"use" not allowed in expression

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Migrated from rt.perl.org#133732 (status was 'rejected')

Searchable as RT133732$

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From Ulrich.Windl@rz.uni-regensburg.de

This is a feature request​:
In Perl 5.18.2 the following code is not allowed (ignore the surrounding {} for now) (``"use" not allowed in expression at /tmp/t.pl line 3, at end of line'')​:
{
use constant VALS => map {
  use integer;
  $_; #...
} (0..255);
}

However this is allowed​:
{
  use integer;
use constant VALS => map {
  $_; #...
} (0..255);
}

I consider both equivalent, with the first (without the surrounding braces) being more elegant. Is there a reason why the first variant does not work?

Regards,
Ulrich

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From @Abigail

On Wed, Dec 19, 2018 at 02​:28​:06AM -0800, Ulrich Windl (via RT) wrote​:

# New Ticket Created by "Ulrich Windl"
# Please include the string​: [perl #133732]
# in the subject line of all future correspondence about this issue.
# <URL​: https://rt-archive.perl.org/perl5/Ticket/Display.html?id=133732 >

This is a feature request​:
In Perl 5.18.2 the following code is not allowed (ignore the surrounding {} for now) (``"use" not allowed in expression at /tmp/t.pl line 3, at end of line'')​:
{
use constant VALS => map {
use integer;
$_; #...
} (0..255);
}

However this is allowed​:
{
use integer;
use constant VALS => map {
$_; #...
} (0..255);
}

I consider both equivalent, with the first (without the surrounding braces) being more elegant. Is there a reason why the first variant does not work?

The problem is that if perl sees 'map {' it has to guess what is coming
next. Either it's the start of a 'map BLOCK LIST' expression, or the start
of a 'map EXPR, LIST' expression, where EXPR happens to be a reference
to hash. Perl uses heuristics, and sometimes, it gets it wrong.

Having said that, I am unable to reproduce your issue​:

  $ /opt/perl/5.29.6/bin/perl5.29.6 -c
  use strict;
  use warnings;
  {
  use constant VALS => map {
  use integer;
  $_; #...
  } (0 .. 255);
  }
  ^D
  - syntax OK
  $

But in general, you can help perl determine whether you have 'map EXPR',
or 'map BLOCK', by using a '+' or a ';' as the first character​:

  map {+... # Start of map ANONHASHREF, LIST
  map {;... # Start of map BLOCK LIST

Abigail

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The RT System itself - Status changed from 'new' to 'open'

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From @sisyphus

On Wed, 19 Dec 2018 08​:10​:35 -0800, abigail@​abigail.be wrote​:

Having said that, I am unable to reproduce your issue​:

Yes, on Windows I had to go back to perl-5.20 to reproduce the reported behaviour.
IIRC the OP was running 5.18.

Cheers,
Rob

[p5pRT]

From Ulrich.Windl@rz.uni-regensburg.de

I can confirm that putting a ';' at the start of the block, prevents the ``"use" not allowed in expression'' error (in my older version of Perl), but I'm unsure about the '+'​: The plus has to be added in front of an expression (like "+1;", I guess. But in that case an empty statement (';') is much simpler...

As the most recent version of Perl seems smarter anyway, your resolved my feature request even before you knew it ;-)

Ok to close it!

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From @jkeenan

On Thu, 20 Dec 2018 08​:27​:55 GMT, Ulrich.Windl@​rz.uni-regensburg.de wrote​:

I can confirm that putting a ';' at the start of the block, prevents
the ``"use" not allowed in expression'' error (in my older version of
Perl), but I'm unsure about the '+'​: The plus has to be added in front
of an expression (like "+1;", I guess. But in that case an empty
statement (';') is much simpler...

As the most recent version of Perl seems smarter anyway, your resolved
my feature request even before you knew it ;-)

Ok to close it!

Per OP, new feature not needed. Closing ticket.

Thank you very much.

--
James E Keenan (jkeenan@​cpan.org)

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@jkeenan - Status changed from 'open' to 'rejected'