IRIX Base Documentation 1998 November/IRIX 6.5.2 Base Documentation November 1998.img/usr/share/catman/u_man/cat1/perlsyn.z/perlsyn

Text File | 1998-10-30 | 27KB | 793 lines

PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))                                                          PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))



NNNNAAAAMMMMEEEE
     perlsyn - Perl syntax

DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN
     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 sssseeeedddd and aaaawwwwkkkk 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 ----nnnn or ----pppp switch.  It's just not the mandatory default
     like it is in sssseeeedddd and aaaawwwwkkkk.)

     DDDDeeeeccccllllaaaarrrraaaattttiiiioooonnnnssss

     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 _m_y(), 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 sub name, thus:

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

     Note that it 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 subroutine as sub myname ($), then myname would functonion 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 the _p_e_r_l_m_o_d manpage for details on this.




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PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))                                                          PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))



     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.

     SSSSiiiimmmmpppplllleeee ssssttttaaaatttteeeemmmmeeeennnnttttssss

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

     CCCCoooommmmppppoooouuuunnnndddd ssssttttaaaatttteeeemmmmeeeennnnttttssss

     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).





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PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))                                                          PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))



     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 _r_e_q_u_i_r_e_d--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.  Because 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 ----wwww 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).

     LLLLoooooooopppp CCCCoooonnnnttttrrrroooollll

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





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PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))                                                          PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))



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

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










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PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))                                                          PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))



         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.

     The form while/if BLOCK BLOCK, available in Perl 4, is no longer
     available.   Replace any occurrence of if BLOCK by if (do BLOCK).

     FFFFoooorrrr LLLLooooooooppppssss

     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++;
         }

     (There is one minor difference: The first form implies a lexical scope
     for variables declared with my in the initialization expression.)

     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.






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PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))                                                          PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))



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


     FFFFoooorrrreeeeaaaacccchhhh LLLLooooooooppppssss

     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 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.  (Note
     that a lexically scoped variable can cause problems with you have
     subroutine or format declarations.)

     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 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 algorithm in Perl:









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PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))                                                          PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))



         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: foreach my $wid (@ary1) {
         INNER:   foreach 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 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.

     BBBBaaaassssiiiicccc BBBBLLLLOOOOCCCCKKKKssss aaaannnndddd SSSSwwwwiiiittttcccchhhh SSSSttttaaaatttteeeemmmmeeeennnnttttssss

     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 _N_O_T true in
     eval{}, sub{}, or contrary to popular belief do{} blocks, which do _N_O_T
     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







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PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))                                                          PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))



         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,





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PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))                                                          PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))



         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+" }
             }
         };


     GGGGoooottttoooo

     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:




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PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))                                                          PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))



         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
     _A_U_T_O_L_O_A_D() 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 _c_a_l_l_e_r()
     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 _d_i_e() for exception processing can also be a prudent
     approach.

     PPPPOOOODDDDssss:::: EEEEmmmmbbbbeeeeddddddddeeeedddd DDDDooooccccuuuummmmeeeennnnttttaaaattttiiiioooonnnn

     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 _p_e_r_l_p_o_d 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 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.






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PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))                                                          PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))



         $a=3;
         =secret stuff
          warn "Neither POD nor CODE!?"
         =cut back
         print "got $a\n";

     You probably shouldn't rely upon the _w_a_r_n() 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.

     PPPPllllaaaaiiiinnnn OOOOlllldddd CCCCoooommmmmmmmeeeennnnttttssss ((((NNNNooootttt!!!!))))

     Much like the C preprocessor, perl can process line directives.  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
     _e_v_a_l()).  The syntax for this mechanism is the same as for most C
     preprocessors: it matches the regular expression
     /^#\s*line\s+(\d+)\s*(?:\s"([^"]*)")?/ with $1 being the line number for
     the next line, and $2 being the optional filename (specified within
     quotes).

     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.





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PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))                                                          PPPPEEEERRRRLLLLSSSSYYYYNNNN((((1111))))























































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