Re: If you believe in Lists in Scalar Context, Clap your Hands

[moritz]

Finally, and for extra points (and points mean prizes), how does one describe the difference between a list and an array ?

That's not that hard. Anything that you can push onto is an array, all other things that look like arrays or lists are actually lists. (Any array method that changes the length works as a probe instead)

Examples for Arrays:

@array
# this works:
push @array, $item;

Examples for Lists:

# things returned from subs:
localtime();

# slices:
@INC[0,1,-1];

# literals
qw(foo bar);

# return values from operators
('12') x 10;

# you can't push() on any of these - and it wouldn't make sense anyway

[gone2015]

OK. Lists cannot be lvalues, though you may have a list of lvalues...

Suppose I refine the points paying question: as an rvalue how does one describe the difference between a list and an array ?

[ikegami]

Lists cannot be lvalues, though you may have a list of lvalues...

Say again? An lvalue list returns a list of lvalues and enforces list context, just like an rvalue list returns a list of rvalues in list context.

In both of the following snippets, you'll see an list as an lvalue ("M" flag).

>perl -MO=Concise -e"($a,$b)=(4,5)"
a  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
9     <2> aassign[t3] vKS ->a
-        <1> ex-list lKP ->6           <-- RHS list
3           <0> pushmark s ->4
4           <$> const[IV 4] s ->5
5           <$> const[IV 5] s ->6
-        <1> ex-list lKPRM* ->9        <-- LHS list w/ "M"
6           <0> pushmark sRM* ->7
-           <1> ex-rv2sv sKRM*/1 ->8
7              <#> gvsv[*a] s ->8
-           <1> ex-rv2sv sKRM*/1 ->-
8              <#> gvsv[*b] s ->9
-e syntax OK

>perl -MO=Concise -e"1 for ($a,$b)"
d  <@> leave[1 ref] vKP/REFC ->(end)
1     <0> enter ->2
2     <;> nextstate(main 1 -e:1) v ->3
3     <;> nextstate(main 1 -e:1) v ->4
c     <2> leaveloop vK/2 ->d
8        <{> enteriter(next->9 last->c redo->9) lK/8 ->a
-           <0> ex-pushmark s ->4
-           <1> ex-list lKPM ->7       <-- lvalue list
4              <0> pushmark sM ->5
-              <1> ex-rv2sv sKM/1 ->6
5                 <#> gvsv[*a] s ->6
-              <1> ex-rv2sv sKM/1 ->-
6                 <#> gvsv[*b] s ->7
7           <#> gv[*_] s ->8
-        <1> null vK/1 ->c
b           <|> and(other->9) vK/1 ->c
a              <0> iter s ->b
-              <@> lineseq vK ->-
-                 <0> ex-const v ->9
9                 <0> unstack v ->a
-e syntax OK

[moritz]

An array is everything that could be used as an lvalue (perhaps with the exception of :lvalue subs, haven't tried those), independently of you actually use it as an lvalue.

That definition so far helped in all real-world cases that I've look at.

[ikegami]

perhaps with the exception of :lvalue subs, haven't tried those

lvalue subs are called in scalar context, so they can't return a list.

use strict;
use warnings;
my ($i,$j);
sub f :lvalue { ($i,$j) }
f() = (4,5);
print("$i,$j\n");

Useless use of a constant in void context at script.pl line 5.
Use of uninitialized value in concatenation (.) or string at script.pl
+ line 6.
,5