=head2 Statement Modifiers
X<statement modifier> X<modifier> X<if> X<unless> X<while>
-X<until> X<foreach> X<for>
+X<until> X<when> X<foreach> X<for>
Any simple statement may optionally be followed by a I<SINGLE> modifier,
just before the terminating semicolon (or block ending). The possible
unless EXPR
while EXPR
until EXPR
+ when EXPR
+ for LIST
foreach LIST
The C<EXPR> following the modifier is referred to as the "condition".
print "Basset hounds got long ears" if length $ear >= 10;
go_outside() and play() unless $is_raining;
+C<when> executes the statement I<when> C<$_> smart matches C<EXPR>, and
+then either C<break>s out if it's enclosed in a C<given> scope or skips
+to the C<next> element when it lies directly inside a C<for> loop.
+See also L</"Switch statements">.
+
+ given ($something) {
+ $abc = 1 when /^abc/;
+ $just_a = 1 when /^a/;
+ $other = 1;
+ }
+
+ for (@names) {
+ admin($_) when [ qw/Alice Bob/ ];
+ regular($_) when [ qw/Chris David Ellen/ ];
+ }
+
The C<foreach> modifier is an iterator: it executes the statement once
for each item in the LIST (with C<$_> aliased to each item in turn).
if (EXPR) BLOCK
if (EXPR) BLOCK else BLOCK
if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
+ unless (EXPR) BLOCK
+ unless (EXPR) BLOCK else BLOCK
+ unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
LABEL while (EXPR) BLOCK
LABEL while (EXPR) BLOCK continue BLOCK
LABEL until (EXPR) BLOCK
The C<if> statement is straightforward. Because BLOCKs are always
bounded by curly brackets, there is never any ambiguity about which
C<if> an C<else> goes with. If you use C<unless> in place of C<if>,
-the sense of the test is reversed.
+the sense of the test is reversed. Like C<if>, C<unless> can be followed
+by C<else>. C<unless> can even be followed by one or more C<elsif>
+statements, though you may want to think twice before using that particular
+language construct, as everyone reading your code will have to think at least
+twice before they can understand what's going on.
The C<while> statement executes the block as long as the expression is
-true (does not evaluate to the null string C<""> or C<0> or C<"0">).
+L<true|/"Truth and Falsehood">.
The C<until> statement executes the block as long as the expression is
false.
The LABEL is optional, and if present, consists of an identifier followed
increment a loop variable, even when the loop has been continued via
the C<next> statement.
+Extension modules can also hook into the Perl parser to define new
+kinds of compound statement. These are introduced by a keyword which
+the extension recognises, and the syntax following the keyword is
+defined entirely by the extension. If you are an implementor, see
+L<perlapi/PL_keyword_plugin> for the mechanism. If you are using such
+a module, see the module's documentation for details of the syntax that
+it defines.
+
=head2 Loop Control
X<loop control> X<loop, control> X<next> X<last> X<redo> X<continue>
when (undef) {
say '$foo is undefined';
}
-
when ("foo") {
say '$foo is the string "foo"';
}
-
when ([1,3,5,7,9]) {
say '$foo is an odd digit';
continue; # Fall through
- }
-
+ }
when ($_ < 100) {
say '$foo is numerically less than 100';
}
-
when (\&complicated_check) {
- say 'complicated_check($foo) is true';
+ say 'a complicated check for $foo is true';
}
-
default {
die q(I don't know what to do with $foo);
}
when($_ ~~ $foo)
-(though you need to enable the "~~" feature before you
-can use the C<~~> operator directly). In fact C<when(EXPR)>
-is treated as an implicit smart match most of the time. The
-exceptions are that when EXPR is:
+Most of the time, C<when(EXPR)> is treated as an implicit smart match of
+C<$_>, i.e. C<$_ ~~ EXPR>. (See L</"Smart matching in detail"> for more
+information on smart matching.) But when EXPR is one of the below
+exceptional cases, it is used directly as a boolean:
=over 4
-=item o
+=item *
a subroutine or method call
-=item o
+=item *
a regular expression match, i.e. C</REGEX/> or C<$foo =~ /REGEX/>,
-or a negated regular expression match C<$foo !~ /REGEX/>.
+or a negated regular expression match (C<!/REGEX/> or C<$foo !~ /REGEX/>).
-=item o
+=item *
a comparison such as C<$_ E<lt> 10> or C<$x eq "abc">
(or of course C<$_ ~~ $c>)
-=item o
+=item *
C<defined(...)>, C<exists(...)>, or C<eof(...)>
-=item o
+=item *
-A negated expression C<!(...)> or C<not (...)>, or a logical
+a negated expression C<!(...)> or C<not (...)>, or a logical
exclusive-or C<(...) xor (...)>.
+=item *
+
+a filetest operator, with the exception of C<-s>, C<-M>, C<-A>, and C<-C>,
+that return numerical values, not boolean ones.
+
+=item *
+
+the C<..> and C<...> flip-flop operators.
+
=back
-then the value of EXPR is used directly as a boolean.
+In those cases the value of EXPR is used directly as a boolean.
+
Furthermore:
=over 4
-=item o
+=item *
If EXPR is C<... && ...> or C<... and ...>, the test
is applied recursively to both arguments. If I<both>
arguments pass the test, then the argument is treated
as boolean.
-=item o
+=item *
-If EXPR is C<... || ...> or C<... or ...>, the test
+If EXPR is C<... || ...>, C<... // ...> or C<... or ...>, the test
is applied recursively to the first argument.
=back
These rules look complicated, but usually they will do what
you want. For example you could write:
- when (/^\d$/ && $_ < 75) { ... }
+ when (/^\d+$/ && $_ < 75) { ... }
Another useful shortcut is that, if you use a literal array
-or hash as the argument to C<when>, it is turned into a
+or hash as the argument to C<given>, it is turned into a
reference. So C<given(@foo)> is the same as C<given(\@foo)>,
for example.
C<default> behaves exactly like C<when(1 == 1)>, which is
to say that it always matches.
-See L</"Smart matching in detail"> for more information
-on smart matching.
-
=head3 Breaking out
You can use the C<break> keyword to break out of the enclosing
given($foo) {
when (/x/) { say '$foo contains an x'; continue }
when (/y/) { say '$foo contains a y' }
- default { say '$foo contains neither an x nor a y' }
+ default { say '$foo does not contain a y' }
}
+=head3 Return value
+
+When a C<given> statement is also a valid expression (e.g.
+when it's the last statement of a block), it evaluates to :
+
+=over 4
+
+=item *
+
+an empty list as soon as an explicit C<break> is encountered.
+
+=item *
+
+the value of the last evaluated expression of the successful
+C<when>/C<default> clause, if there's one.
+
+=item *
+
+the value of the last evaluated expression of the C<given> block if no
+condition is true.
+
+=back
+
+In both last cases, the last expression is evaluated in the context that
+was applied to the C<given> block.
+
+Note that, unlike C<if> and C<unless>, failed C<when> statements always
+evaluate to an empty list.
+
+ my $price = do { given ($item) {
+ when ([ 'pear', 'apple' ]) { 1 }
+ break when 'vote'; # My vote cannot be bought
+ 1e10 when /Mona Lisa/;
+ 'unknown';
+ } };
+
+Currently, C<given> blocks can't always be used as proper expressions. This
+may be addressed in a future version of perl.
+
=head3 Switching in a loop
Instead of using C<given()>, you can use a C<foreach()> loop.
}
print "\@array contains $count copies of 'foo'\n";
-On exit from the C<when> block, there is an implicit C<next>.
+At the end of all C<when> blocks, there is an implicit C<next>.
You can override that with an explicit C<last> if you're only
interested in the first match.
=head3 Smart matching in detail
-The behaviour of a smart match depends on what type of thing
-its arguments are. It is always commutative, i.e. C<$a ~~ $b>
-behaves the same as C<$b ~~ $a>. The behaviour is determined
-by the following table: the first row that applies, in either
-order, determines the match behaviour.
+The behaviour of a smart match depends on what type of thing its arguments
+are. The behaviour is determined by the following table: the first row
+that applies determines the match behaviour (which is thus mostly
+determined by the type of the right operand). Note that the smart match
+implicitly dereferences any non-blessed hash or array ref, so the "Hash"
+and "Array" entries apply in those cases. (For blessed references, the
+"Object" entries apply.)
+Note that the "Matching Code" column is not always an exact rendition. For
+example, the smart match operator short-circuits whenever possible, but
+C<grep> does not.
$a $b Type of Match Implied Matching Code
====== ===== ===================== =============
- (overloading trumps everything)
+ Any undef undefined !defined $a
- Code[+] Code[+] referential equality $a == $b
- Any Code[+] scalar sub truth $b->($a)
+ Any Object invokes ~~ overloading on $object, or dies
- Hash Hash hash keys identical [sort keys %$a]~~[sort keys %$b]
- Hash Array hash value slice truth grep $_, @$a{@$b}
- Hash Regex hash key grep grep /$b/, keys %$a
- Hash Any hash entry existence exists $a->{$b}
+ Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a
+ Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a
+ Any CodeRef scalar sub truth $b->($a)
- Array Array arrays are identical[*]
- Array Regex array grep grep /$b/, @$a
- Array Num array contains number grep $_ == $b, @$a
- Array Any array contains string grep $_ eq $b, @$a
+ Hash Hash hash keys identical (every key is found in both hashes)
+ Array Hash hash keys intersection grep { exists $b->{$_} } @$a
+ Regex Hash hash key grep grep /$a/, keys %$b
+ undef Hash always false (undef can't be a key)
+ Any Hash hash entry existence exists $b->{$a}
- Any undef undefined !defined $a
+ Hash Array hash keys intersection grep { exists $a->{$_} } @$b
+ Array Array arrays are comparable[2]
+ Regex Array array grep grep /$a/, @$b
+ undef Array array contains undef grep !defined, @$b
+ Any Array match against an array element[3]
+ grep $a ~~ $_, @$b
+
+ Hash Regex hash key grep grep /$b/, keys %$a
+ Array Regex array grep grep /$b/, @$a
Any Regex pattern match $a =~ /$b/
- Code() Code() results are equal $a->() eq $b->()
- Any Code() simple closure truth $b->() # ignoring $a
- Num numish[!] numeric equality $a == $b
- Any Str string equality $a eq $b
- Any Num numeric equality $a == $b
+ Object Any invokes ~~ overloading on $object, or falls back:
+ Any Num numeric equality $a == $b
+ Num numish[4] numeric equality $a == $b
+ undef Any undefined !defined($b)
Any Any string equality $a eq $b
-
- + - this must be a code reference whose prototype (if present) is not ""
- (subs with a "" prototype are dealt with by the 'Code()' entry lower down)
- * - if a circular reference is found, we fall back to referential equality
- ! - either a real number, or a string that looks like a number
-
-The "matching code" doesn't represent the I<real> matching code,
-of course: it's just there to explain the intended meaning. Unlike
-C<grep>, the smart match operator will short-circuit whenever it can.
+ 1 - empty hashes or arrays will match.
+ 2 - that is, each element smart-matches the element of same index in the
+ other array. [3]
+ 3 - If a circular reference is found, we fall back to referential equality.
+ 4 - either a real number, or a string that looks like a number
=head3 Custom matching via overloading
You can change the way that an object is matched by overloading
-the C<~~> operator. This trumps the usual smart match semantics.
+the C<~~> operator. This may alter the usual smart match semantics.
+
+It should be noted that C<~~> will refuse to work on objects that
+don't overload it (in order to avoid relying on the object's
+underlying structure).
+
+Note also that smart match's matching rules take precedence over
+overloading, so if C<$obj> has smart match overloading, then
+
+ $obj ~~ X
+
+will not automatically invoke the overload method with X as an argument;
+instead the table above is consulted as normal, and based in the type of X,
+overloading may or may not be invoked.
+
See L<overload>.
+=head3 Differences from Perl 6
+
+The Perl 5 smart match and C<given>/C<when> constructs are not
+absolutely identical to their Perl 6 analogues. The most visible
+difference is that, in Perl 5, parentheses are required around
+the argument to C<given()> and C<when()> (except when this last
+one is used as a statement modifier). Parentheses in Perl 6
+are always optional in a control construct such as C<if()>,
+C<while()>, or C<when()>; they can't be made optional in Perl
+5 without a great deal of potential confusion, because Perl 5
+would parse the expression
+
+ given $foo {
+ ...
+ }
+
+as though the argument to C<given> were an element of the hash
+C<%foo>, interpreting the braces as hash-element syntax.
+
+The table of smart matches is not identical to that proposed by the
+Perl 6 specification, mainly due to the differences between Perl 6's
+and Perl 5's data models.
+
+In Perl 6, C<when()> will always do an implicit smart match
+with its argument, whilst it is convenient in Perl 5 to
+suppress this implicit smart match in certain situations,
+as documented above. (The difference is largely because Perl 5
+does not, even internally, have a boolean type.)
+
=head2 Goto
X<goto>
# example: '# line 42 "new_filename.plx"'
/^\# \s*
line \s+ (\d+) \s*
- (?:\s("?)([^"]+)\2)? \s*
+ (?:\s("?)([^"]+)\g2)? \s*
$/x
with C<$1> being the line number for the next line, and C<$3> being
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