Need tests for goto block-that-evaluates-to-code-ref

[jkeenan]

As part of my research on Fatalization of currently deprecated goto
construct I have been
re-reading our current documentation of goto in blead and have been trying
to locate in our test suite examples of each claim made in that documentation.
What we find -- or fail to find -- there will have an impact on the
documentation updates provided as part of a pull request like the current
#23782. I've posted the preliminary results
of that documentation review
here.

This is the ninth in a series of GH issues addressing the items which I
believe are lacking in that documentation.

perldoc -f goto states:

The "goto &NAME" form is quite different from the other forms of
"goto". In fact, it isn't a goto in the normal sense at all, and
doesn't have the stigma associated with other gotos. Instead, it
exits the current subroutine (losing any changes set by "local")
and immediately calls in its place the named subroutine using
the current value of @_. This is used by "AUTOLOAD" subroutines
that wish to load another subroutine and then pretend that the
other subroutine had been called in the first place (except that
any modifications to @_ in the current subroutine are propagated
to the other subroutine.) After the "goto", not even "caller"
will be able to tell that this routine was called first.

NAME needn't be the name of a subroutine; it can be a scalar
variable containing a code reference or a block that evaluates
to a code reference.

Do we have any examples of goto BLOCK-THAT-EVALUATES-TO-CODE-REFERENCE?
Probably not; ack '\bgoto\s*\{' t/op/goto* turned up nothing.

Write some unit tests that exercise this. Place them in `t/op/goto-sub.t'.

[jkeenan]

I have not been able to figure this one out. This compiles and runs:

$ cat gh-23811-goto-block.pl 
#!/usr/bin/env perl
use 5.14.0;
use warnings;

my $hw = 'hello world';
my $coderef = sub { return $hw; };
say "<", ref($coderef), ">";

my $rv = $coderef->();
say "<$rv>";

#$rv = goto { $coderef; };
#$rv = goto {; $coderef; };

$ perl gh-23811-goto-block.pl 
<CODE>
<hello world>

But if I un-comment one or the other of the last two statements and try to compile, I get an exception:

$ perl -c gh-23811-goto-block.pl 
syntax error at gh-23811-goto-block.pl line 12, near "; }"
gh-23811-goto-block.pl had compilation errors.

[mauke]

Do we have any examples of goto BLOCK-THAT-EVALUATES-TO-CODE-REFERENCE?

Yes:

perl5/t/op/goto-sub.t

Line 345 in b7b77ff

goto &{ sub {} } unless $i++;

Your attempts are missing the & part of the syntax, so { is parsed as a hashref constructor (with a list inside) and { LIST ; } is a syntax error.

[ap]

That documentation should be improved.

It’s describing the part that immediately follows the & in a very textual way: the shown syntax uses a subroutine name (goto &NAME) but it says instead of a subroutine name you can use “a scalar variable containing a code reference” ($coderef, so by replacing NAME you get goto &$coderef) “or a block that evaluates to a code reference” ({ get_coderef() }, so by replacing NAME you get goto &{ get_coderef() }).

I don’t think that’s at all helpful. If someone’s understanding of Perl is basic enough to need these options spelled out, then I don’t think these descriptions will be at all clear to them, and if their understanding of Perl is advanced enough to get what is being said, then they wouldn’t need this level of spelling out.

I don’t have a good idea at this time for how to phrase this instead, but the current phrasing is not fit for purpose.