Recursive lazy lists?
Sorry if this has come up before and I missed it, but I just came across this Haskell function to convert from Gray code back to normal binary (as lists of 1s and 0s; full code at http://rosettacode.org/mw/index.php/Gray_code#Haskell): gray2bin [] = [] gray2bin (x:xs) = bin where bin = x : zipWith http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#v:zipWith xor2 xs bin Notice how the definition of 'bin' is recursive. Does Perl6's variety of laziness support this sort of definition? It's not an infinite list; zipWith stops zipping as soon as either list is empty. But the self-reference in the definition means it still has to be computed lazily. -- Mark J. Reed markjr...@gmail.com
Re: Recursive lazy lists?
On Jul 30, 2011, at 8:30 AM, Mark J. Reed wrote:
--snip--
Does Perl6's variety of
laziness support this sort of definition?
It's not an infinite list; zipWith stops zipping as soon as either
list is
empty. But the self-reference in the definition means it still has
to be
computed lazily.
Yes, Perl 6 does support laziness in this sort of definition, via run-
time binding.
I was delighted to find that it already works in Rakudo!
(We also support Haskell's cool pictogram-style declarations.)
Here is a quick conversion of the Haskell solution into Perl 6; its
output
(when run in Rakudo) exactly matches the output of my iterative Perl 6
solution (http://rosettacode.org/mw/index.php/Gray_code#Perl_6):
our multi sub infix:xor2 ( $x, $y ) { ( $x + $y ) % 2 };
multi bin_to_gray ( [] ) { [] }
multi bin_to_gray ( [$head, *@tail] ) {
return [ $head, ( @tail Zxor2 ($head, @tail) ) ];
}
multi gray_to_bin ( [] ) { [] }
multi gray_to_bin ( [$head, *@tail] ) {
my @bin := $head, (@tail Zxor2 @bin); # Note the recursive
definition via bound array
return @bin.flat;
}
for ^32 - $n {
my @b = $n.fmt('%b').comb;
my $g = bin_to_gray( @b );
my $d = gray_to_bin( $g );
printf %2d: %5b = %5s = %5s: %2d\n, $n, $n, $g.join, $d.join, :
2($d.join);
die if :2($d.join) != $n;
}
--
Hope this helps,
Bruce Gray (Util of PerlMonks)
Re: Recursive lazy lists?
On Sat, Jul 30, 2011 at 6:02 PM, Bruce Gray bruce.g...@acm.org wrote:
Yes, Perl 6 does support laziness in this sort of definition, via run-time
binding.
Very cool. Thanks!
Here is a quick conversion of the Haskell solution into Perl 6; its output
(when run in Rakudo) exactly matches the output of my iterative Perl 6
solution
(http://rosettacode.org/mw/**index.php/Gray_code#Perl_6http://rosettacode.org/mw/index.php/Gray_code#Perl_6
):
our multi sub infix:xor2 ( $x, $y ) { ( $x + $y ) % 2 };
Why did you need to define this yourself instead of just using +^ ?
--
Mark J. Reed markjr...@gmail.com
Re: Recursive lazy lists?
On Jul 30, 2011, at 6:40 PM, Mark J. Reed wrote:
On Sat, Jul 30, 2011 at 6:02 PM, Bruce Gray bruce.g...@acm.org
wrote:
--snip--
our multi sub infix:xor2 ( $x, $y ) { ( $x + $y ) % 2 };
Why did you need to define this yourself instead of just using +^ ?
Umm, because that is what was done in the Haskell code I was
translating?
That is my only excuse for my oversight. You are correct; in fact,
Solomon
Foster already pointed this out to me, and I have already posted the
revised
version on RC as a second solution:
http://rosettacode.org/wiki/Gray_code#Perl_6
--
Hope this helps,
Bruce Gray (Util of PerlMonks)
