Skunkware 5/Skunkware 5.iso/man/cat.1/perlsub.1

Text File | 1995-07-26 | 11.7 KB | 331 lines

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          perlsub - Perl subroutines

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          To declare subroutines:

              sub NAME;      # A "forward" declaration.
              sub NAME BLOCK # A declaration and a definition.

          To import subroutines:

              use PACKAGE qw(NAME1 NAME2 NAME3);

          To call subroutines:

              &NAME          # Passes current @_ to subroutine.
              &NAME(LIST);   # Parens required with & form.
              NAME(LIST);    # & is optional with parens.
              NAME LIST;     # Parens optional if predeclared/imported.


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          Any arguments passed to the routine come in as array @_,
          that is ($_[0], $_[1], ...).  The array @_ is a local array,
          but its values are references to the actual scalar
          parameters.  The return value of the subroutine is the value
          of the last expression evaluated, and can be either an array
          value or a scalar value.  Alternately, a return statement
          may be used to specify the returned value and exit the
          subroutine.  To create local variables see the _l_o_c_a_l() and
          _m_y() operators.

          A subroutine may called using the "&" prefix.  The "&" is
          optional in Perl 5, and so are the parens if the subroutine
          has been predeclared.  (Note, however, that the "&" is _N_O_T
          optional when you're just naming the subroutine, such as
          when it's used as an argument to _d_e_f_i_n_e_d() or _u_n_d_e_f().  Nor
          is it optional when you want to do an indirect subroutine
          call with a subroutine name or reference using the
          &$subref() or &{$subref}() constructs.  See the _p_e_r_l_r_e_f
          manpage for more on that.)

          Example:

              sub MAX {
                  my $max = pop(@_);
                  foreach $foo (@_) {
                      $max = $foo if $max < $foo;
                  }
                  $max;
              }




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              ...
              $bestday = &MAX($mon,$tue,$wed,$thu,$fri);

          Example:

              # get a line, combining continuation lines
              #  that start with whitespace

              sub get_line {
                  $thisline = $lookahead;
                  LINE: while ($lookahead = <STDIN>) {
                      if ($lookahead =~ /^[ \t]/) {
                          $thisline .= $lookahead;
                      }
                      else {
                          last LINE;
                      }
                  }
                  $thisline;
              }

              $lookahead = <STDIN>;       # get first line
              while ($_ = get_line()) {
                  ...
              }

          Use array assignment to a local list to name your formal
          arguments:

              sub maybeset {
                  my($key, $value) = @_;
                  $foo{$key} = $value unless $foo{$key};
              }

          This also has the effect of turning call-by-reference into
          call-by-value, since the assignment copies the values.

          Subroutines may be called recursively.  If a subroutine is
          called using the "&" form, the argument list is optional.
          If omitted, no @_ array is set up for the subroutine; the @_
          array at the time of the call is visible to subroutine
          instead.

              &foo(1,2,3);        # pass three arguments
              foo(1,2,3);         # the same

              foo();              # pass a null list
              &foo();             # the same
              &foo;               # pass no arguments--more efficient






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          [Note:  The mechanism described in this section works fine
          in Perl 5, but the new reference mechanism is generally
          easier to work with.  See the _p_e_r_l_r_e_f manpage.]

          Sometimes you don't want to pass the value of an array to a
          subroutine but rather the name of it, so that the subroutine
          can modify the global copy of it rather than working with a
          local copy.  In perl you can refer to all the objects of a
          particular name by prefixing the name with a star: *foo.
          This is often known as a "type glob", since the star on the
          front can be thought of as a wildcard match for all the
          funny prefix characters on variables and subroutines and
          such.

          When evaluated, the type glob produces a scalar value that
          represents all the objects of that name, including any
          filehandle, format or subroutine.  When assigned to, it
          causes the name mentioned to refer to whatever "*" value was
          assigned to it.  Example:

              sub doubleary {
                  local(*someary) = @_;
                  foreach $elem (@someary) {
                      $elem *= 2;
                  }
              }
              doubleary(*foo);
              doubleary(*bar);

          Note that scalars are already passed by reference, so you
          can modify scalar arguments without using this mechanism by
          referring explicitly to $_[0] etc.  You can modify all the
          elements of an array by passing all the elements as scalars,
          but you have to use the * mechanism (or the equivalent
          reference mechanism) to push, pop or change the size of an
          array.  It will certainly be faster to pass the typeglob (or
          reference).

          Even if you don't want to modify an array, this mechanism is
          useful for passing multiple arrays in a single LIST, since
          normally the LIST mechanism will merge all the array values
          so that you can't extract out the individual arrays.

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          Many builtin functions may be overridden, though this should
          only be tried occasionally and for good reason.  Typically
          this might be done by a package attempting to emulate
          missing builtin functionality on a non-Unix system.




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          Overriding may only be done by importing the name from a
          module--ordinary predeclaration isn't good enough.  However,
          the subs pragma (compiler directive) lets you, in effect,
          predeclare subs via the import syntax, and these names may
          then override the builtin ones:

              use subs 'chdir', 'chroot', 'chmod', 'chown';
              chdir $somewhere;
              sub chdir { ... }

          Library modules should not in general export builtin names
          like "open" or "chdir" as part of their default @EXPORT
          list, since these may sneak into someone else's namespace
          and change the semantics unexpectedly.  Instead, if the
          module adds the name to the @EXPORT_OK list, then it's
          possible for a user to import the name explicitly, but not
          implicitly.  That is, they could say

              use Module 'open';

          and it would import the open override, but if they said

              use Module;

          they would get the default imports without the overrides.

          AAAAuuuuttttoooollllooooaaaaddddiiiinnnngggg

          If you call a subroutine that is undefined, you would
          ordinarily get an immediate fatal error complaining that the
          subroutine doesn't exist.  (Likewise for subroutines being
          used as methods, when the method doesn't exist in any of the
          base classes of the class package.) If, however, there is an
          AUTOLOAD subroutine defined in the package or packages that
          were searched for the original subroutine, then that
          AUTOLOAD subroutine is called with the arguments that would
          have been passed to the original subroutine.  The fully
          qualified name of the original subroutine magically appears
          in the $AUTOLOAD variable in the same package as the
          AUTOLOAD routine.  The name is not passed as an ordinary
          argument because, er, well, just because, that's why...

          Most AUTOLOAD routines will load in a definition for the
          subroutine in question using eval, and then execute that
          subroutine using a special form of "goto" that erases the
          stack frame of the AUTOLOAD routine without a trace.  (See
          the standard AutoLoader module, for example.) But an
          AUTOLOAD routine can also just emulate the routine and never
          define it.  A good example of this is the standard Shell
          module, which can treat undefined subroutine calls as calls
          to Unix programs.




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          There are mechanisms available for modules to help them
          split themselves up into autoloadable files to be used with
          the standard AutoLoader module.  See the document on
          extensions.



















































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