=head1 Title
Synopsis 4: a Summary of Apocalypse 4
=head1 Author
Larry Wall <[EMAIL PROTECTED]>
=head1 Version
Maintainer:
Date:
Last Modified:
Number: 4
Version: 0
This document summarizes Apocalypse 4, which covers the block and
statement syntax of Perl.
=head1 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
C<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 $^b } # placeholder arguments
A bare closure without placeholder arguments that uses C<$_>
(either explicitly or implicitly) is treated as though C<$_> were a
placeholder argument:
$func = { print if $_ };
$func("printme");
In any case, all formal parameters are the equivalent of C<my> variables
within the block. See S6 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 = $x;
will no longer see the value of the outer C<$x>; you'll need to say
my $x = $OUTER::x;
instead. (It's illegal to declare C<$x> twice in the same scope.)
As in Perl 5, "C<our $foo>" introduces a lexically scoped alias for
a variable in the current package.
There is a new C<state> declarator that introduces a lexically scoped
variable like C<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 "C<local>" function has been renamed to C<temp> to better
reflect what it does. There is also a C<let> function that sets a
hypothetical value. It works exactly like C<temp>, except that the
value will be restored only if the current block exits unsuccessfully.
(See Definition of Success below for more.)
=head1 Conditional statements
The C<if> and C<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 C<elsunless> in Perl 6--except that it's spelled C<elsif not>.
C<:-)>
=head1 Loop statements
The C<while> and C<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 C<do> block is specifically
disallowed. Instead of
do {
...
} while $x;
you must write
loop {
...
last unless $x;
}
Loop modifiers C<next>, C<last>, and C<redo> work as in Perl 5.
There is no longer a C<continue> block. Instead, use a C<NEXT> block
within the loop. See below.
=head1 The general loop statement
The C<loop> statement is the C-style C<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.
=head1 The C<for> statement
There is no C<foreach> statement any more. It's always spelled C<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 either the zip function:
for zip(@a,@b) -> $a, $b { print "[$a, $b]\n" }
or the "zipper" operator to interleave them:
for @a ¥ @b ¥ @c -> $a, $b, $c { print "[$a, $b, $c]\n" }
The list is evaluated lazily by default, so instead of using a C<while>
to read a file a line at a time:
while my $line = <$*IN> {...}
you should use a C<for> instead:
for <$*IN> -> $line {...}
This has the added benefit of limiting the scope of the C<$line>
parameter to the block it's bound to. (The C<while>'s declaration of
C<$line> continues to be visible past the end of the block. Remember,
there are no implicit block scopes in Perl 6.) It is possible to write
while <$*IN> -> $line {...}
but you have to be careful that the object being evaluated for truth
can also behave as a line after behaving as a boolean. If you write:
while ?<$*IN> -> $line {...}
then C<$line> will only ever contain 0 or 1, because that's what ? returns.
Note also that Perl 5's special rule causing
while (<>) {...}
to automatically assign to C<$_> is not carried over to Perl 6. That's
what
for <> {...}
is for.
Parameters are by default constant within the block. You can
declare a parameter read/write by including the "C<is rw>" trait.
If you rely on C<$_> as the implicit parameter to a block, then
then C<$_> is considered read/write by default. That is,
the construct:
for @foo {...}
is actually short for:
for @foo -> $_ is rw {...}
so you can modify the current list element in that case. However,
any time you specify the arguments, they default to read only.
When used as a statement modifers, C<for> and C<given> use a private
instance of C<$_> for the left side of the statement. The outer C<$_>
can be referred to as C<$OUTER::_>. (And yes, this implies that the
compiler may have to retroactively change the binding of <$_> on the
left side. But it's what people expect of a pronoun like "it".)
=head1 Switch statements
A switch statement is a means of topicalizing, so the switch keyword
is the English topicalizer, C<given>. The keyword for individual
cases is C<when>:
given EXPR {
when EXPR { ... }
when EXPR { ... }
default { ... }
}
The current topic is always aliased to the special variable C<$_>.
The C<given> block is just one way to set the current topic, but a
switch statement can be any block that sets C<$_>, including a C<for>
loop (in which the first loop parameter is the topic) or the body
of a method (in which the object itself is the topic). So switching
behavior is actually caused by the C<when> statements in the block,
not by the nature of the block itself. A C<when> statement implicitly
does a "smart match" between the current topic (C<$_>) and the argument
of the C<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 C<when> statement. Since C<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 C<when>
statements (though it helps the optimizer to have a sequence of
contiguous C<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 C<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 C<when> into ordinary code. But use of a C<default>
block is good documentation.
If you use a C<for> loop with a named parameter, the parameter is
also aliased to C<$_> so that it can function as the topic of any
C<when> statements within the loop. If you use a C<for> statement
with multiple parameters, only the first parameter is aliased to C<$_>
as the topic.
You can explicitly break out of a C<when> block (and its surrounding
switch) early using the C<break> verb. You can explicitly break out
of a C<when> block and go to the next statement by using C<continue>.
(Note that, unlike with C's idea of falling through, subsequent C<when>
conditions are evaluated.)
If you have a switch that is the main block of a C<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 C<last> to break
out of the entire loop early. Of course, an explicit C<next> would
be clearer than a C<break> in that case.
=head1 Exception handlers
Unlike many other languages, Perl 6 specifies exception handlers by
placing a C<CATCH> block I<within> that block that is having its exceptions
handled.
The Perl 6 equivalent to Perl 5's C<eval {...}> is C<try {...}>.
(Perl 6's C<eval> function only evaluates strings, not blocks.)
A C<try> block by default has a C<CATCH> block that handles all
exceptions by ignoring them. If you define a C<CATCH> block within
the C<try>, it replaces the default C<CATCH>. It also makes the C<try>
keyword redundant, because any block can function as a C<try> block
if you put a C<CATCH> block within it.
An exception handler is just a switch statement on an implicit topic
supplied within the C<CATCH> block. That implicit topic is the current
exception object, also known as C<$!>. Inside the C<CATCH> block, it's
also bound to C<$_>, since it's the topic. Because of smart matching,
ordinary C<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 C<CATCH> handles the exception, the exception is rethrown.
To ignore all unhandled exceptions, use an empty C<default> case.
(In other words, there is an implicit C<die $!> just inside the end
of the C<CATCH> block. Handled exceptions break out past this implicit
rethrow.)
=head1 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 C<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 C<next>
are abnormal, but looping because you hit the end of the block is not.
The implicit break of a C<when> block is abnormal.
A C<CATCH> block handles only "bad" exceptions, and lets control
exceptions pass unhindered. Control exceptions may be caught with a
C<CONTROL> block. Generally you don't need to worry about this unless
you're defining a control construct. You may have one C<CATCH> block
and one C<CONTROL> block, since some user-defined constructs may wish to
supply an implicit C<CONTROL> block to your closure, but let you define
your own C<CATCH> block.
A C<return> always exits from the lexically surrounding sub
or method definition (that is, from a function officially declared
with the C<sub>, C<method>, or C<submethod> keywords). Pointy subs
and bare closures are transparent to C<return>. If you pass a reference
to a closure outside of its official "sub" scope, it is illegal to
return from it.
To return a value from a pointy sub or bare closure, you either
just mention the value last that you want to return, or you can
use C<leave>. A C<leave> by default exits from the innermost block.
But you may change the behavior of C<leave> with selector adverbs:
leave :from(Loop) :label«LINE» <== 1,2,3;
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;
=head1 Exceptions
As in Perl 5, many built-in functions simply return undef when you ask for
a value out of range. 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 C<die $!> will turn an unthrown
exception into a thrown exception. (A bare C<die> does the same.)
You can cause built-ins to automatically throw exceptions on failure using
use fatal;
The C<fail> function responds to the caller's "use fatal" state. It
either returns an unthrown exception, or throws the exception.
If an exception is raised while C<$!> already contains an exception
that is active and "unclean", no information is discarded. The old
exception is pushed onto the exception stack within the new exception,
which is then bound to C<$!> and, hopefully, propagated. The default
printout for the new exception should include the old exception
information so that the user can trace back to the original error.
(Likewise, rethrown exceptions add information about how the exception
is propagated.)
Exception objects are born "unclean". The C<$!> object keeps track of
whether it's currently "clean" or "unclean". The exception in C<$!> still
exists after it has been caught, but catching it marks it as clean
if any of the cases in the switch matched. Clean exceptions don't
require their information to be preserved if another exception occurs.
=head1 Closure traits
A C<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 C<*> can also be used within an expression:
my $compiletime = BEGIN { localtime };
our $temphandle = FIRST { maketemp() };
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 C<CATCH>, which can only occur once, most of these can occur
multiple times within the block. So they aren't really traits,
exactly--they actually add themselves onto a list stored in the
actual trait. So if you examine the C<ENTER> trait of a block, you'll
find that it's really a list of closures rather than a single closure.
The semantics of C<INIT> and C<FIRST> are not equivalent to each
other in the case of cloned closures. An C<INIT> only runs once for
all copies of a cloned closure. A C<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 C<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 C<$x>, because that's
what C<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 C<PRE>/C<POST> with C<ENTER>/C<LEAVE>.
These are now separate notions. C<ENTER> and C<LEAVE> are used only for
their side effects. C<PRE> and C<POST> must return boolean values that are
evaluated according to the usual Design by Contract rules. (Plus,
if you use C<ENTER>/C<LEAVE> in a class block, they only execute when the
class block is executed, but C<PRE>/C<POST> in a class block are evaluated
around every method in the class.)
C<LEAVE> blocks are evaluated after C<CATCH> and C<CONTROL> blocks, including
the C<LEAVE> variants, C<KEEP> and C<UNDO>. C<POST> blocks are evaluated after
everything else, to guarantee that even C<LEAVE> blocks can't violate DBC.
Likewise C<PRE> blocks fire off before any C<ENTER> or C<FIRST> (though not
before C<BEGIN>, C<CHECK>, or C<INIT>, since those are done at compile or
process initialization time).
=head1 Statement parsing
In this statement:
given EXPR {
when EXPR { ... }
when EXPR { ... }
...
}
the parentheses aren't necessary around C<EXPR> because the whitespace
between C<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 C<hash> list operator will
explicitly evaluate a list and compose a hash of the returned value,
while C<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)
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 C<else>.)
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:if ($expr, &ifblock) {...}
macro statement:while ($expr, &whileblock) {...}
macro statement: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 C<do {...}> or C<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 C<statement:BEGIN> does not preclude us from
also defining a C<prefix:BEGIN> that I<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 C<statement:BEGIN> hides C<prefix:BEGIN> at the start of a statement.
You could also conceivably define a C<prefix:if>, but then you would
get a syntax error when you say:
print if $foo
since C<prefix:if> would hide C<statement_modifier:if>.
=head1 Smart matching
Here is the current table of smart matches (which probably belongs in
S3). The list is intended to reflect forms that can be recognized at
compile time. If none of these forms is recognized at compile time, it
falls through to a multiple dispatch to C<infix:~~()>, which presumably
reflects similar semantics, but can finesse things that aren't exact
type matches. Note that all types are scalarized here. Both C<~~>
and C<given>/C<when> provide scalar contexts to their arguments.
(You can always hyperize C<~~> explicitly, though.) So both C<$_>
and C<$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 »eq« $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 $_ »~~« $x
Array any(list) list intersection match if any(@$_) ~~ 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 $_
Any Any run-time multi call match if infix:~~($_, $x)
Matches marked with * are non-reversible, typically because C<~~> takes
its left side as the topic for the right side, and sets the topic to a
private instance of C<$_> for its right side, so C<$_> means something
different on either side. Such non-reversible constructs can be made
reversible by putting the leading term into a closure to defer the
binding of C<$_>. 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 C<given> and C<when>:
given $x { when .does(Storeable) {...} } # okay
given .does(Storeable) { when $x {...} } # not okay
given { .does(Storeable) } { when $x {...} } # okay
Boolean expressions are those known to return a boolean value, such
as comparisons, or the unary C<?> operator. They may reference C<$_>
explicitly or implicitly. If they don't reference C<$_> 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.
XXX How close can the final run-time multi call get to reproducing
the ordering of the compile-time table, which breaks ties according
to that ordering? Seems to say that multiple dispatch should have a
way to break ties by precedence, not just by "is default". Or maybe
the default routine itself can use precedence of some sort. Seems
like it would be breaking ties that don't even arise in the original
table, however. It's not clear, for instance, whether
%hash ~~ { foo }
would resolve as infix:~~<Hash,Any> or infix:~~<Any,Code> under
multiple dispatch. With the ordered table it clearly prefers the
latter interpretation.
=head1 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 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 C<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
C<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 C<let>. Backtracking over a closure in a rule is also
considered failure, which is how hypothetical variables are managed
by rules. (And on the flip side, use of C<fail> within a rule closure
initiates backtracking of the rule.)
