Andy Fugard wrote:
> My main question is really what facilities of the language I should be
> looking at to make this code more elegant! As you can see I currently know
> only the basics of currying, and some list operations.
Definitely list comprehensions! I digged out some old code:
> module Perms where
Permutations.
> perms :: [a] -> [[a]]
> perms [] = [ [] ]
> perms (a : x) = [ z | y <- perms x, z <- insertions a y ]
>
> insertions :: a -> [a] -> [[a]]
> insertions a [] = [ [a] ]
> insertions a x@(b : y) = (a : x) : [ b : z | z <- insertions a y ]
Using deletions instead of insertions; generates the permutations
in lexicographic order, but is a bit slower.
> perms' :: [a] -> [[a]]
> perms' [] = [ [] ]
> perms' x = [ a : z | (a, y) <- deletions x, z <- perms' y ]
> deletions :: [a] -> [(a, [a])]
> deletions [] = []
> deletions (a : x) = (a, x) : [ (b, a : y) | (b, y) <- deletions x ]
Cheers, Ralf
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