Flattening out arguments to splice

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

Oh enlightened ones, please share your wisdom on the following trivial topic:

use 5.14.0;
use warnings;

sub my_splice_like_func
{
    my ($list, @args) = @_;
    my @copy = @$list;
    return splice @copy, @args;         # this doesn't actually work :
+-(
}

$, = ' ';                               # just keepin' it pretty

my $list = [qw< one two three four >];
say my_splice_like_func($list, 2);      # want "three four"; get "two 
+three four"
say my_splice_like_func($list, 2, -1);  # want "three"; get "three fou
+r"
[download]

Now, I see what's actually happening: splice is forcing scalar context on @args, so all I'm really passing in is the number of args, which is obviously not what I want. So I see what's wrong, but I have no clue how to fix it. AFAIK there's no way to force list context (and Google seems to back me up on that). I suppose if this were Perl6 I could do something like *@args to flatten out the array, but in Perl5 I can't see any options other than something like:

    if (@args == 0)
    {   
        return @copy;
    }
    elsif (@args == 1)
    {   
        return splice @copy, $args[0];
    }
    elsif (@args == 2)
    {   
        return splice @copy, $args[0], $args[1];
    }
    else
    {   
        die("too many args");
    }
}
[download]

... which works, but damn, that's clumsy. (Other than checking for too many args, which the original version should have had, granted.)

Does anyone see a way that I've misssed? As always, I humbly await illumination from the collective consciousness.

Comment on Flattening out arguments to splice Select or Download Code

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Re: Flattening out arguments to splice (updated) by haukex (Archbishop) on May 30, 2018 at 06:25 UTC
What you're seeing is because splice has a prototype, and you can use `prototype` to see it (this is on Perl v5.26): `$ perl -le 'print prototype("CORE::splice")//"undef"' \@;$$@` [download] (From Perl v5.14 thru v5.22, the prototype was `+;$$@`, because of the removed autoderef.) As of 5.26, you can use the `&` sub calling to avoid the prototype, as in `&CORE::splice(@args);`. But since that's not available in all versions of Perl, you may in fact need to resort to conditionals, even if it seems "clumsy" :-) For example, because some functions change their behavior based on how many arguments they get, I had to do exactly that in Tie::Handle::Base - see my `binmode` and `open` wrappers. However, the arguments to splice have clear defaults, so as tybalt89 and AnomalousMonk already showed you don't necessarily need to resort to a long `if`-`elsif` chain. This passes all the tests in `t/op/splice.t`: `sub mysplice (\@;$$@) { my ($arr,$off,$len,@list) = @_; splice @$arr, $off//0, $len // @$arr - ($off//0), @list; }` [download] And unlike `CORE::splice`, you can use a `&mysplice` call to ignore the prototype (before v5.26), or change the prototype and arguments however you like. Full test code: Read more... full test code (6 kB)	[reply] [d/l] [select]
Re: Flattening out arguments to splice by tybalt89 (Monsignor) on May 29, 2018 at 22:42 UTC
`#!/usr/bin/perl use 5.14.0; use strict; use warnings; sub my_splice_like_func { my ($list, $offset, $length) = @_; my @copy = @$list; return splice @copy, $offset // 0, $length // @copy - $offset; } $, = ' '; # just keepin' it pretty my $list = [qw< one two three four >]; say my_splice_like_func($list, 2); # want "three four"; get "two +three four" say my_splice_like_func($list, 2, -1); # want "three"; get "three fou +r"` [download]	[reply] [d/l]
Re^2: Flattening out arguments to splice by jwkrahn (Abbot) on May 30, 2018 at 00:34 UTC
splice ARRAY,OFFSET,LENGTH,LIST You forgot the fourth argument: 'LIST'. `my ($list, $offset, $length, @list) = @_;` [download]	[reply] [d/l]
Re^3: Flattening out arguments to splice by AnomalousMonk (Archbishop) on May 30, 2018 at 02:39 UTC
New splicer function operates on external referent (although I'm not sure what this gets you: it's almost exactly like splice) (update: needs Perl 5.10+ for // operator): c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le "use 5.010; ;; sub my_splicer { my ($ar, $offset, $len, @list) = @_; $offset //= 0; $len //= @$ar - $offset; return splice @$ar, $offset, $len, @list; } ;; use constant TEST => qw(one two three four five six seven eight); ;; my $ar; my @spl; my @args; ;; $ar = [ TEST ]; @args = (); @spl = my_splicer($ar, @args); dd \@spl, $ar; ;; $ar = [ TEST ]; @args = (2); @spl = my_splicer($ar, @args); dd \@spl, $ar; ;; $ar = [ TEST ]; @args = (2, 3); @spl = my_splicer($ar, @args); dd \@spl, $ar; ;; $ar = [ TEST ]; @args = (2, 3, qw(FOO BAR)); @spl = my_splicer($ar, @args); dd \@spl, $ar; " ( ["one", "two", "three", "four", "five", "six", "seven", "eight"], [], ) ( ["three", "four", "five", "six", "seven", "eight"], ["one", "two"], ) ( ["three", "four", "five"], ["one", "two", "six", "seven", "eight"], ) ( ["three", "four", "five"], ["one", "two", "FOO", "BAR", "six", "seven", "eight"], ) [download] Update: Note this code will also work as expected with something like `@args = ($ar, 2, 3, qw(FOO BAR));` `@spl = my_splicer(@args);` Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^3: Flattening out arguments to splice by AnomalousMonk (Archbishop) on May 30, 2018 at 01:58 UTC
I haven't tested this, but I believe that in tybalt89's code (and in your OP code), because the splice is operating on a copy of the `$list` array reference argument, so a fourth argument `LIST` would be assigned to the lexical array `@copy` and then almost immediately lost along with that array. Do you mean that you want your function to operate on the external referent of the `$list` array reference? Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re: Flattening out arguments to splice by vr (Curate) on May 30, 2018 at 15:52 UTC
Splicing... >perl -wE "@a=qw/a b c/; say splice @a, 0, 1" a >perl -wE "@a=qw/a b c/; say splice @a, 0, 1e18" abc >perl -wE "@a=qw/a b c/; say splice @a, 0, 1e19" >perl -wE "@a=qw/a b c/; say splice @a, 0, 1e20" ab >perl -wE "@a=qw/a b c/; say splice @a, 0, 1" a >perl -wE "@a=qw/a b c/; say splice @a, 1e18, 1" splice() offset past end of array at -e line 1. >perl -wE "@a=qw/a b c/; say splice @a, 1e19, 1" Modification of non-creatable array value attempted, subscript -198128 +4352 at -e line 1. >perl -wE "@a=qw/a b c/; say splice @a, 1e20, 1" c [download]	[reply] [d/l]