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

In reply to Re: Re: Re: An anomaly with List::Util::reduce and/or eval? by blokhead
in thread An anomaly with List::Util::reduce and/or eval? by BrowserUk

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