when {} is a syntax error

[schwern]

This works fine.

func foo( $thing = [1,2,3] when [] ) {
    print @$thing
}

This is a messy parse error.

func foo( $thing = { foo => 23 } when {} ) {
    print keys %$thing;
}

Not enough arguments for grep at /Users/schwern/tmp/test.plx line 3, near "} $arg"
syntax error at /Users/schwern/tmp/test.plx line 3, near "} $arg"
syntax error at /Users/schwern/tmp/test.plx line 5, near "}"
Execution of /Users/schwern/tmp/test.plx aborted due to compilation errors.

Something's gone wrong with the injection.

my $thing = !(@_ > 0) ? ( { foo => 23 } ) : do{ no warnings; my $arg = $_[0]; (grep  {} $arg) ? ( { foo => 23 } ) : $arg}; Method::Signatures->too_many_args_error(1) if @_ > 1; 

[schwern]

OIC it thinks its a block. Patch on the way.

[schwern]

Hrm. I can special case an empty hash, but what about hashes in general? I don't think there will be much call for checking for anything but an empty hash, so I'm inclined to just special case it. Anything more complicated can be done as a full block.

[thoughtstream]

Hashes in general are already supported.

PPI uses the usual perlish guesswork to determine
whether a pair of curlies are a hash or a block.

Empty curlies are arguably a useless special case
as an empty hash will never smartmatch anything.

Of course, treating {} as an empty block would be
equally useless, so special casing them as empty
hash is probably for the best (i.e. least surprising).

[barefootcoder]

Well, we should probably special-case it as whatever-doesn't-generate-a-syntax-error. :-D

This fix will go into the next dev release. Just waiting now for CPANTesters to throw up some reports for me to look at ...

[schwern]

@thoughtstream PPI might have that guesswork, but inject_for_sig() isn't using it. It's using a regex. I'll look at switching it to PPI, but this fix to the existing regex should go in.

And an empty hash ref smart matches, surprise, an empty hash ref.

perl -wle '$h = {};  print $h ~~ {} ? "Match" : "No Match" '
Match

perl -wle '$h = { foo => 23 };  print $h ~~ {} ? "Match" : "No Match" '
No Match

This is handy for providing a default if the user passes in an empty hash ref, similar to using when "" for a string parameter.

[thoughtstream]

Very good point re empty hashref matching empty hashes.
This definitely should be special-cased.

[barefootcoder]

Fixed in version 20121201.