"splice" and "shift" aren't strictly equivalent #17104

p5pRT · 2019-07-23T15:53:47Z

Migrated from rt.perl.org#134304 (status was 'open')

Searchable as RT134304$

p5pRT · 2019-07-23T15:53:47Z

From choroba@matfyz.cz

Created by choroba@matfyz.cz

"perldoc -f splice" says:

The following equivalences hold (assuming "$#a >= $i" )

push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)

They are equivalent in the sense of what happens to array @a. They even return
the same thing in scalar context. But in list context, their return value is
different if the array is empty:

#!/usr/bin/perl
use warnings;
use strict;

use Test::More tests => 4;

my @y = my @x = qw( a b );

is_deeply [scalar splice @x, 0, 1], [scalar shift @y];
is_deeply [splice @x, 0, 1], [shift @y];

@x = @y = ();

is_deeply [scalar splice @x, 0, 1], [scalar shift @y];
is_deeply [splice @x, 0, 1], [shift @y]; # BOOM!

This behaviour is documented under "splice" and "shift", but the word
"equivalences" might be a bit misleading. Maybe we should assume "@a > 0" as
well, or document that the equivalences are valid except for the return value.

Ch.

Perl Info


Flags:
     category=docs
     severity=low

Site configuration information for perl 5.31.1:

Configured by choroba at Tue Jun 18 09:46:42 CEST 2019.

Summary of my perl5 (revision 5 version 31 subversion 1) configuration:
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@INC for perl 5.31.1:
     /home/choroba/blead/lib/perl5/site_perl/5.31.1/x86_64-linux-thread-multi
     /home/choroba/blead/lib/perl5/site_perl/5.31.1
     /home/choroba/blead/lib/perl5/5.31.1/x86_64-linux-thread-multi
     /home/choroba/blead/lib/perl5/5.31.1


Environment for perl 5.31.1:
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p5pRT · 2019-10-06T00:32:31Z

From @jkeenan

On Tue, 23 Jul 2019 15:53:47 GMT, choroba@matfyz.cz wrote:

This is a bug report for perl from choroba@matfyz.cz,
generated with the help of perlbug 1.41 running under perl 5.31.1.

-----------------------------------------------------------------
[Please describe your issue here]

"perldoc -f splice" says:

The following equivalences hold (assuming "$#a >= $i" )

push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)

They are equivalent in the sense of what happens to array @a. They
even return
the same thing in scalar context. But in list context, their return
value is
different if the array is empty:

#!/usr/bin/perl
use warnings;
use strict;

use Test::More tests => 4;

my @y = my @x = qw( a b );

is_deeply [scalar splice @x, 0, 1], [scalar shift @y];
is_deeply [splice @x, 0, 1], [shift @y];

@x = @y = ();

is_deeply [scalar splice @x, 0, 1], [scalar shift @y];
is_deeply [splice @x, 0, 1], [shift @y]; # BOOM!

This behaviour is documented under "splice" and "shift", but the word
"equivalences" might be a bit misleading. Maybe we should assume "@a >
0" as
well, or document that the equivalences are valid except for the
return value.

I concur that this discussion of "equivalences" is highly misleading. The fact that 'splice' has 3 different cases for return value means that those return values are not at all comparable to those of the other 4 built-in functions. When the original list is empty, calls to 'pop' need to anticipate an exception.

See the 4 scripts attached.

In principle, we could modify the documentation to say that the original list must (sometimes) be non-empty and that the "equivalence" applies only to the result list and not to the return values -- but that's so complicated that I think we're better off just removing all the discussion of "equivalence".

Thank you very much.

--
James E Keenan (jkeenan@cpan.org)