c.nehren/p5p at shadowcat
Jul 4, 2012, 12:55 PM
Post #6 of 11
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Re: What would having a & prototype after the first position break?
[In reply to]
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On Wed, Jul 04, 2012 at 21:33:04 +0200 , Robert Sedlacek wrote:
> On Wed, 2012-07-04 at 12:23 -0600, Darin McBride wrote:
> > sub do_stuff($;&); # implementation not important, I think.
> >
> > my %foo;
> > my $foo;
> > do_stuff $foo { 'something here' };
> >
> > Now, is that passing in $foo as the first parameter and a code ref that returns
> > a static string as the second, or passing in a value contained in %foo as that
> > first parameter, and no code ref?
> >
> > Don't get me wrong, I've wanted to be able to put & somewhere other than the
> > first parameter on many occassions. But first the syntax has to not surprise
> > the reader[0], and we use enough squiggles already for so many different
> > purposes, especially the braces, so a bit of care might be required.
> >
> > [0] Surprise with new syntax is fine. Surprise with something that could be
> > one of multiple syntaxes, probably not so much.
>
> I'm not sure you'd even need to make it optional, since perl would have
> to decide if the '{' belongs to the '$foo' and is a hash access or is a
> whole new argument.
>
> However, only '&' prototypes seem to not require a comma afterwards, so
> shouldn't the above always parse as hash access? I'd assume the code for
> the above prototype would be
>
> do_stuff $foo, { 'something here' };
>
> Or am I missing something?
I do believe you're correct there, Robert. To demonstrate with existing
code:
$ cat p
#!/usr/bin/env perl
sub foo (&;$) {my ($code, $arg) = @_; $code->($arg) }
foo { print "$_[0]\n" } 'hello'
$ perl p
hello
$
More interestingly, if we place a , after the block in the invocation,
strange things happen:
$ cat p
#!/usr/bin/env perl
sub foo (&;$) {my ($code, $arg) = @_; $code->($arg) }
foo { print "$_[0]\n" }, 'hello'
$ perl p
$
For those following along at home, perl prints a sole \n (consider od
-bc or the like on the output). Let's peek at what's really happening:
$ perl -MO=Deparse p
sub foo (&;$) {
my($code, $arg) = @_;
&$code($arg);
}
foo(sub {
print "$_[0]\n";
}
), '???';
p syntax OK
$
If we make the $ parameter required:
$ cat p
#!/usr/bin/env perl
sub foo (&$) {my ($code, $arg) = @_; $code->($arg) }
foo { print "$_[0]\n" }, 'hello'
$ perl p
Not enough arguments for main::foo at p line 5, near "},"
Execution of p aborted due to compilation errors.
$
I think this covers all cases of ambiguity Darrin pointed out above. Are
there any others I've missed?
--
Chris Nehren | Coder, Sysadmin, Masochist
Shadowcat Systems Ltd. | http://shadowcat.co.uk/
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