=head1 NAME
+X<syntax>
perlsyn - Perl syntax
=head1 DESCRIPTION
-A Perl script consists of a sequence of declarations and statements.
-The only things that need to be declared in Perl are report formats
-and subroutines. See the sections below for more information on those
-declarations. All uninitialized user-created objects are assumed to
-start with a C<null> or C<0> value until they are defined by some explicit
-operation such as assignment. (Though you can get warnings about the
-use of undefined values if you like.) The sequence of statements is
-executed just once, unlike in B<sed> and B<awk> scripts, where the
-sequence of statements is executed for each input line. While this means
-that you must explicitly loop over the lines of your input file (or
-files), it also means you have much more control over which files and
-which lines you look at. (Actually, I'm lying--it is possible to do an
-implicit loop with either the B<-n> or B<-p> switch. It's just not the
-mandatory default like it is in B<sed> and B<awk>.)
+A Perl program consists of a sequence of declarations and statements
+which run from the top to the bottom. Loops, subroutines and other
+control structures allow you to jump around within the code.
+
+Perl is a B<free-form> language, you can format and indent it however
+you like. Whitespace mostly serves to separate tokens, unlike
+languages like Python where it is an important part of the syntax.
+
+Many of Perl's syntactic elements are B<optional>. Rather than
+requiring you to put parentheses around every function call and
+declare every variable, you can often leave such explicit elements off
+and Perl will figure out what you meant. This is known as B<Do What I
+Mean>, abbreviated B<DWIM>. It allows programmers to be B<lazy> and to
+code in a style with which they are comfortable.
+
+Perl B<borrows syntax> and concepts from many languages: awk, sed, C,
+Bourne Shell, Smalltalk, Lisp and even English. Other
+languages have borrowed syntax from Perl, particularly its regular
+expression extensions. So if you have programmed in another language
+you will see familiar pieces in Perl. They often work the same, but
+see L<perltrap> for information about how they differ.
=head2 Declarations
+X<declaration> X<undef> X<undefined> X<uninitialized>
+
+The only things you need to declare in Perl are report formats and
+subroutines (and sometimes not even subroutines). A variable holds
+the undefined value (C<undef>) until it has been assigned a defined
+value, which is anything other than C<undef>. When used as a number,
+C<undef> is treated as C<0>; when used as a string, it is treated as
+the empty string, C<"">; and when used as a reference that isn't being
+assigned to, it is treated as an error. If you enable warnings,
+you'll be notified of an uninitialized value whenever you treat
+C<undef> as a string or a number. Well, usually. Boolean contexts,
+such as:
+
+ my $a;
+ if ($a) {}
+
+are exempt from warnings (because they care about truth rather than
+definedness). Operators such as C<++>, C<-->, C<+=>,
+C<-=>, and C<.=>, that operate on undefined left values such as:
+
+ my $a;
+ $a++;
-Perl is, for the most part, a free-form language. (The only exception
-to this is format declarations, for obvious reasons.) Text from a
-C<"#"> character until the end of the line is a comment, and is
-ignored. If you attempt to use C</* */> C-style comments, it will be
-interpreted either as division or pattern matching, depending on the
-context, and C++ C<//> comments just look like a null regular
-expression, so don't do that.
+are also always exempt from such warnings.
A declaration can be put anywhere a statement can, but has no effect on
the execution of the primary sequence of statements--declarations all
take effect at compile time. Typically all the declarations are put at
the beginning or the end of the script. However, if you're using
-lexically-scoped private variables created with C<my()>, you'll have to make sure
+lexically-scoped private variables created with C<my()>, you'll
+have to make sure
your format or subroutine definition is within the same block scope
as the my if you expect to be able to access those private variables.
Declaring a subroutine allows a subroutine name to be used as if it were a
list operator from that point forward in the program. You can declare a
subroutine without defining it by saying C<sub name>, thus:
+X<subroutine, declaration>
sub myname;
$me = myname $0 or die "can't get myname";
-Note that my() functions as a list operator, not as a unary operator; so
-be careful to use C<or> instead of C<||> in this case. However, if
+Note that myname() functions as a list operator, not as a unary operator;
+so be careful to use C<or> instead of C<||> in this case. However, if
you were to declare the subroutine as C<sub myname ($)>, then
C<myname> would function as a unary operator, so either C<or> or
C<||> would work.
statements as if it were an ordinary statement. That means it actually
has both compile-time and run-time effects.
-=head2 Simple statements
+=head2 Comments
+X<comment> X<#>
+
+Text from a C<"#"> character until the end of the line is a comment,
+and is ignored. Exceptions include C<"#"> inside a string or regular
+expression.
+
+=head2 Simple Statements
+X<statement> X<semicolon> X<expression> X<;>
The only kind of simple statement is an expression evaluated for its
side effects. Every simple statement must be terminated with a
semicolon, unless it is the final statement in a block, in which case
-the semicolon is optional. (A semicolon is still encouraged there if the
-block takes up more than one line, because you may eventually add another line.)
-Note that there are some operators like C<eval {}> and C<do {}> that look
-like compound statements, but aren't (they're just TERMs in an expression),
-and thus need an explicit termination if used as the last item in a statement.
+the semicolon is optional. (A semicolon is still encouraged if the
+block takes up more than one line, because you may eventually add
+another line.) Note that there are some operators like C<eval {}> and
+C<do {}> that look like compound statements, but aren't (they're just
+TERMs in an expression), and thus need an explicit termination if used
+as the last item in a statement.
+
+=head2 Truth and Falsehood
+X<truth> X<falsehood> X<true> X<false> X<!> X<not> X<negation> X<0>
+
+The number 0, the strings C<'0'> and C<''>, the empty list C<()>, and
+C<undef> are all false in a boolean context. All other values are true.
+Negation of a true value by C<!> or C<not> returns a special false value.
+When evaluated as a string it is treated as C<''>, but as a number, it
+is treated as 0.
+
+=head2 Statement Modifiers
+X<statement modifier> X<modifier> X<if> X<unless> X<while>
+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
- foreach EXPR
+ when EXPR
+ for LIST
+ foreach LIST
+
+The C<EXPR> following the modifier is referred to as the "condition".
+Its truth or falsehood determines how the modifier will behave.
+
+C<if> executes the statement once I<if> and only if the condition is
+true. C<unless> is the opposite, it executes the statement I<unless>
+the condition is true (i.e., if the condition is false).
+
+ 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">.
-The C<if> and C<unless> modifiers have the expected semantics,
-presuming you're a speaker of English. The C<foreach> modifier is an
-iterator: For each value in EXPR, it aliases C<$_> to the value and
-executes the statement. The C<while> and C<until> modifiers have the
-usual "C<while> loop" semantics (conditional evaluated first), except
-when applied to a C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE
-statement), in which case the block executes once before the
-conditional is evaluated. This is so that you can write loops like:
+ given ($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).
+
+ print "Hello $_!\n" foreach qw(world Dolly nurse);
+
+C<while> repeats the statement I<while> the condition is true.
+C<until> does the opposite, it repeats the statement I<until> the
+condition is true (or while the condition is false):
+
+ # Both of these count from 0 to 10.
+ print $i++ while $i <= 10;
+ print $j++ until $j > 10;
+
+The C<while> and C<until> modifiers have the usual "C<while> loop"
+semantics (conditional evaluated first), except when applied to a
+C<do>-BLOCK (or to the deprecated C<do>-SUBROUTINE statement), in
+which case the block executes once before the conditional is
+evaluated. This is so that you can write loops like:
do {
$line = <STDIN>;
loop labels. Sorry. You can always put another block inside of it
(for C<next>) or around it (for C<last>) to do that sort of thing.
For C<next>, just double the braces:
+X<next> X<last> X<redo>
do {{
next if $x == $y;
}} until $x++ > $z;
For C<last>, you have to be more elaborate:
+X<last>
LOOP: {
do {
} while $x++ <= $z;
}
-=head2 Compound statements
+B<NOTE:> The behaviour of a C<my> statement modified with a statement
+modifier conditional or loop construct (e.g. C<my $x if ...>) is
+B<undefined>. The value of the C<my> variable may be C<undef>, any
+previously assigned value, or possibly anything else. Don't rely on
+it. Future versions of perl might do something different from the
+version of perl you try it out on. Here be dragons.
+X<my>
+
+=head2 Compound Statements
+X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace>
+X<{> X<}> X<if> X<unless> X<while> X<until> X<foreach> X<for> X<continue>
In Perl, a sequence of statements that defines a scope is called a block.
Sometimes a block is delimited by the file containing it (in the case
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
+ LABEL until (EXPR) BLOCK continue BLOCK
LABEL for (EXPR; EXPR; EXPR) BLOCK
LABEL foreach VAR (LIST) BLOCK
LABEL foreach VAR (LIST) BLOCK continue 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
by a colon. The LABEL identifies the loop for the loop control
statements C<next>, C<last>, and C<redo>.
If the LABEL is omitted, the loop control statement
refers to the innermost enclosing loop. This may include dynamically
looking back your call-stack at run time to find the LABEL. Such
-desperate behavior triggers a warning if you use the B<-w> flag.
+desperate behavior triggers a warning if you use the C<use warnings>
+pragma or the B<-w> flag.
If there is a C<continue> BLOCK, it is always executed just before the
-conditional is about to be evaluated again, just like the third part of a
-C<for> loop in C. Thus it can be used to increment a loop variable, even
-when the loop has been continued via the C<next> statement (which is
-similar to the C C<continue> statement).
+conditional is about to be evaluated again. Thus it can be used to
+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>
-The C<next> command is like the C<continue> statement in C; it starts
-the next iteration of the loop:
+The C<next> command starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
-The C<last> command is like the C<break> statement in C (as used in
-loops); it immediately exits the loop in question. The
+The C<last> command immediately exits the loop in question. The
C<continue> block, if any, is not executed:
LINE: while (<STDIN>) {
# now process $line
}
-Note that if there were a C<continue> block on the above code, it would get
-executed even on discarded lines. This is often used to reset line counters
-or C<?pat?> one-time matches.
+Note that if there were a C<continue> block on the above code, it would
+get executed only on lines discarded by the regex (since redo skips the
+continue block). A continue block is often used to reset line counters
+or C<?pat?> one-time matches:
# inspired by :1,$g/fred/s//WILMA/
while (<>) {
they aren't loops. You can double the braces to make them such, though.
if (/pattern/) {{
- next if /fred/;
- next if /barney/;
- # so something here
+ last if /fred/;
+ next if /barney/; # same effect as "last", but doesn't document as well
+ # do something here
}}
+This is caused by the fact that a block by itself acts as a loop that
+executes once, see L<"Basic BLOCKs">.
+
The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
=head2 For Loops
+X<for> X<foreach>
-Perl's C-style C<for> loop works exactly like the corresponding C<while> loop;
+Perl's C-style C<for> loop works like the corresponding C<while> loop;
that means that this:
for ($i = 1; $i < 10; $i++) {
$i++;
}
-(There is one minor difference: The first form implies a lexical scope
-for variables declared with C<my> in the initialization expression.)
+There is one minor difference: if variables are declared with C<my>
+in the initialization section of the C<for>, the lexical scope of
+those variables is exactly the C<for> loop (the body of the loop
+and the control sections).
+X<my>
Besides the normal array index looping, C<for> can lend itself
to many other interesting applications. Here's one that avoids the
problem you get into if you explicitly test for end-of-file on
an interactive file descriptor causing your program to appear to
hang.
+X<eof> X<end-of-file> X<end of file>
$on_a_tty = -t STDIN && -t STDOUT;
sub prompt { print "yes? " if $on_a_tty }
# do something
}
+Using C<readline> (or the operator form, C<< <EXPR> >>) as the
+conditional of a C<for> loop is shorthand for the following. This
+behaviour is the same as a C<while> loop conditional.
+X<readline> X<< <> >>
+
+ for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
+ # do something
+ }
+
=head2 Foreach Loops
+X<for> X<foreach>
The C<foreach> loop iterates over a normal list value and sets the
variable VAR to be each element of the list in turn. If the variable
implicitly local to the loop and regains its former value upon exiting
the loop. If the variable was previously declared with C<my>, it uses
that variable instead of the global one, but it's still localized to
-the loop.
+the loop. This implicit localisation occurs I<only> in a C<foreach>
+loop.
+X<my> X<local>
The C<foreach> keyword is actually a synonym for the C<for> keyword, so
you can use C<foreach> for readability or C<for> for brevity. (Or because
the Bourne shell is more familiar to you than I<csh>, so writing C<for>
comes more naturally.) If VAR is omitted, C<$_> is set to each value.
-If any element of LIST is an lvalue, you can modify it by modifying VAR
-inside the loop. That's because the C<foreach> loop index variable is
-an implicit alias for each item in the list that you're looping over.
+X<$_>
+
+If any element of LIST is an lvalue, you can modify it by modifying
+VAR inside the loop. Conversely, if any element of LIST is NOT an
+lvalue, any attempt to modify that element will fail. In other words,
+the C<foreach> loop index variable is an implicit alias for each item
+in the list that you're looping over.
+X<alias>
If any part of LIST is an array, C<foreach> will get very confused if
you add or remove elements within the loop body, for example with
C<splice>. So don't do that.
+X<splice>
C<foreach> probably won't do what you expect if VAR is a tied or other
special variable. Don't do that either.
for (@ary) { s/foo/bar/ }
- foreach my $elem (@elements) {
+ for my $elem (@elements) {
$elem *= 2;
}
Whereas here's how a Perl programmer more comfortable with the idiom might
do it:
- OUTER: foreach my $wid (@ary1) {
- INNER: foreach my $jet (@ary2) {
+ OUTER: for my $wid (@ary1) {
+ INNER: for my $jet (@ary2) {
next OUTER if $wid > $jet;
$wid += $jet;
}
Perl executes a C<foreach> statement more rapidly than it would the
equivalent C<for> loop.
-=head2 Basic BLOCKs and Switch Statements
+=head2 Basic BLOCKs
+X<block>
A BLOCK by itself (labeled or not) is semantically equivalent to a
loop that executes once. Thus you can use any of the loop control
C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
block is optional.
-The BLOCK construct is particularly nice for doing case
-structures.
+The BLOCK construct can be used to emulate case structures.
SWITCH: {
if (/^abc/) { $abc = 1; last SWITCH; }
$nothing = 1;
}
-There is no official C<switch> statement in Perl, because there are
-already several ways to write the equivalent. In addition to the
-above, you could write
+Such constructs are quite frequently used, because older versions
+of Perl had no official C<switch> statement.
- SWITCH: {
- $abc = 1, last SWITCH if /^abc/;
- $def = 1, last SWITCH if /^def/;
- $xyz = 1, last SWITCH if /^xyz/;
- $nothing = 1;
- }
+=head2 Switch statements
+X<switch> X<case> X<given> X<when> X<default>
-(That's actually not as strange as it looks once you realize that you can
-use loop control "operators" within an expression, That's just the normal
-C comma operator.)
+Starting from Perl 5.10, you can say
-or
+ use feature "switch";
- SWITCH: {
- /^abc/ && do { $abc = 1; last SWITCH; };
- /^def/ && do { $def = 1; last SWITCH; };
- /^xyz/ && do { $xyz = 1; last SWITCH; };
- $nothing = 1;
+which enables a switch feature that is closely based on the
+Perl 6 proposal.
+
+The keywords C<given> and C<when> are analogous
+to C<switch> and C<case> in other languages, so the code
+above could be written as
+
+ given($_) {
+ when (/^abc/) { $abc = 1; }
+ when (/^def/) { $def = 1; }
+ when (/^xyz/) { $xyz = 1; }
+ default { $nothing = 1; }
}
-or formatted so it stands out more as a "proper" C<switch> statement:
+This construct is very flexible and powerful. For example:
- SWITCH: {
- /^abc/ && do {
- $abc = 1;
- last SWITCH;
- };
-
- /^def/ && do {
- $def = 1;
- last SWITCH;
- };
-
- /^xyz/ && do {
- $xyz = 1;
- last SWITCH;
- };
- $nothing = 1;
+ use feature ":5.10";
+ given($foo) {
+ 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 'a complicated check for $foo is true';
+ }
+ default {
+ die q(I don't know what to do with $foo);
+ }
}
-or
+C<given(EXPR)> will assign the value of EXPR to C<$_>
+within the lexical scope of the block, so it's similar to
- SWITCH: {
- /^abc/ and $abc = 1, last SWITCH;
- /^def/ and $def = 1, last SWITCH;
- /^xyz/ and $xyz = 1, last SWITCH;
- $nothing = 1;
+ do { my $_ = EXPR; ... }
+
+except that the block is automatically broken out of by a
+successful C<when> or an explicit C<break>.
+
+Most of the power comes from implicit smart matching:
+
+ when($foo)
+
+is exactly equivalent to
+
+ when($_ ~~ $foo)
+
+Most of the time, C<when(EXPR)> is treated as an implicit smart match of
+C<$_>, i.e. C<$_ ~~ EXPR>. (See L</"Smart matching in detail"> for more
+information on smart matching.) But when EXPR is one of the below
+exceptional cases, it is used directly as a boolean:
+
+=over 4
+
+=item *
+
+a subroutine or method call
+
+=item *
+
+a regular expression match, i.e. C</REGEX/> or C<$foo =~ /REGEX/>,
+or a negated regular expression match (C<!/REGEX/> or C<$foo !~ /REGEX/>).
+
+=item *
+
+a comparison such as C<$_ E<lt> 10> or C<$x eq "abc">
+(or of course C<$_ ~~ $c>)
+
+=item *
+
+C<defined(...)>, C<exists(...)>, or C<eof(...)>
+
+=item *
+
+a negated expression C<!(...)> or C<not (...)>, or a logical
+exclusive-or C<(...) xor (...)>.
+
+=item *
+
+a filetest operator, with the exception of C<-s>, C<-M>, C<-A>, and C<-C>,
+that return numerical values, not boolean ones.
+
+=item *
+
+the C<..> and C<...> flip-flop operators.
+
+=back
+
+In those cases the value of EXPR is used directly as a boolean.
+
+Furthermore:
+
+=over 4
+
+=item *
+
+If EXPR is C<... && ...> or C<... and ...>, the test
+is applied recursively to both arguments. If I<both>
+arguments pass the test, then the argument is treated
+as boolean.
+
+=item *
+
+If EXPR is C<... || ...>, C<... // ...> or C<... or ...>, the test
+is applied recursively to the first argument.
+
+=back
+
+These rules look complicated, but usually they will do what
+you want. For example you could write:
+
+ when (/^\d+$/ && $_ < 75) { ... }
+
+Another useful shortcut is that, if you use a literal array
+or hash as the argument to C<given>, it is turned into a
+reference. So C<given(@foo)> is the same as C<given(\@foo)>,
+for example.
+
+C<default> behaves exactly like C<when(1 == 1)>, which is
+to say that it always matches.
+
+=head3 Breaking out
+
+You can use the C<break> keyword to break out of the enclosing
+C<given> block. Every C<when> block is implicitly ended with
+a C<break>.
+
+=head3 Fall-through
+
+You can use the C<continue> keyword to fall through from one
+case to the next:
+
+ given($foo) {
+ when (/x/) { say '$foo contains an x'; continue }
+ when (/y/) { say '$foo contains a y' }
+ default { say '$foo does not contain a y' }
}
-or even, horrors,
-
- if (/^abc/)
- { $abc = 1 }
- elsif (/^def/)
- { $def = 1 }
- elsif (/^xyz/)
- { $xyz = 1 }
- else
- { $nothing = 1 }
-
-A common idiom for a C<switch> statement is to use C<foreach>'s aliasing to make
-a temporary assignment to C<$_> for convenient matching:
-
- SWITCH: for ($where) {
- /In Card Names/ && do { push @flags, '-e'; last; };
- /Anywhere/ && do { push @flags, '-h'; last; };
- /In Rulings/ && do { last; };
- die "unknown value for form variable where: `$where'";
- }
+=head3 Return value
-Another interesting approach to a switch statement is arrange
-for a C<do> block to return the proper value:
+When a C<given> statement is also a valid expression (e.g.
+when it's the last statement of a block), it returns :
- $amode = do {
- if ($flag & O_RDONLY) { "r" } # XXX: isn't this 0?
- elsif ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
- elsif ($flag & O_RDWR) {
- if ($flag & O_CREAT) { "w+" }
- else { ($flag & O_APPEND) ? "a+" : "r+" }
- }
- };
-
-Or
-
- print do {
- ($flags & O_WRONLY) ? "write-only" :
- ($flags & O_RDWR) ? "read-write" :
- "read-only";
- };
-
-Or if you are certainly that all the C<&&> clauses are true, you can use
-something like this, which "switches" on the value of the
-C<HTTP_USER_AGENT> envariable.
-
- #!/usr/bin/perl
- # pick out jargon file page based on browser
- $dir = 'http://www.wins.uva.nl/~mes/jargon';
- for ($ENV{HTTP_USER_AGENT}) {
- $page = /Mac/ && 'm/Macintrash.html'
- || /Win(dows )?NT/ && 'e/evilandrude.html'
- || /Win|MSIE|WebTV/ && 'm/MicroslothWindows.html'
- || /Linux/ && 'l/Linux.html'
- || /HP-UX/ && 'h/HP-SUX.html'
- || /SunOS/ && 's/ScumOS.html'
- || 'a/AppendixB.html';
+=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 was true.
+
+=back
+
+Note that, unlike C<if> and C<unless>, both C<when> and C<default> always
+themselves return 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';
+ } };
+
+C<given> blocks can't currently 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.
+For example, here's one way to count how many times a particular
+string occurs in an array:
+
+ my $count = 0;
+ for (@array) {
+ when ("foo") { ++$count }
}
- print "Location: $dir/$page\015\012\015\012";
+ print "\@array contains $count copies of 'foo'\n";
+
+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.
+
+This doesn't work if you explicitly specify a loop variable,
+as in C<for $item (@array)>. You have to use the default
+variable C<$_>. (You can use C<for my $_ (@array)>.)
+
+=head3 Smart matching in detail
+
+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
+ ====== ===== ===================== =============
+ Any undef undefined !defined $a
+
+ Any Object invokes ~~ overloading on $object, or dies
+
+ Hash CodeRef sub truth for each key[1] !grep { !$b->($_) } keys %$a
+ Array CodeRef sub truth for each elt[1] !grep { !$b->($_) } @$a
+ Any CodeRef scalar sub truth $b->($a)
+
+ 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}
+
+ 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/
+
+ 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 $b
+
+ 1 - empty hashes or arrays will match.
+ 2 - that is, each element smart-matches the element of same index in the
+ other array. [3]
+ 3 - If a circular reference is found, we fall back to referential equality.
+ 4 - either a real number, or a string that looks like a number
+
+=head3 Custom matching via overloading
+
+You can change the way that an object is matched by overloading
+the C<~~> operator. This may alter the usual smart match semantics.
+
+It should be noted that C<~~> will refuse to work on objects that
+don't overload it (in order to avoid relying on the object's
+underlying structure).
+
+Note also that smart match's matching rules take precedence over
+overloading, so if C<$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 $foo {
+ ...
+ }
+
+as though the argument to C<given> were an element of the hash
+C<%foo>, interpreting the braces as hash-element syntax.
-That kind of switch statement only works when you know the C<&&> clauses
-will be true. If you don't, the previous C<?:> example should be used.
+The table of smart matches is not identical to that proposed by the
+Perl 6 specification, mainly due to the differences between Perl 6's
+and Perl 5's data models.
-You might also consider writing a hash of subroutine references
-instead of synthesizing a C<switch> statement.
+In Perl 6, C<when()> will always do an implicit smart match
+with its argument, whilst it is convenient in Perl 5 to
+suppress this implicit smart match in certain situations,
+as documented above. (The difference is largely because Perl 5
+does not, even internally, have a boolean type.)
=head2 Goto
+X<goto>
Although not for the faint of heart, Perl does support a C<goto>
statement. There are three forms: C<goto>-LABEL, C<goto>-EXPR, and
dynamically. This allows for computed C<goto>s per FORTRAN, but isn't
necessarily recommended if you're optimizing for maintainability:
- goto ("FOO", "BAR", "GLARCH")[$i];
+ goto(("FOO", "BAR", "GLARCH")[$i]);
The C<goto>-&NAME form is highly magical, and substitutes a call to the
named subroutine for the currently running subroutine. This is used by
C<eval{}> and die() for exception processing can also be a prudent approach.
=head2 PODs: Embedded Documentation
+X<POD> X<documentation>
Perl has a mechanism for intermixing documentation with source code.
While it's expecting the beginning of a new statement, if the compiler
of code.
=head2 Plain Old Comments (Not!)
+X<comment> X<line> X<#> X<preprocessor> X<eval>
-Much like the C preprocessor, Perl can process line directives. Using
+Perl can process line directives, much like the C preprocessor. Using
this, one can control Perl's idea of filenames and line numbers in
error or warning messages (especially for strings that are processed
with C<eval()>). The syntax for this mechanism is the same as for most
C preprocessors: it matches the regular expression
-C</^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/> with C<$1> being the line
-number for the next line, and C<$2> being the optional filename
-(specified within quotes).
+
+ # example: '# line 42 "new_filename.plx"'
+ /^\# \s*
+ line \s+ (\d+) \s*
+ (?:\s("?)([^"]+)\2)? \s*
+ $/x
+
+with C<$1> being the line number for the next line, and C<$3> being
+the optional filename (specified with or without quotes).
+
+There is a fairly obvious gotcha included with the line directive:
+Debuggers and profilers will only show the last source line to appear
+at a particular line number in a given file. Care should be taken not
+to cause line number collisions in code you'd like to debug later.
Here are some examples that you should be able to type into your command
shell:
https://perl5.git.perl.org / perl5.git / blobdiff