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perlsyn



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 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 impor­
       tant part of the syntax.

       Many of Perl's syntactic elements are optional.  Rather
       than requiring you to put parentheses around every func­
       tion 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 Do What I Mean, abbreviated
       DWIM.  It allows programmers to be lazy and to code in a
       style with which they are comfortable.

       Perl borrows syntax and concepts from many languages: awk,
       sed, C, Bourne Shell, Smalltalk, Lisp and even English.
       Other languages have borrowed syntax from Perl, particu­
       larly 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 perl­
       trap for information about how they differ.

       Declarations

       The only things you need to declare in Perl are report
       formats and subroutines--and even undefined subroutines
       can be handled through AUTOLOAD.  A variable holds the
       undefined value ("undef") until it has been assigned a
       defined value, which is anything other than "undef".  When
       used as a number, "undef" is treated as 0; when used as a
       string, it is treated the empty string, ""; 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 "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 "++", "--",
       "+=", "-=", and ".=", that operate on undefined left val­
       ues such as:

           my $a;
           $a++;

       as if it were a list operator from that point forward in
       the program.  You can declare a subroutine without defin­
       ing it by saying "sub name", thus:

           sub myname;
           $me = myname $0             or die "can't get myname";

       Note that myname() functions as a list operator, not as a
       unary operator; so be careful to use "or" instead of "||"
       in this case.  However, if you were to declare the subrou­
       tine as "sub myname ($)", then "myname" would function as
       a unary operator, so either "or" or "||" would work.

       Subroutines declarations can also be loaded up with the
       "require" statement or both loaded and imported into your
       namespace with a "use" statement.  See perlmod for details
       on this.

       A statement sequence may contain declarations of lexi­
       cally-scoped variables, but apart from declaring a vari­
       able name, the declaration acts like an ordinary state­
       ment, and is elaborated within the sequence of statements
       as if it were an ordinary statement.  That means it actu­
       ally has both compile-time and run-time effects.

       Comments

       Text from a "#" character until the end of the line is a
       comment, and is ignored.  Exceptions include "#" inside a
       string or regular expression.

       Simple Statements

       The only kind of simple statement is an expression evalu­
       ated for its side effects.  Every simple statement must be
       terminated with a semicolon, unless it is the final state­
       ment 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
       "eval {}" and "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.

       Any simple statement may optionally be followed by a SIN­
       GLE modifier, just before the terminating semicolon (or
       block ending).  The possible modifiers are:

           if EXPR
           unless EXPR
           while EXPR
           do {
               $line = <STDIN>;
               ...
           } until $line  eq ".\n";

       See "do" in perlfunc.  Note also that the loop control
       statements described later will NOT work in this con­
       struct, because modifiers don't take loop labels.  Sorry.
       You can always put another block inside of it (for "next")
       or around it (for "last") to do that sort of thing.  For
       "next", just double the braces:

           do {{
               next if $x == $y;
               # do something here
           }} until $x++ > $z;

       For "last", you have to be more elaborate:

           LOOP: {
                   do {
                       last if $x = $y**2;
                       # do something here
                   } while $x++ <= $z;
           }

       NOTE: The behaviour of a "my" statement modified with a
       statement modifier conditional or loop construct (e.g. "my
       $x if ...") is undefined.  The value of the "my" variable
       may be "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.

       Compound Statements

       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 con­
       struct a BLOCK.

       The following compound statements may be used to control
       flow:

           if (EXPR) BLOCK
           if (EXPR) BLOCK else BLOCK
           if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
           if (!open(FOO)) { die "Can't open $FOO: $!"; }
           die "Can't open $FOO: $!" unless open(FOO);
           open(FOO) or die "Can't open $FOO: $!";     # FOO or bust!
           open(FOO) ? 'hi mom' : die "Can't open $FOO: $!";
                               # a bit exotic, that last one

       The "if" statement is straightforward.  Because BLOCKs are
       always bounded by curly brackets, there is never any ambi­
       guity about which "if" an "else" goes with.  If you use
       "unless" in place of "if", the sense of the test is
       reversed.

       The "while" statement executes the block as long as the
       expression is true (does not evaluate to the null string
       "" or 0 or "0").  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
       "next", "last", and "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 "use warnings"
       pragma or the -w flag.

       If there is a "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 "next" statement.

       Loop Control

       The "next" command starts the next iteration of the loop:

           LINE: while (<STDIN>) {
               next LINE if /^#/;      # discard comments
               ...
           }

       The "last" command immediately exits the loop in question.
       The "continue" block, if any, is not executed:

           LINE: while (<STDIN>) {
               last LINE if /^$/;      # exit when done with header
               ...
           }

       The "redo" command restarts the loop block without evalu­
       ating the conditional again.  The "continue" block, if
       any, is not executed.  This command is normally used by
       programs that want to lie to themselves about what was
       just input.

           }

       which is Perl short-hand 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 "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 "?pat?" one-
       time matches:

           # inspired by :1,$g/fred/s//WILMA/
           while (<>) {
               ?(fred)?    && s//WILMA $1 WILMA/;
               ?(barney)?  && s//BETTY $1 BETTY/;
               ?(homer)?   && s//MARGE $1 MARGE/;
           } continue {
               print "$ARGV $.: $_";
               close ARGV  if eof();           # reset $.
               reset       if eof();           # reset ?pat?
           }

       If the word "while" is replaced by the word "until", the
       sense of the test is reversed, but the conditional is
       still tested before the first iteration.

       The loop control statements don't work in an "if" or
       "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 "Basic BLOCKs and Switch
       Statements".

       The form "while/if BLOCK BLOCK", available in Perl 4, is
       no longer available.   Replace any occurrence of "if
       BLOCK" by "if (do BLOCK)".
           while ($i < 10) {
               ...
           } continue {
               $i++;
           }

       There is one minor difference: if variables are declared
       with "my" in the initialization section of the "for", the
       lexical scope of those variables is exactly the "for" loop
       (the body of the loop and the control sections).

       Besides the normal array index looping, "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.

           $on_a_tty = -t STDIN && -t STDOUT;
           sub prompt { print "yes? " if $on_a_tty }
           for ( prompt(); <STDIN>; prompt() ) {
               # do something
           }

       Using "readline" (or the operator form, "<EXPR>") as the
       conditional of a "for" loop is shorthand for the follow­
       ing.  This behaviour is the same as a "while" loop condi­
       tional.

           for ( prompt(); defined( $_ = <STDIN> ); prompt() ) {
               # do something
           }

       Foreach Loops

       The "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 is preceded with the keyword "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 exit­
       ing the loop.  If the variable was previously declared
       with "my", it uses that variable instead of the global
       one, but it's still localized to the loop.  This implicit
       localisation occurs only in a "foreach" loop.

       The "foreach" keyword is actually a synonym for the "for"
       keyword, so you can use "foreach" for readability or "for"
       for brevity.  (Or because the Bourne shell is more famil­
       iar to you than csh, so writing "for" comes more natu­
       rally.)  If VAR is omitted, $_ is set to each value.

       If any element of LIST is an lvalue, you can modify it by

           for (@ary) { s/foo/bar/ }

           for my $elem (@elements) {
               $elem *= 2;
           }

           for $count (10,9,8,7,6,5,4,3,2,1,'BOOM') {
               print $count, "\n"; sleep(1);
           }

           for (1..15) { print "Merry Christmas\n"; }

           foreach $item (split(/:[\\\n:]*/, $ENV{TERMCAP})) {
               print "Item: $item\n";
           }

       Here's how a C programmer might code up a particular algo­
       rithm 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 exe­
       cuted.  The "next" explicitly iterates the other loop
       rather than merely terminating the inner one.  And it's
       faster because Perl executes a "foreach" statement more
       rapidly than it would the equivalent "for" loop.

       Basic BLOCKs and Switch Statements

       A BLOCK by itself (labeled or not) is semantically equiva­
               $nothing = 1;
           }

       There is no official "switch" statement in Perl, because
       there are already several ways to write the equivalent.

       However, starting from Perl 5.8 to get switch and case one
       can use the Switch extension and say:

               use Switch;

       after which one has switch and case.  It is not as fast as
       it could be because it's not really part of the language
       (it's done using source filters) but it is available, and
       it's very flexible.

       In addition to the above BLOCK construct, you could write

           SWITCH: {
               $abc = 1, last SWITCH  if /^abc/;
               $def = 1, last SWITCH  if /^def/;
               $xyz = 1, last SWITCH  if /^xyz/;
               $nothing = 1;
           }

       (That's actually not as strange as it looks once you real­
       ize that you can use loop control "operators" within an
       expression.  That's just the binary comma operator in
       scalar context.  See "Comma Operator" in perlop.)

       or

           SWITCH: {
               /^abc/ && do { $abc = 1; last SWITCH; };
               /^def/ && do { $def = 1; last SWITCH; };
               /^xyz/ && do { $xyz = 1; last SWITCH; };
               $nothing = 1;
           }

       or formatted so it stands out more as a "proper" "switch"
       statement:

           SWITCH: {
               /^abc/      && do {
                                   $abc = 1;
                                   last SWITCH;
                              };

               /^def/      && do {
                                   $def = 1;
                                   last SWITCH;
                              };

               $nothing = 1;
           }

       or even, horrors,

           if (/^abc/)
               { $abc = 1 }
           elsif (/^def/)
               { $def = 1 }
           elsif (/^xyz/)
               { $xyz = 1 }
           else
               { $nothing = 1 }

       A common idiom for a "switch" statement is to use "fore­
       ach"'s aliasing to make a temporary assignment to $_ 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'";
                   }

       Another interesting approach to a switch statement is
       arrange for a "do" block to return the proper value:

           $amode = do {
               if     ($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 certain that all the "&&" clauses are true,
       you can use something like this, which "switches" on the
       value of the "HTTP_USER_AGENT" environment variable.

                        || /Linux/          && 'l/Linux.html'
                        || /HP-UX/          && 'h/HP-SUX.html'
                        || /SunOS/          && 's/ScumOS.html'
                        ||                     'a/AppendixB.html';
           }
           print "Location: $dir/$page\015\012\015\012";

       That kind of switch statement only works when you know the
       "&&" clauses will be true.  If you don't, the previous
       "?:" example should be used.

       You might also consider writing a hash of subroutine ref­
       erences instead of synthesizing a "switch" statement.

       Goto

       Although not for the faint of heart, Perl does support a
       "goto" statement.  There are three forms: "goto"-LABEL,
       "goto"-EXPR, and "goto"-&NAME.  A loop's LABEL is not
       actually a valid target for a "goto"; it's just the name
       of the loop.

       The "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 "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 bet­
       ter to use some other construct such as "last" or "die".
       The author of Perl has never felt the need to use this
       form of "goto" (in Perl, that is--C is another matter).

       The "goto"-EXPR form expects a label name, whose scope
       will be resolved dynamically.  This allows for computed
       "goto"s per FORTRAN, but isn't necessarily recommended if
       you're optimizing for maintainability:

           goto(("FOO", "BAR", "GLARCH")[$i]);

       The "goto"-&NAME form is highly magical, and substitutes a
       call to the named subroutine for the currently running
       subroutine.  This is used by "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 @_ in the current sub­
       routine are propagated to the other subroutine.)  After
       the "goto", not even "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

           =head1 Here There Be Pods!

       Then that text and all remaining text up through and
       including a line beginning with "=cut" will be ignored.
       The format of the intervening text is described in perl­
       pod.

       This allows you to intermix your source code and your doc­
       umentation 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 "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.

       Plain Old Comments (Not!)

       Perl can process line directives, much like the C prepro­
       cessor.  Using this, one can control Perl's idea of file­
       names and line numbers in error or warning messages (espe­
       cially for strings that are processed with "eval()").  The
       syntax for this mechanism is the same as for most C pre­
       processors: it matches the regular expression
       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.

perl v5.8.1                 2003-09-02                 PERLSYN(1)
  
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