Haskell道 その１

Haskellはforやwhileのような繰り返し構文がないんですね。なかなか戸惑いますが慣れでしょうか。

早速、コンビネーションを得る関数を作ってみた。

module Main (main) where

combination :: Integer -> [Integer] -> [[Integer]]
combination n elems | n <= 0 || length elems == 0 = []
combination 1 elems     = map (\e -> [e]) elems
combination n (e:elems) = current ++ others
    where current = map (e:) (combination (n-1) elems)
          others  = combination n elems

main :: IO ()
main = do
    print (combination 3 [1,2,3,4,5])

出力

*Main> main
[[1,2,3],[1,2,4],[1,2,5],[1,3,4],[1,3,5],[1,4,5],[2,3,4],[2,3,5],[2,4,5],[3,4,5]]