I needed a scan function for some data processing I am doing. Unfortunately Sub::Curried does not work, so I cannot use it's scan function.

Language::Functional has a broken test suite. And I can't figure out how to fix its issues.

fp does not have a scanl function. I considered contributing this code to it, but am turned off by the semantics of the reduce function... I think that would be better named behead. It also potentially conflicts with List::Util in that respect.

So here we have the scanl function, a staple of functional programming, available in standalone fashion. As you can see, it behaves the same as scanl for haskell


use strict; use warnings;
use Data::Dumper;

=for comment

 scanl f q ls = q : case ls of
     []   -> []
     x:xs -> scanl f (f q x) xs

=cut

sub scanl {
    my ($f, $seed, @list)=@_;

    my @tail = @list[ 1 .. $#list ] ;

    my @rest = scalar @list ? scanl($f, $f->($seed, $list[0]), @tail) 
+: ();

    ($seed, @rest);
}


my @lis = (4,2,4);


sub add { $_[0] + $_[1] }
sub divide { $_[0] / $_[1] }

my @result = scanl \&divide, 64, @lis;

warn Dumper(\@result);

 @result = scanl \&divide, 3, ();

warn Dumper(\@result);

use List::Util 'max';

 @result = scanl \&List::Util::max, 5, (1 .. 7) ;

warn Dumper(\@result);

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In reply to functional programming: scan function by metaperl

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