=head1 NAME
X<syntax>
perlsyn - Perl syntax
=head1 DESCRIPTION
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 serves mostly to separate tokens, unlike
languages like Python where it is an important part of the syntax,
or Fortran where it is immaterial.
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 scalar 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:
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 variables such as:
undef $a;
$a++;
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. All declarations are typically put at
the beginning or the end of the script. However, if you're using
lexically-scoped private variables created with C<my()>,
C<state()>, or C<our()>, 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 $0 or die "can't get myname";
A bare declaration like that declares the function to be a list operator,
not a unary operator, so you have to be careful to use parentheses (or
C<or> instead of C<||>.) The C<||> operator binds too tightly to use after
list operators; it becomes part of the last element. You can always use
parentheses around the list operators arguments to turn the list operator
back into something that behaves more like a function call. Alternatively,
you can use the prototype C<($)> to turn the subroutine into a unary
operator:
sub myname ($);
$me = myname $0 || die "can't get myname";
That now parses as you'd expect, but you still ought to get in the habit of
using parentheses in that situation. For more on prototypes, see
L<perlsub>.
Subroutines declarations can also be loaded up with the C<require> statement
or both loaded and imported into your namespace with a C<use> statement.
See L<perlmod> for details on this.
A statement sequence may contain declarations of lexically-scoped
variables, but apart from declaring a variable name, the declaration acts
like an ordinary statement, and is elaborated within the sequence of
statements as if it were an ordinary statement. That means it actually
has both compile-time and run-time effects.
=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. But put the semicolon in anyway if the
block takes up more than one line, because you may eventually add
another line. Note that there are operators like C<eval {}>, C<sub {}>, and
C<do {}> that I<look> like compound statements, but aren't--they're just
TERMs in an expression--and thus need an explicit termination when used
as the last item in a statement.
=head2 Statement Modifiers
X<statement modifier> X<modifier> X<if> X<unless> X<while>
X<until> X<whereis> X<whereso> 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
modifiers are:
if EXPR
unless EXPR
while EXPR
until EXPR
for LIST
foreach LIST
whereis EXPR
whereso EXPR
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 (that is, if the condition is false).
print "Basset hounds got long ears" if length $ear >= 10;
go_outside() and play() unless $is_raining;
The C<for(each)> modifier is an iterator: it executes the statement once
for each item in the LIST (with C<$_> aliased to each item in turn).
print "Hello $_!\n" for 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 Perl4 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 {
$line = <STDIN>;
...
} until !defined($line) || $line eq ".\n"
See L<perlfunc/do>. Note also that the loop control statements described
later will I<NOT> work in this construct, because modifiers don't take
loop labels. Sorry. You can always put another block inside of it
(for C<next>/C<redo>) or around it (for C<last>) to do that sort of thing.
X<next> X<last> X<redo>
For C<next> or C<redo>, just double the braces:
do {{
next if $x == $y;
# do something here
}} until $x++ > $z;
For C<last>, you have to be more elaborate and put braces around it:
X<last>
{
do {
last if $x == $y**2;
# do something here
} while $x++ <= $z;
}
If you need both C<next> and C<last>, you have to do both and also use a
loop label:
LOOP: {
do {{
next if $x == $y;
last LOOP if $x == $y**2;
# do something here
}} until $x++ > $z;
}
B<NOTE:> The behaviour of a C<my>, C<state>, or
C<our> modified with a statement modifier conditional
or loop construct (for example, C<my $x if ...>) is
B<undefined>. The value of the C<my> variable may be C<undef>, any
previously assigned value, or possibly anything else. Don't rely on
it. Future versions of perl might do something different from the
version of perl you try it out on. Here be dragons.
X<my>
The C<whereis> and C<whereso> modifiers are an experimental feature
that first appeared with this spelling in Perl 5.28. To use them, you
should include a C<use feature 'switch'> declaration, or a declaration
that implies it. They behave like the full C<whereis> or C<whereso>
statement with block, described in L</"Switch Statements"> below.
They executes the statement only if the I<EXPR> is true for C<whereso>,
or C<$_> smartmatches the I<EXPR> for C<whereis>. If the statement
executes, control then implicitly jumps to the end of the dynamically
enclosing loop (usually a C<given> block).
=head2 Compound Statements
X<statement, compound> X<block> X<bracket, curly> X<curly bracket> X<brace>
X<{> X<}> X<if> X<unless> X<given> 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
of a required file, or the program as a whole), and sometimes a block
is delimited by the extent of a string (in the case of an eval).
But generally, a block is delimited by curly brackets, also known as
braces. We will call this syntactic construct a BLOCK. Because enclosing
braces are also the syntax for hash reference constructor expressions
(see L<perlref>), you may occasionally need to disambiguate by placing a
C<;> immediately after an opening brace so that Perl realises the brace
is the start of a block. You will more frequently need to disambiguate
the other way, by placing a C<+> immediately before an opening brace to
force it to be interpreted as a hash reference constructor expression.
It is considered good style to use these disambiguating mechanisms
liberally, not only when Perl would otherwise guess incorrectly.
The following compound statements may be used to control flow:
if (EXPR) BLOCK
if (EXPR) BLOCK else BLOCK
if (EXPR) BLOCK elsif (EXPR) BLOCK ...
if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
unless (EXPR) BLOCK
unless (EXPR) BLOCK else BLOCK
unless (EXPR) BLOCK elsif (EXPR) BLOCK ...
unless (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
given (EXPR) BLOCK
whereis (EXPR) BLOCK
whereso (EXPR) 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 for VAR (LIST) BLOCK
LABEL for VAR (LIST) BLOCK continue BLOCK
LABEL foreach (EXPR; EXPR; EXPR) BLOCK
LABEL foreach VAR (LIST) BLOCK
LABEL foreach VAR (LIST) BLOCK continue BLOCK
LABEL BLOCK
LABEL BLOCK continue BLOCK
PHASE BLOCK
The experimental C<given>, C<whereis>, and C<whereso> statements are I<not
automatically enabled>; see L</"Switch Statements"> below for how to do
so, and the attendant caveats.
Unlike in C and Pascal, in Perl these are all defined in terms of BLOCKs,
not statements. This means that the curly brackets are I<required>--no
dangling statements allowed. If you want to write conditionals without
curly brackets, there are several other ways to do it. The following
all do the same thing:
if (!open(FOO)) { die "Can't open $FOO: $!" }
die "Can't open $FOO: $!" unless open(FOO);
open(FOO) || die "Can't open $FOO: $!";
open(FOO) ? () : die "Can't open $FOO: $!";
# a bit exotic, that last one
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. 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.
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 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. Thus it can be used to
increment a loop variable, even when the loop has been continued via
the C<next> statement.
When a block is preceding by a compilation phase keyword such as C<BEGIN>,
C<END>, C<INIT>, C<CHECK>, or C<UNITCHECK>, then the block will run only
during the corresponding phase of execution. See L<perlmod> for more details.
Extension modules can also hook into the Perl parser to define new
kinds of compound statements. These are introduced by a keyword which
the extension recognizes, 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 starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
...
}
The C<last> command immediately exits the loop in question. The
C<continue> block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
...
}
The C<redo> command restarts the loop block without evaluating the
conditional again. The C<continue> block, if any, is I<not> executed.
This command is normally used by programs that want to lie to themselves
about what was just input.
For example, when processing a file like F</etc/termcap>.
If your input lines might end in backslashes to indicate continuation, you
want to skip ahead and get the next record.
while (<>) {
chomp;
if (s/\\$//) {
$_ .= <>;
redo unless eof();
}
# now process $_
}
which is Perl shorthand for the more explicitly written version:
LINE: while (defined($line = <ARGV>)) {
chomp($line);
if ($line =~ s/\\$//) {
$line .= <ARGV>;
redo LINE unless eof(); # not eof(ARGV)!
}
# now process $line
}
Note that if there were a C<continue> block on the above code, it would
get executed only on lines discarded by the regex (since redo skips the
continue block). A continue block is often used to reset line counters
or C<m?pat?> one-time matches:
# inspired by :1,$g/fred/s//WILMA/
while (<>) {
m?(fred)? && s//WILMA $1 WILMA/;
m?(barney)? && s//BETTY $1 BETTY/;
m?(homer)? && s//MARGE $1 MARGE/;
} continue {
print "$ARGV $.: $_";
close ARGV if eof; # reset $.
reset if eof; # reset ?pat?
}
If the word C<while> is replaced by the word C<until>, the sense of the
test is reversed, but the conditional is still tested before the first
iteration.
Loop control statements don't work in an C<if> or C<unless>, since
they aren't loops. You can double the braces to make them such, though.
if (/pattern/) {{
last if /fred/;
next if /barney/; # same effect as "last",
# but doesn't document as well
# do something here
}}
This is caused by the fact that a block by itself acts as a loop that
executes once, see L</"Basic BLOCKs">.
The form C<while/if BLOCK BLOCK>, available in Perl 4, is no longer
available. Replace any occurrence of C<if BLOCK> by C<if (do BLOCK)>.
=head2 For Loops
X<for> X<foreach>
Perl's C-style C<for> loop works like the corresponding C<while> loop;
that means that this:
for ($i = 1; $i < 10; $i++) {
...
}
is the same as this:
$i = 1;
while ($i < 10) {
...
} continue {
$i++;
}
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>
As a special case, if the test in the C<for> loop (or the corresponding
C<while> loop) is empty, it is treated as true. That is, both
for (;;) {
...
}
and
while () {
...
}
are treated as infinite loops.
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 }
for ( prompt(); <STDIN>; prompt() ) {
# 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( $_ = <STDIN> ); prompt() ) {
# do something
}
=head2 Foreach Loops
X<for> X<foreach>
The C<foreach> loop iterates over a normal list value and sets the scalar
variable VAR to be each element of the list in turn. If the variable
is preceded with the keyword C<my>, then it is lexically scoped, and
is therefore visible only within the loop. Otherwise, the variable is
implicitly local to the loop and regains its former value upon exiting
the loop. If the variable was previously declared with C<my>, it uses
that variable instead of the global one, but it's still localized to
the loop. This implicit localization occurs I<only> in a C<foreach>
loop.
X<my> X<local>
The C<foreach> keyword is actually a synonym for the C<for> keyword, so
you can use either. If VAR is omitted, C<$_> is set to each value.
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.
As of Perl 5.22, there is an experimental variant of this loop that accepts
a variable preceded by a backslash for VAR, in which case the items in the
LIST must be references. The backslashed variable will become an alias
to each referenced item in the LIST, which must be of the correct type.
The variable needn't be a scalar in this case, and the backslash may be
followed by C<my>. To use this form, you must enable the C<refaliasing>
feature via C<use feature>. (See L<feature>. See also L<perlref/Assigning
to References>.)
Examples:
for (@ary) { s/foo/bar/ }
for my $elem (@elements) {
$elem *= 2;
}
for $count (reverse(1..10), "BOOM") {
print $count, "\n";
sleep(1);
}
for (1..15) { print "Merry Christmas\n"; }
foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
print "Item: $item\n";
}
use feature "refaliasing";
no warnings "experimental::refaliasing";
foreach \my %hash (@array_of_hash_references) {
# do something which each %hash
}
Here's how a C programmer might code up a particular algorithm in Perl:
for (my $i = 0; $i < @ary1; $i++) {
for (my $j = 0; $j < @ary2; $j++) {
if ($ary1[$i] > $ary2[$j]) {
last; # can't go to outer :-(
}
$ary1[$i] += $ary2[$j];
}
# this is where that last takes me
}
Whereas here's how a Perl programmer more comfortable with the idiom might
do it:
OUTER: for my $wid (@ary1) {
INNER: for my $jet (@ary2) {
next OUTER if $wid > $jet;
$wid += $jet;
}
}
See how much easier this is? It's cleaner, safer, and faster. It's
cleaner because it's less noisy. It's safer because if code gets added
between the inner and outer loops later on, the new code won't be
accidentally executed. The C<next> explicitly iterates the other loop
rather than merely terminating the inner one. And it's faster because
Perl executes a C<foreach> statement more rapidly than it would the
equivalent C<for> loop.
Perceptive Perl hackers may have noticed that a C<for> loop has a return
value, and that this value can be captured by wrapping the loop in a C<do>
block. The reward for this discovery is this cautionary advice: The
return value of a C<for> loop is unspecified and may change without notice.
Do not rely on it.
=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
statements in it to leave or restart the block. (Note that this is
I<NOT> true in C<eval{}>, C<sub{}>, or contrary to popular belief
C<do{}> blocks, which do I<NOT> count as loops.) The C<continue>
block is optional.
The BLOCK construct can be used to emulate case structures.
SWITCH: {
if (/^abc/) { $abc = 1; last SWITCH; }
if (/^def/) { $def = 1; last SWITCH; }
if (/^xyz/) { $xyz = 1; last SWITCH; }
$nothing = 1;
}
You'll also find the C<foreach> loop used to establish a topic for
a switch:
SWITCH:
for ($var) {
if (/^abc/) { $abc = 1; last SWITCH; }
if (/^def/) { $def = 1; last SWITCH; }
if (/^xyz/) { $xyz = 1; last SWITCH; }
$nothing = 1;
}
Such constructs are quite frequently used, both because older versions of
Perl had no official C<switch> statement, and also because the new version
described immediately below remains experimental.
=head2 Switch Statements
X<switch> X<case> X<given> X<whereis> X<whereso>
C<given>, C<whereso>, and related keywords make up an experimental feature
that first appeared in Perl 5.10, but behaved quite differently from
its present form prior to Perl 5.28. To use it, you should declare
use feature "switch";
You also get the switch feature whenever you declare that your
code prefers to run under a version of Perl that is 5.10 or later. For
example:
use v5.14;
Under the "switch" feature, Perl gains the experimental keywords C<given>,
C<whereis>, and C<whereso>. Starting from Perl 5.16, one can
prefix the switch keywords with C<CORE::> to access the feature without
a C<use feature> statement.
The "switch" feature is considered highly experimental; it is subject
to change with little notice. Uses of the C<given>, C<whereis>, and
C<whereso> keywords will by default warn about their experimental status.
These warnings are in the same category as warnings about the C<~~>
(smartmatch) operator being experimental.
The keywords C<given> and C<whereis> or C<whereso>
are analogous to C<switch> and C<case>
in C. They're meant to be used together, but can actually be used
independently and mixed with other kinds of compound statement.
C<given> evaluates its argument in scalar context, and executes its block
with the C<$_> variable locally aliased to the result of evaluating the
argument expression. It is much like a C<foreach> loop that always has
exactly one item to iterate over.
A C<given> construct even counts as a one-iteration loop for the purposes
of loop control, so the C<redo> operator can be used to restart its block,
and C<next> or C<last> can be used to exit the block early.
C<whereso> evaluates its argument as a truth value. If the argument
was false then it does not execute its block, and proceeds to the
following statement. If the argument was true, it executes the block,
then implicitly performs a C<next>, jumping to the end of the closest
dynamically enclosing C<given> block or other kind of loop.
C<whereis> evaluates its argument and uses it as a smartmatch object,
checking whether C<$_> matches it. If C<$_> did not match then it does
not execute its block, and proceeds to the following statement. If C<$_>
did match, it executes the block, then implicitly performs a C<next>,
jumping to the end of the closest dynamically enclosing C<given> block
or other kind of loop. This is exactly like C<whereso>, except for the
implicit use of smartmatch.
Putting this together, the code in the previous section could be
rewritten as
use v5.10.1;
given ($var) {
whereso (/^abc/) { $abc = 1 }
whereso (/^def/) { $def = 1 }
whereso (/^xyz/) { $xyz = 1 }
$nothing = 1;
}
Or if you prefer the modifier form of C<whereso>, it can be written with
less punctuation as
use v5.14;
given ($var) {
$abc = 1 whereso /^abc/;
$def = 1 whereso /^def/;
$xyz = 1 whereso /^xyz/;
$nothing = 1;
}
You can use the C<continue> keyword to exit a C<whereis> or C<whereso>
block, proceeding to the following statement. This is most commonly
done last thing inside the block, to override the implicit C<next>.
For example
given($foo) {
whereso (/x/) { say '$foo contains an x'; continue }
whereso (/y/) { say '$foo contains a y' }
say '$foo does not contain a y';
}
When a C<given> statement is executed in a position where it will provide
a value, for example when it's the last statement of a subroutine and
so providing the subroutine's return value, it evaluates to:
=over 4
=item *
An empty list as soon as an explicit C<next> or C<last> is encountered.
=item *
The value of the last evaluated expression of the successful
C<whereis> or C<whereso> clause, if there happens to be 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<whereis>/C<whereso> statements always
evaluate to an empty list.
On versions of Perl preceding Perl 5.28, C<given> and the related keywords
behave quite differently from their present behaviour. If your code needs
to run on older versions, avoid C<given>, C<whereis>, and C<whereso>.
=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
C<goto>-&NAME. A loop's LABEL is not actually a valid target for
a C<goto>; it's just the name of the loop.
The C<goto>-LABEL form finds the statement labeled with LABEL and resumes
execution there. It may not be used to go into any construct that
requires initialization, such as a subroutine or a C<foreach> loop. It
also can't be used to go into a construct that is optimized away. It
can be used to go almost anywhere else within the dynamic scope,
including out of subroutines, but it's usually better to use some other
construct such as C<last> or C<die>. The author of Perl has never felt the
need to use this form of C<goto> (in Perl, that is--C is another matter).
The C<goto>-EXPR form expects a label name, whose scope will be resolved
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]);
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<AUTOLOAD()> subroutines that wish to load another subroutine and then
pretend that the other subroutine had been called in the first place
(except that any modifications to C<@_> in the current subroutine are
propagated to the other subroutine.) After the C<goto>, not even C<caller()>
will be able to tell that this routine was called first.
In almost all cases like this, it's usually a far, far better idea to use the
structured control flow mechanisms of C<next>, C<last>, or C<redo> instead of
resorting to a C<goto>. For certain applications, the catch and throw pair of
C<eval{}> and die() for exception processing can also be a prudent approach.
=head2 The Ellipsis Statement
X<...>
X<... statement>
X<ellipsis operator>
X<elliptical statement>
X<unimplemented statement>
X<unimplemented operator>
X<yada-yada>
X<yada-yada operator>
X<... operator>
X<whatever operator>
X<triple-dot operator>
Beginning in Perl 5.12, Perl accepts an ellipsis, "C<...>", as a
placeholder for code that you haven't implemented yet.
When Perl 5.12 or later encounters an ellipsis statement, it parses this
without error, but if and when you should actually try to execute it, Perl
throws an exception with the text C<Unimplemented>:
use v5.12;
sub unimplemented { ... }
eval { unimplemented() };
if ($@ =~ /^Unimplemented at /) {
say "I found an ellipsis!";
}
You can only use the elliptical statement to stand in for a complete
statement. Syntactically, "C<...;>" is a complete statement, but,
as with other kinds of semicolon-terminated statement, the semicolon
may be omitted if "C<...>" appears immediately before a closing brace.
These examples show how the ellipsis works:
use v5.12;
{ ... }
sub foo { ... }
...;
eval { ... };
sub somemeth {
my $self = shift;
...;
}
$x = do {
my $n;
...;
say "Hurrah!";
$n;
};
The elliptical statement cannot stand in for an expression that
is part of a larger statement.
These examples of attempts to use an ellipsis are syntax errors:
use v5.12;
print ...;
open(my $fh, ">", "/dev/passwd") or ...;
if ($condition && ... ) { say "Howdy" };
... if $a > $b;
say "Cromulent" if ...;
$flub = 5 + ...;
There are some cases where Perl can't immediately tell the difference
between an expression and a statement. For instance, the syntax for a
block and an anonymous hash reference constructor look the same unless
there's something in the braces to give Perl a hint. The ellipsis is a
syntax error if Perl doesn't guess that the C<{ ... }> is a block.
Inside your block, you can use a C<;> before the ellipsis to denote that the
C<{ ... }> is a block and not a hash reference constructor.
Note: Some folks colloquially refer to this bit of punctuation as a
"yada-yada" or "triple-dot", but its true name
is actually an ellipsis.
=head2 PODs: Embedded Documentation
X<POD> X<documentation>
Perl has a mechanism for intermixing documentation with source code.
While it's expecting the beginning of a new statement, if the compiler
encounters a line that begins with an equal sign and a word, like this
=head1 Here There Be Pods!
Then that text and all remaining text up through and including a line
beginning with C<=cut> will be ignored. The format of the intervening
text is described in L<perlpod>.
This allows you to intermix your source code
and your documentation text freely, as in
=item snazzle($)
The snazzle() function will behave in the most spectacular
form that you can possibly imagine, not even excepting
cybernetic pyrotechnics.
=cut back to the compiler, nuff of this pod stuff!
sub snazzle($) {
my $thingie = shift;
.........
}
Note that pod translators should look at only paragraphs beginning
with a pod directive (it makes parsing easier), whereas the compiler
actually knows to look for pod escapes even in the middle of a
paragraph. This means that the following secret stuff will be
ignored by both the compiler and the translators.
$a=3;
=secret stuff
warn "Neither POD nor CODE!?"
=cut back
print "got $a\n";
You probably shouldn't rely upon the C<warn()> being podded out forever.
Not all pod translators are well-behaved in this regard, and perhaps
the compiler will become pickier.
One may also use pod directives to quickly comment out a section
of code.
=head2 Plain Old Comments (Not!)
X<comment> X<line> X<#> X<preprocessor> X<eval>
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 almost the same as for
most C preprocessors: it matches the regular expression
# example: '# line 42 "new_filename.plx"'
/^\# \s*
line \s+ (\d+) \s*
(?:\s("?)([^"]+)\g2)? \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). Note that
no whitespace may precede the C<< # >>, unlike modern C preprocessors.
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:
% perl
# line 200 "bzzzt"
# the '#' on the previous line must be the first char on line
die 'foo';
__END__
foo at bzzzt line 201.
% perl
# line 200 "bzzzt"
eval qq[\n#line 2001 ""\ndie 'foo']; print $@;
__END__
foo at - line 2001.
% perl
eval qq[\n#line 200 "foo bar"\ndie 'foo']; print $@;
__END__
foo at foo bar line 200.
% perl
# line 345 "goop"
eval "\n#line " . __LINE__ . ' "' . __FILE__ ."\"\ndie 'foo'";
print $@;
__END__
foo at goop line 345.
=cut