Re^2: constants wont optimize

I think I found a solution, but I dont know why it works. The fix is to throw another BEGIN around the part that sets the constant. I would like to know why it works.

C:\Documents and Settings\Owner\Desktop>cat consttest.pl & echo: & ech
+o: & echo:
& perl -MO=Deparse,  consttest.pl & perl consttest.pl
#!/usr/bin/perl -w
#BEGIN {
#     package mod;
#     $VERSION = 1;
#     package main;
#}

BEGIN {
     BEGIN {
     if (scalar(%mod::)
         && $mod::VERSION
         ) {
          eval " sub haveMod () { 1; }"
            ;}
     else {
          eval "sub haveMod () { 0; }"
           ;}
     }
     sub checkMod () {
          if(haveMod())
          {return "goodMod";}
          else {return "badMod";}
     }
}
print checkMod;




BEGIN { $^W = 1; }
sub BEGIN {
    sub BEGIN {
        sub checkMod () {
            do {
                return 'badMod'
            };
        }

    }
    if (scalar %{'mod::'} and $mod::VERSION) {
        eval ' sub haveMod () { 1; }';
    }
    else {
        eval 'sub haveMod () { 0; }';
    }
}
print checkMod;
consttest.pl syntax OK
badMod
C:\Documents and Settings\Owner\Desktop>
[download]

Getting rid of the returns doesn't fix the problem, the return is always implied. Last statement of a subroutine is the return result in Perl.

C:\Documents and Settings\Owner\Desktop>cat consttest.pl & echo: & ech
+o: & echo:
& perl -MO=Deparse,  consttest.pl & perl consttest.pl
#!/usr/bin/perl -w
#BEGIN {
#     package mod;
#     $VERSION = 1;
#     package main;
#}

BEGIN {
     if (scalar(%mod::)
         && $mod::VERSION
         ) {
          eval " sub haveMod () { 1; }"
            ;}
     else {
          eval "sub haveMod () { 0; }"
           ;}
     sub checkMod () {
          if(haveMod())
          {return "goodMod";}
          else {return "badMod";}
     }
}
print checkMod;




BEGIN { $^W = 1; }
sub BEGIN {
    if (scalar %{'mod::'} and $mod::VERSION) {
        eval ' sub haveMod () { 1; }';
    }
    else {
        eval 'sub haveMod () { 0; }';
    }
}
sub checkMod () {
    if (haveMod) {
        return 'goodMod';
    }
    else {
        return 'badMod';
    }
}
print checkMod;
consttest.pl syntax OK
badMod
C:\Documents and Settings\Owner\Desktop>
[download]

Comment on Re^2: constants wont optimize Select or Download Code

Replies are listed 'Best First'.
Re^3: constants wont optimize by chromatic (Archbishop) on Jul 11, 2011 at 05:13 UTC
The fix is to throw another BEGIN around the part that sets the constant. I would like to know why it works. eval is a runtime operation. Sub declaration is a compile time operation. `BEGIN` does nothing to change that unless you use a `BEGIN` to perform the runtime eval before the compile time declaration occurs. All of the operations within a single `BEGIN` block occur, with respect to each other, in the order they would occur if not for the `BEGIN` block.	[reply] [d/l] [select]

Replies are listed 'Best First'.

Re^3: constants wont optimize
by chromatic (Archbishop) on Jul 11, 2011 at 05:13 UTC

The fix is to throw another BEGIN around the part that sets the constant. I would like to know why it works.

eval is a runtime operation. Sub declaration is a compile time operation. BEGIN does nothing to change that unless you use a BEGIN to perform the runtime eval before the compile time declaration occurs.

All of the operations within a single BEGIN block occur, with respect to each other, in the order they would occur if not for the BEGIN block.

[reply]
[d/l]
[select]