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functional programming: scan function

by metaperl (Curate)
on Mar 12, 2009 at 19:49 UTC ( #750246=snippet: print w/ replies, xml ) Need Help??

Description: 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);

Comment on functional programming: scan function
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Re: functional programming: scan function
by ikegami (Pope) on Mar 12, 2009 at 20:20 UTC
    The expensive recursion can be optimized away. Also, you might also want to specify the "&" prototype to also allow map-like syntax.
    sub scanl(&@) { my ($f, @rv) = @_; $rv[$_] = $f->($rv[$_-1], $rv[$_]) for 1..$#rv; return @rv; } my @rv = scanl \&divide, 64, 4,2,4; -or- my @rv = scanl { $_[0] / $_[1] } 64, 4,2,4;

    Finally, you could use $a and $b instead of $_[0] and $_[1], but that would prevent the use of existing functions directly. If that's not a problem, using $a and $b can be done by assigning references to *$ap and *$bp after doing

    my $pkg = caller(); my $ap = do { no strict 'refs'; \*{$pkg.'::a'} }; local *$ap; my $bp = do { no strict 'refs'; \*{$pkg.'::b'} }; local *$bp;

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