/ 
mem-0.4.7
River stage one • 3 direct dependents • 6 total dependents
/ mem

NAME

    •ḟmem - use modules in "mem"ory (already declared in same file)

VERSION

Version "0.4.7"

SYNOPSIS

use mem;
use mem(@COMPILE_TIME_DEFINES=qw(a b c));

mem is a syntactic-sugar pragma that allows use-ing a package as it is previously defined, in the same file. This allows easy declaration of specialized, typed data structures (like C struct definitions) that increase code legibility and concept clarity. In a similar usage, the constants pragma allows defining low-overhead, runtime constants.

Allowing use of packages in the same file allows calling code to access classes in a clean, object oriented manner, allowing for identical code to exist either in the same file, or in a separate file without making code changes or requiring use of non-portable, language specific features to accomplish the same thing.

In the 2nd form, it can allow in-lined BEGIN's for single line assignments. While one could use it as a replacement for multiple lines, an actual BEGIN block can often look as much or more tidy.

In many cases, these compile time assignments are essential to take full advantage of perl's strengths. For example, without compile time assignment of '@EXPORT', you can't use perl's function prototypes. Due the overhead and difficulty in getting them right, new perl programmers are dissuaded from using such featues.

When used to force assignments into the early parsing stages of perl, Using dynamically allocated, pre-initialized and type-checked data structures become possible.

EXAMPLE

Following, is a sample program, showing two uses of mem . This first example allows declaring a run-time keyword 'ARRAY', that can check to see if it's argument is an ARRAY reference, and provide a runtime literal, ARRAY , that can be used without quotes.

  use strict; use warnings;

  { package Ar_Type;
      #
      use mem;                                    #1st usage 
      our (@EXPORT);
      sub ARRAY (;*) {
          my $p = $_[0]; my $t="ARRAY";
          return @_ ? (ref $p && (1+index($p, $t))) : $t;
      }
      #
      use mem( @EXPORT=qw(ARRAY) );               #2nd usage 
      
			use Xporter;
  }

  package main;
  use Ar_Type;
  use P;

  my @a=(1,2,3);
  my ($ed, $light);
      (@$ed, @$light) = (@a, @a);  #ed & light point to copies of @a
  bless $ed, "bee";

  P "\@a = ref of array" if ARRAY \@a;
  P "ref of \$ed is \"%s\".", ref $ed;
  P "ed still points to underlying type, 'array'" if ARRAY $ed;
  P "Is ref \$light, an ARRAY?: %s", (ref $light eq ARRAY) ? 'yes':'no';
  P "Does \"ref \$ed\" eq ARRAY?: %s", (ref $ed eq ARRAY) ? 'yes':'no';
  P "%s", "#  (Because \"ref \$ed\" is really a bless \"ed\" bee)"

  • Now, to show what happens using mem, and the errors that occur if you do not. First, the correct output:

    @a = ref of array
ref of $ed is "bee".
ed still points to underlying type, 'array'
Is ref $light, an ARRAY?: yes
Does ref $ed eq ARRAY?: no
#  (Because ref "ed" is really a bless"ed" bee)

  • Second, without the first " use mem ", presuming the line was commented out:

    Can't locate Ar_Type.pm in @INC (@INC contains: 
  /usr/lib/perl5/5.18.2 ...   /usr/lib/perl5/site_perl .) 
  at /tmp/ex line 18.
BEGIN failed--compilation aborted at /tmp/ex line 18.

    This is due to package AR_Type, the package already declared and in memory>, being ignored by Perl's use statement because some Perl-specific, "internal flag" is not set for package Ar_Type. The first use mem causes this flag, normally set with the path of the of a used file, to be set with the containing file path and an added comment, containing the line number.

    This tells perl to use the definition of the package that is already in memory.

      and

  • Third, instead of dropping the 1st " use mem ", you drop (or comment out) the 2nd usage in the above example, you get:

    Bareword "ARRAY" not allowed while "strict subs" 
  in use at /tmp/ex line 27.
syntax error at /tmp/ex line 27, near "ARRAY \"
Bareword "ARRAY" not allowed while "strict subs" 
  in use at /tmp/ex line 30.
Bareword "ARRAY" not allowed while "strict subs" 
  in use at /tmp/ex line 31.
Execution of /tmp/ex aborted due to compilation errors.

    This happens because when use Xporter is called, the contents of @EXPORT is not known. Even with the assignment to @EXPORT, the "@EXPORT=qw(ARRAY)" being right above the use Exporter statement. Similarly to the first error, above, Perl doesn't use the value of @EXPORT just above it. Having use mem in the second position forces Perl to put the assignment to @EXPORT in mem ory, so that when use Exporter is called, it can pick up the name of ARRAY as already being "exported" and defined.

    Without use mem putting the value of @EXPORT in memory, ARRAY isn't defined, an you get the errors shown above.

Summary

The first usage allows 'main' to find package Ar_Type, already in memory.

The second usage forces the definition of 'ARRAY' into memory so they can be exported by an exporter function.

In both cases, mem allows your already-in-memory code to be used. Thsi allows simplified programming and usage without knowledge of or references to Perl's internal-flags or internal run phases.

SEE ALSO

See Xporter for more help with exporting features from your modules, or the older Exporter for the cadillac of exporting that will do everything you want (and a bit more). See P for more details about the generic print operator that is actually user friendly, and see Types::Core for a more complete treatment of the CORE Types (with helpers for other perl data types besides ARRAY's.