shift on empty array in list context broken

LanX has asked for the wisdom of the Perl Monks concerning the following question:

Why does a list context shift on an empty array return (undef) ?

Shouldn't shift behave like one element splice

  DB<16> @array =()

  DB<17> (@x) = splice @array,0,1

  DB<18> x @x
  empty array
  DB<19> (@x) = shift @array

  DB<20> x @x
0  undef                                   # WTF
[download]

update motivation

please note how loops over shift can't handle false values in scalar context, but become endless in list context.

This behaviour breaks conventions of other iterators ...

  DB<1> @x = (1..3,0,undef,1..3)

  DB<2> $max=100

  DB<3> while (my ($x) = shift @x) { print "$x,"; last if $max-- < 0 }
1,2,3,0,,1,2,3,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
+,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
  DB<4> @x = (1..3,0,undef,1..3)

  DB<5> while (my $x = shift @x) { print "$x," }
1,2,3,
[download]

Cheers Rolf
_{(addicted to the Perl Programming Language :)

Wikisyntax for the Monastery
FootballPerl is like chess, only without the dice}

Comment on shift on empty array in list context broken Select or Download Code

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Re: shift on empty array in list context broken by tybalt89 (Monsignor) on Jul 13, 2019 at 21:05 UTC
It doesn't look like shift has a list context, at least according to perldoc. That makes some sense, since it can only return one thing, unlike splice, which can return zero to many. I'd use `while( @x ) { print shift @x, ',' }` [download] for your last example.	[reply] [d/l]
Re^2: shift on empty array in list context broken by LanX (Saint) on Jul 13, 2019 at 21:22 UTC
> It doesn't look like shift has a list context, at least according to perldoc. well `splice` also says The following equivalences hold `... shift(@a) splice(@a,0,1)` [download] Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply] [d/l]
Re^3: shift on empty array in list context broken by tybalt89 (Monsignor) on Jul 13, 2019 at 21:30 UTC
`splice` lied! It's almost sort-of nearly true, but not completely true :)	[reply] [d/l]
Re^4: shift on empty array in list context broken by Anonymous Monk on Jul 13, 2019 at 21:48 UTC
Re^5: shift on empty array in list context broken by ikegami (Patriarch) on Jul 15, 2019 at 11:15 UTC
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Re^4: shift on empty array in list context broken by LanX (Saint) on Jul 13, 2019 at 21:36 UTC
Re^3: shift on empty array in list context broken by tybalt89 (Monsignor) on Jul 13, 2019 at 21:38 UTC
On the other hand, try `my @x = (); my $x = splice @x, 0, 1;` [download] What splice says is true in scalar context.	[reply] [d/l]
Re^4: shift on empty array in list context broken by LanX (Saint) on Jul 13, 2019 at 22:21 UTC
Re^5: shift on empty array in list context broken by dsheroh (Monsignor) on Jul 14, 2019 at 08:14 UTC
Re^3: shift on empty array in list context broken by ikegami (Patriarch) on Jul 15, 2019 at 11:17 UTC
That should be fixed. Feel free to use `perlbug` to submit a bug report. Please include a suggested fix.	[reply] [d/l]
Re^4: shift on empty array in list context broken by choroba (Cardinal) on Jul 23, 2019 at 15:55 UTC
Re^5: shift on empty array in list context broken by LanX (Saint) on Jul 23, 2019 at 16:00 UTC
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Re^3: shift on empty array in list context broken by Don Coyote (Hermit) on Jul 21, 2019 at 14:08 UTC
>well splice also says > The following equivalences hold > ... > shift(@a) splice(@a,0,1) It would appear shift is doing something like this: `DB<232> x $x=4; while( ($y)= ( $u = splice( @t,0,1 ))){ last unless +$x--; print ":$_: " } :: :: :: :: empty array` [download] which would make the equivalence: `shift(@a) === ( $t = splice( @a,0,1 ) )` [download]	[reply] [d/l] [select]
Re: shift on empty array in list context broken by ikegami (Patriarch) on Jul 15, 2019 at 11:12 UTC
No, it shouldn't. Operators and subs that one expects to return a scalar should do so in list context too. It leads to weird and sometimes subtle (i.e. hard to debug) problems if they sometimes return an empty list instead. The purpose of `shift` is to return a scalar, and it should do so.	[reply] [d/l]
Re: shift on empty array in list context broken by Don Coyote (Hermit) on Jul 16, 2019 at 01:32 UTC
shift may not be the culprit, if you look at how the assignment is evaluated. Both the scalar and array assignment are evaluated in scalar context `DB<41> x { $max=100; while ( () = undef ) {print "$x, "; last if $ +max-- == 0 } } , , , , , , , , , , , , , , , ... , , , , , , , , , , , , , , , , , , +, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , + , , , , , , , , , , , empty array DB<42> x { $max=100; while ( $_ = undef ) {print "$x, "; last if $ma +x-- == 0 } } 0 undef` [download] I dont know if this might be affecting how shift is working, but may shed some light. I did investigate the splice/shift difference a little, they behave the same when both @x and @array are undef, but different when, as you highlighted they are empty. Possibly shift and splice are optimised at that level due to their expected usage. Update: this may be something to consider. `DB<60> x { $max=100; while ( @x = splice( @{[]},0,1 )) {print "$x, " +; last if $max-- == 0 } } 0 0 DB<61> x { $max=100; while ( @x = shift @{[]} ) {print "$x, "; last +if $max-- == 0 } } , , , , , , , , , , , , , , ... , , , , , , , , , , , , , , , , , , , +, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , + , , , , , , , empty array` [download]	[reply] [d/l] [select]
Re^2: shift on empty array in list context broken by LanX (Saint) on Jul 16, 2019 at 02:07 UTC
> they behave the same when both @x and @array are undef, not sure what you mean here...(?) anyway I doubt it's possible to "fix" this, much code will rely on shift returning undef. Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}	[reply]
Re^3: shift on empty array in list context broken by Don Coyote (Hermit) on Jul 16, 2019 at 12:29 UTC
I'm not exactly sure either. I attempted to write some Test scripts. Getting only as far as comparing a couple of variations. heres two scripts that start out looking to compare list and scalar context asignment. I'm not sure they have picked up what you are viewing, but could possibly get there? A B my @x @y Scalar Context Read more... (3 kB) C D my (@x),(@y) List Context Read more... (3 kB)	[reply] [d/l] [select]
Re^4: shift on empty array in list context broken by LanX (Saint) on Jul 16, 2019 at 13:25 UTC
Re^5: shift on empty array in list context broken by Don Coyote (Hermit) on Jul 16, 2019 at 14:25 UTC
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