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Re: Hamming Sequences and Lazy Lists

by Roy Johnson (Monsignor)
on Mar 21, 2005 at 17:45 UTC ( #441248=note: print w/replies, xml ) Need Help??

in reply to Hamming Sequences and Lazy Lists

This solution is extremely similar to the Haskell one in both structure and technique -- as much of a direct translation as I could manage. I had to roll a couple of tools, but they are elegantly in keeping with functional programming style.

I was really pleased with how this all came together.

use strict; use warnings; # A stream is an array ref, in which the first element is an ordinary # value, and the second element is a continuation: a sub that yields # a stream. In other words, a lazy list. # Takes an ordinary list and turns it into a stream sub stream_up { my ($x, @rest) = @_; return [ $x, sub { @rest ? stream_up(@rest) : [] } ]; } # Yields the first N elements from a stream sub take { my ($N, $x_xs) = @_; $N > 0 or return (); if (!@$x_xs) { return () } my ($x, $xs) = @$x_xs; return($x, take($N-1, $xs->())); } # merge takes two streams, and returns one sub merge { my ($x_xs, $y_ys) = @_; if (!@$x_xs) { return $y_ys } if (!@$y_ys) { return $x_xs } my ($x, $xs) = @$x_xs; my ($y, $ys) = @$y_ys; if ($x < $y) { return [$x, sub { merge($xs->(), $y_ys) } ] } if ($x > $y) { return [$y, sub { merge($x_xs , $ys->())} ] } if ($x == $y) { return [$x, sub { merge($xs->(), $ys->())} ] } } # Like map, but applies coderef to stream, returning a stream sub stream_map { my ($coderef, $x_xs) = @_; if (!@$x_xs) { return $x_xs } my ($x, $xs) = @$x_xs; local $_ = $x; return [$coderef->($x), sub { stream_map($coderef, $xs->()) }]; } # genHam takes a stream and returns a stream sub genHam { my ($x_xs) = @_; if (!@$x_xs) { return $x_xs } my ($x, $xs) = @$x_xs; my $out; $out = merge([1, sub {stream_map(sub{$_ * $x}, $out)}], genHam($xs +->())); } print "$_\n" for take 20, genHam stream_up(2,3,5);

Caution: Contents may have been coded under pressure.

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