing it the
iteration number and the expression:
Calc::calc(i=>$i, expr=>$expr)
Note that we used named arguments, so passing them in the wrong order
didn't matter.
We then interpolate the result back into the output string using the
"$(...)" scalar interpolator:
print "$i> $( Calc::calc(i=>$i, expr=>$expr) )\n";
We could even simplify that a little further, by taking advatage of the
fact that subroutine calls interpolate directly into strings in Perl 6,
provided we use the "&" prefix:
print "$i> &Calc::calc(i=>$i, expr=>$expr)\n";
Either way, that's it: we're done.
Summing Up
In terms of control structures, Perl 6:
· provides far more support for exceptions and exception handling,
· cleans up and extends the "for" loop syntax in several ways,
· unifies the notions of blocks and closures and makes them
interchangeable,
· provides hooks for attaching various kinds of automatic handlers to
a block/closure,
· re-factors the concept of a switch statement into two far more
general ideas: marking a value/variable as the current topic, and
then doing "smart matching" against that topic.
These extensions and cleanups offer us far more power and control, and
-- amazingly -- in most cases require far less syntax. For example,
here's (almost) the same program, written in Perl 5:
package Err::BadData;
use base 'Exception'; # which you'd have to write yourself
package NoData; # not lexical
use base 'Exception';
sub warn { die @_ }
package Calc;
my %var;
sub get_data {
my $data = shift;
if ($data =~ /^\d+$/) { return $var{""} = $data }
elsif ($data eq 'previous') { return defined $var{""}
? $var{""}
: die NoData->new()
}
elsif ($var{$data}) { return $var{""} = $var{$data} }
else { die Err::BadData->new(
{msg=>"Don't understand $data"}
)
}
}
sub calc {
my %data = @_;
my ($i, $expr) = @data{'i', 'expr'};
my %operator = (
'*' => sub { $_[0] * $_[1] },
'/' => sub { $_[0] / $_[1] },
'~' => sub { ($_[0] + $_[1]) / 2 },
);
my @stack;
my $toknum = 1;
LOOP: for my $token (split /\s+/, $expr) {
defined eval {
TRY: if ($operator{$token}) {
my @args = splice @stack, -2;
push @stack, $operator{$token}->(@args);
last TRY;
}
last LOOP if $token eq '.' || $token eq ';' || $token eq '=';
push @stack, get_data($token);
} || do {
if ($@->isa(Err::Reportable)) { warn $@; }
if ($@->isa(Err::BadData)) { $@->{at} = $i; die $@ }
elsif ($@->isa(NoData)) { push @stack, 0 }
elsif ($@ =~ /division by zero/) { push @stack, ~0 }
}
}
continue { $toknum++ }
die Err::BadData->new(msg=>"Too many operands") if @stack > 1;
$var{'$'.$i} = $stack[-1] . ' but true';
return 0+pop(@stack);
}
package main;
for (my $i=1; 1; $i++) {
print "$i> ";
defined( my $expr = <> ) or last;
print "$i> ${\Calc::calc(i=>$i, expr=>$expr)}\n";
}
Hmmmmmmm. I know which version I'd rather maintain.
perl v5.14.0 2006-02-28 Perl6::Bible::E04(3)
