in reply to Re: The Case for Macros in Perl
in thread The Case for Macros in Perl

I don't understand ... what are macros?

Textual substitutions made, once, inline, at compile time.


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Re^3: The Case for Macros in Perl
by einhverfr (Friar) on Sep 13, 2014 at 11:45 UTC

    Actually I think evals and strings would be a good match for this use case because the primary use here is only done once, on module load (this could be done at near-compile time by putting the method generators in a BEGIN block).

    However this specific case would be more generally solvable as a compile time macro, i.e. expanding the macro call at compile time into exactly what you want. The only reason I haven't seriously considered going with eval and strings is I haven't had the time to put together a good proof of concept for comparison's sake. When I do, I may ask about clarity here or in the seekers forum.

Re^3: The Case for Macros in Perl
by Anonymous Monk on Sep 13, 2014 at 07:55 UTC

    evals are made every time they are encountered, at runtime.

    Sure, right, but

    use MyFoo -macronic => <<'YO'; ... YO;
    ???
      Sure, right, but
      use MyFoo -macronic => <<'YO'; ... YO;
      ???

      What is that meant to prove or disprove?

      All you've succeeded in doing is pass a multi-line string as an argument to a modules import list.

      A (trivial) example macro might look something like this:

      use Macro ADD( x, y ) { x + y; } ... my $p = getPfromSomewhere(); my $q = getQfromSomewhere(); my $r = ADD( $p, $q );

      Which would substitute out as:

      my $p = getPfromSomewhere(); my $q = getQfromSomewhere(); $r = $p + $q;

      And better yet:

      use Macro ADD( x, y ) { x + y; } ... my $r = ADD( getPfromSomewhere(), getQfromSomewhere() ); ... my $s = ADD( geta(), getb() );

      Resulting in just:

      my $r = getPfromSomewhere() + getQfromSomewhere(); my $s = geta() + getb();

      Allowing the efficient use of small "functions and methods" at the source level without the runtime overhead of the calls.


      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
      functions and methods

        Way back, in perl4, there was (from perl.man)

        -Idirectory may be used in conjunction with -P to tell the C prep +rocessor where to look for include files. By default /usr/incl +ude and /usr/lib/perl are searched. -P causes your script to be run through the C preprocessor + before compilation by perl. (Since both comments and cpp di +rectives begin with the # character, you should avoid starting +comments with any words recognized by the C preprocessor such +as "if", "else" or "define".)
        which was dropped (in favour of source filters, I guess). But hey, you can still run your code through cpp:
        #define ADD(x,y) x + y my $r = ADD( getPfromSomewhere(), getQfromSomewhere() ); ... my $s = ADD( geta(), getb() );
        # 1 "macro.pl" # 1 "<command-line>" # 1 "/usr/include/stdc-predef.h" 1 3 4 # 30 "/usr/include/stdc-predef.h" 3 4 # 1 "/usr/include/x86_64-linux-gnu/bits/predefs.h" 1 3 4 # 31 "/usr/include/stdc-predef.h" 2 3 4 # 1 "<command-line>" 2 # 1 "macro.pl" my $r = getPfromSomewhere() + getQfromSomewhere(); ... my $s = geta() + getb();

        "can you tell me the way to Tipperary?" - "well, I wouldn't start from here..."
        ;-)

        perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'

        What is that meant to prove or disprove?

        That eval doesn't have to be repeatedly called at runtime ... that you can have it at "compile time" ...

        not sure about "Macro" module but it looks like that macro pragma already provides this kind of thing, so what is missing? We have macros right?