I was using a string because the '+' in the example was actually a runtime supplied argument (see Re: array problems).

I see the same errors, but I don't understand why eval would return undef.... Or maybe I do.

I think you hit the nail on the head liz!

<DEEP SPECULATION>Maybe eval is using wantarray to detect a void context and returning undef? But that would mean wantarray is failing to detect the body of reduce as a LIST context?

reduce { print         wantarray ? "array\n"  :
               defined wantarray ? "scalar\n" : "void\n" } 1 .. 2;

# prints "void"
[download]

So the eval must optimize itself based on the context, but a normal non-eval'ed block ending with $a + $b would not optimize away, and (I think) still perform and return the addition, even though being in void context.

Certainly I agree that you are right, that this must be a bug in List::Util. The block should be in scalar context since the return value is the important part of this operation. I really wonder how the module can get the return value of something that it executes in void context??

Also, I got some strange behavoir when I add a return statement to the eval. Both should behave the same way, but don't. Why on earth $a gets set to 4 for every addition is a mystery...

my $result = reduce { print "eval: $a + $b\n"; 
                      eval "$a + $b" }
             1 .. 4;
print "result=$result\n\n";
 
# eval: 1 + 2
# eval:  + 3
# eval:  + 4
# result=4


$result = reduce { print "eval: return $a + $b\n";
                   eval "return $a + $b" }
          1 .. 4;
print "result=$result\n\n";

# eval: return 1 + 2
# eval: return 4 + 3
# eval: return 4 + 4
# result=4
[download]

Update: sgifford claims to see scalar context from the block in his reply above -- running his code, I still get void context reported.. This is with Perl 5.8, and List::Util 1.07_00. But even in scalar context, he still gets the weird behavoir? The wonders never cease!

blokhead

Your code prints "scalar" for me as well.

I'm using Perl 5.6.1, List::Util 1.11.

Very strange indeed.

I thought the same thing, but it doesn't seem so:

#!/usr/bin/perl -w

use List::Util qw[ reduce ];

print reduce{ print "a = '$a', b = '$b'\n"; eval "showcontext($a + $b)
+";} 1,2,3;

sub showcontext
{
  print "Context: ";
  if (wantarray) {
    print "array\n";
  }
  elsif (defined(wantarray)) {
    print "scalar\n";
  }
  else {
    print "void\n";
  }
  $_[0];
}
[download]

produces:

$ perl /tmp/t76
a = '1', b = '2'
Context: scalar
Use of uninitialized value in concatenation (.) or string at /tmp/t76 line 5.
a = '', b = '3'
Use of uninitialized value in concatenation (.) or string at /tmp/t76 line 5.
Context: scalar
3

...failing to detect the body of reduce as a LIST context?

But it is in scalar context: try forcing a list contect by prefixing the eval with () = ;-)

Liz