eval'ing coderef with ampersand

hbm has asked for the wisdom of the Perl Monks concerning the following question:

Given a simple subroutine:

sub q { print "q" };

What is it that causes the following to output nothing?

eval \&q;

While these others output as expected?

eval \q;
eval \q();
eval \&q();
[download]

I've read about the ampersand and prototyping, but I'm not understanding this.

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Re: eval'ing coderef with ampersand by jwkrahn (Abbot) on Sep 04, 2010 at 03:36 UTC
`\q`, `\q()` and `\&q()` first call the subroutine and then take a reference to its return value while `\&q` takes a reference to the subroutine itself.	[reply] [d/l] [select]
Re^2: eval'ing coderef with ampersand by hbm (Hermit) on Sep 07, 2010 at 13:13 UTC
That is a subtle and significant difference. What I'm really doing is forking a list of processes; and I was alarmed that one process executed when I defined the list, prior to forking. Do you know where this is documented? I haven't seen it in perlref, my Cookbook, or elsewhere. Thanks!	[reply]
Re^3: eval'ing coderef with ampersand by ikegami (Patriarch) on Sep 07, 2010 at 17:45 UTC
perlop says "Unary '\' creates a reference to whatever follows it." and refers you to perlref. perlref shows special that `\&handler` is the means of getting a reference to a function. `\handler` and `\handler()` are clearly not special cases, so the `\` applies to the value returned by the expressions that follows. They could be written `\( handler )` and `\( handler() )`. The behaviour of `\&handler()` is not documented, but there's only one parsing that wouldn't result in an error: `\( &handler() )`.	[reply] [d/l] [select]
Re: eval'ing coderef with ampersand by AnomalousMonk (Archbishop) on Sep 04, 2010 at 07:51 UTC
... and just as an addendum to jwkrahn's reply, prototyping is not used in and has no bearing on the behavior of any of the OPed statements. Update: This may also throw some light on what is happening: `>perl -wMstrict -le "sub S { print 's' } print '[', eval \S, ']'; print $@; " s [] Undefined subroutine &main::SCALAR called at (eval 1) line 1.` [download]	[reply] [d/l]
Re^2: eval'ing coderef with ampersand by hbm (Hermit) on Sep 07, 2010 at 13:24 UTC
prototyping is not used in and has no bearing Agreed. My point was, prototyping is the only thing I can find regarding ampersands and subroutines. Thanks!	[reply]