AmigActive 2/AACD 2.iso/AACD/Programming/perlman/man/perlsyn.txt

Text File | 1999-09-09 | 21.1 KB | 556 lines

NAME
       perlsyn - Perl syntax

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 null or 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 sed and 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 -n or -p switch.  It's just not the
       mandatory default like it is in sed and awk.)

       Declarations

       Perl is, for the most part, a free-form language.  (The
       only exception to this is format declarations, for obvious
       reasons.) Comments are indicated by the "#" character, and
       extend to the end of the line.  If you attempt to use /*
       */ C-style comments, it will be interpreted either as
       division or pattern matching, depending on the context,
       and C++ // comments just look like a null regular
       expression, so don't do that.

       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 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 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 (prototyped to
       take one scalar parameter) without defining it by saying
       just:

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

       Note that it functions as a list operator though, not as a
       unary operator, so be careful to use or instead of ||
       there.

       Subroutines declarations can also be loaded up with the
       require statement or both loaded and imported into your
       namespace with a use statement.  See the perlmod manpage
       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.

       Simple statements

       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, since 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
       SINGLE modifier, just before the terminating semicolon (or
       block ending).  The possible modifiers are:

           if EXPR
           unless EXPR
           while EXPR
           until EXPR

       The if and unless modifiers have the expected semantics,
       presuming you're a speaker of English.  The while and
       until modifiers also have the usual "while loop" semantics
       (conditional evaluated first), except when applied to a
       do-BLOCK (or to the now-deprecated 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 $line  eq ".\n";

       See the do entry in the perlfunc manpage.  Note also that
       the loop control statements described later will NOT work
       in this construct, since modifiers don't take loop labels.
       Sorry.  You can always wrap another block around it to do
       that sort of thing.

       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
       construct 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
           LABEL while (EXPR) BLOCK
           LABEL while (EXPR) BLOCK continue BLOCK
           LABEL for (EXPR; EXPR; EXPR) BLOCK
           LABEL foreach VAR (LIST) BLOCK
           LABEL BLOCK continue BLOCK

       Note that, unlike C and Pascal, these are defined in terms
       of BLOCKs, not statements.  This means that the curly
       brackets are 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) 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.  Since BLOCKs are
       always bounded by curly brackets, there is never any
       ambiguity 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 -w flag.

       If there is a continue BLOCK, it is always executed just
       before the conditional is about to be evaluated again,
       just like the third part of a for loop in C.  Thus it can
       be used to increment a loop variable, even when the loop
       has been continued via the next statement (which is
       similar to the C continue statement).

       Loop Control

       The next command is like the continue statement in C; it
       starts the next iteration of the loop:

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

       The last command is like the break statement in C (as used
       in loops); it 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
       evaluating 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.

       For example, when processing a file like /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 short-hand for the more explicitly written
       version:



           LINE: while ($line = <ARGV>) {
               chomp($line);
               if ($line =~ s/\\$//) {
                   $line .= <ARGV>;
                   redo LINE unless eof(); # not eof(ARGV)!
               }
               # now process $line
           }

       Or here's a a simpleminded Pascal comment stripper
       (warning: assumes no { or } in strings)

           LINE: while (<STDIN>) {
               while (s|({.*}.*){.*}|$1 |) {}
               s|{.*}| |;
               if (s|{.*| |) {
                   $front = $_;
                   while (<STDIN>) {
                       if (/}/) {      # end of comment?
                           s|^|$front{|;
                           redo LINE;
                       }
                   }
               }
               print;
           }

       Note that if there were a continue block on the above
       code, it would get executed even on discarded lines.

       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.

       In either the if or the while statement, you may replace
       "(EXPR)" with a BLOCK, and the conditional is true if the
       value of the last statement in that block is true.  While
       this "feature" continues to work in version 5, it has been
       deprecated, so please change any occurrences of "if BLOCK"
       to "if (do BLOCK)".

       For Loops

       Perl's C-style for loop works exactly like the
       corresponding while loop; that means that this:

           for ($i = 1; $i < 10; $i++) {
               ...
           }

       is the same as this:



           $i = 1;
           while ($i < 10) {
               ...
           } continue {
               $i++;
           }

       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
           }


       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.  The variable is implicitly local to the loop and
       regains its former value upon exiting 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 can cause problems if you
       have subroutine or format declarations within that block's
       scope.

       The foreach keyword is actually a synonym for the for
       keyword, so you can use foreach for readability or for for
       brevity.  If VAR is omitted, $_ is set to each value.  If
       LIST is an actual array (as opposed to an expression
       returning a list value), you can modify each element of
       the array by modifying VAR inside the loop.  That's
       because the foreach loop index variable is an implicit
       alias for each item in the list that you're looping over.

       Examples:

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

           foreach $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
       algorithm in Perl:

           for ($i = 0; $i < @ary1; $i++) {
               for ($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 confortable with
       the idiom might do it:

           OUTER: foreach $wid (@ary1) {
           INNER:   foreach $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 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
       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 NOT true in eval{},
       sub{}, or contrary to popular belief do{} blocks, which do
       NOT count as loops.)  The continue block is optional.

       The BLOCK construct is particularly nice for doing case
       structures.

           SWITCH: {
               if (/^abc/) { $abc = 1; last SWITCH; }
               if (/^def/) { $def = 1; last SWITCH; }
               if (/^xyz/) { $xyz = 1; last SWITCH; }
               $nothing = 1;
           }
       There is no official switch statement in Perl, because
       there are already several ways to write the equivalent.
       In addition to the above, 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
       realize that you can use loop control "operators" within
       an expression,  That's just the normal C comma operator.)

       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;
                              };

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

       or

           SWITCH: {
               /^abc/ and $abc = 1, last SWITCH;
               /^def/ and $def = 1, last SWITCH;
               /^xyz/ and $xyz = 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 foreach'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" }
               elsif  ($flag & O_WRONLY) { ($flag & O_APPEND) ? "a" : "w" }
               elsif  ($flag & O_RDWR)   {
                   if ($flag & O_CREAT)  { "w+" }
                   else                  { ($flag & O_APPEND) ? "a+" : "r+" }
               }
           };


       Goto

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

       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 better 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 gotos
       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
       subroutine 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
       of next, last, or redo instead of resorting to a goto.
       For certain applications, the catch and throw pair of
       eval{} and die() for exception processing can also be a
       prudent approach.

       PODs: Embedded 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 =cut will be ignored.  The
       format of the intervening text is described in the perlpod
       manpage.

       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 only look at paragraphs
       beginning with a pod diretive (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.