Re^2: Variable scope

From my experience, using state in a named subroutine is a code smell. Sooner or later, you'll need to call the same subroutine for a different purpose, and you'll need a way to reinitialize the state variable with a different value. I usually use two closures over the same my variable, one of them being the setter.

my $state;
sub init_state { $state = shift }
sub foo {
    if ($state) {
        ...
[download]

Where state makes sense, in my opinion, is in anonymous subs, because an anonymous sub has a defined life span:

for my $i (1 .. 3) {
    my $s = sub { state $x = $_; say "$i $x" };
    $s->($_) for qw( a b );
}
[download]

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
[download]

Comment on Re^2: Variable scope Select or Download Code

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Re^3: Variable scope by shmem (Chancellor) on Apr 05, 2018 at 12:28 UTC
From my experience, using state in a named subroutine is a code smell. That depends on the life span of the code. The older it gets, the more it reeks of foul. perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'	[reply]
Re^3: Variable scope by Anonymous Monk on Apr 06, 2018 at 09:19 UTC
Static variables have their uses. For example, one might generate a crc32 table on-demand. This has the advantages of: (1) not using globals, (2) not relying on BEGIN, (3) on-demand construction, (4) conciseness. It replaces the less expressive construct `{ my $table; sub foo { $table //= init_tables(0x123); ... } }` [download] There's nothing wrong with procedural programming.	[reply] [d/l]
Re^3: Variable scope by Anonymous Monk on Apr 06, 2018 at 09:36 UTC
Oh, and another use for state variables — caching/memoization. For example: `sub sort_network { state @mem; my $cmp = $mem[@_] //= do { ... }; my @res = @_[ @$cmp ]; }` [download] I'm sure there are plenty more uses.	[reply] [d/l]
Re^4: Variable scope by choroba (Cardinal) on Apr 06, 2018 at 09:42 UTC
That's exactly what I warned against. Imagine you now need to sort two networks: how do you clear the cache before the second run? ($q=q:Sq=~/;[c](.)(.)/;chr(-\|\|-\|5+lengthSq)`"S\|oS2"`map{chr \|+ord }map{substrSq`S_+\|`\|}3E\|-\|`7**2-3:)=~y+S\|`+$1,++print+eval$q,q,a, [download]	[reply] [d/l]
Re^5: Variable scope by Anonymous Monk on Apr 06, 2018 at 10:33 UTC
Pardon me? That example would presumably return a sorting network or certain size and type; I'm not sure what you mean by "sort two networks". Memoization helps there because of recursive structure. I'll give another example that requires no explanation. `sub fibonacci { my $n = shift; return 0+($n==1) if $n < 2; state @mem; $mem[$n] //= fibonacci($n-1) + fibonacci($n-2); }` [download] And before you ask: no, I do not foresee a need to produce two (different) Fibonacci sequences.	[reply] [d/l]
Re^6: Variable scope by choroba (Cardinal) on Apr 06, 2018 at 10:56 UTC
Re^3: Variable scope by Anonymous Monk on May 15, 2018 at 17:11 UTC
Hm. One more example. Here `state` is used to enumerate unique string instances (over the whole lifetime of a program): `#! /usr/bin/perl use warnings; use 5.010; sub enumerate { my ($property, $key) = @_; state (%hash, %enum); $hash{$property}{$key} //= ++$enum{$property}; } print "\n", "Edibles:\n"; print "$_ => @{[enumerate edible => $_]}\n" for qw( apple spam pear orange spam spam spam kebab spam caterpill +ar ); print "\n", "Colors:\n"; print "$_ => @{[enumerate color => $_]}\n" for qw( red yellow red red red green );` [download]	[reply] [d/l] [select]
Re^4: Variable scope by choroba (Cardinal) on May 15, 2018 at 17:51 UTC
Thank you for giving me another counter-example. Add "orange" to the colours. ($q=q:Sq=~/;[c](.)(.)/;chr(-\|\|-\|5+lengthSq)`"S\|oS2"`map{chr \|+ord }map{substrSq`S_+\|`\|}3E\|-\|`7**2-3:)=~y+S\|`+$1,++print+eval$q,q,a, [download]	[reply] [d/l]