引数の数の自動判定

お題: 関数引数のpermutationの蛇足。

*TestFunArgPerm> [f 1 2 3 | f <- permArgs' (\a b c -> [a, b, c])]
[[1,2,3],[1,3,2],[2,1,3],[2,3,1],[3,1,2],[3,2,1]]

{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}
{-# OPTIONS -fallow-overlapping-instances #-}

permArgs' f = permArgs (nargs f) f
class Nargs f where
  nargs :: f -> [f -> f]
instance Nargs (a -> b) => Nargs (a -> a -> b) where
  nargs _ = suc (nargs (undefined :: a -> b))
instance Nargs (a -> b) where
  nargs _ = one