アレイのuniq

cf. どう書く？.org – アレイのuniq

再帰で

uniq [] = []
uniq (x:xs) = x:uniq (filter (/=x) xs)

と書いてから，こんな関数ありそうだなぁと思ったらやっぱりあった。

Data.List nub

高階関数版：

uniq2 :: (Eq a) => [a] -> [a]
uniq2 = foldl (\a e -> if (elem e a) then a else a ++ [e]) []

型を明示しないとダメ。

Prelude> :l uniq2.hs
[1 of 1] Compiling Main             ( uniq2.hs, interpreted )

uniq2.hs:2:27:
    Ambiguous type variable b' in the constraint:
      Eq b' arising from use of `elem' at uniq2.hs:2:27-34
    Possible cause: the monomorphism restriction applied to the following:
      uniq2 :: [b] -> [b] (bound at uniq2.hs:2:0)
    Probable fix: give these definition(s) an explicit type signature
                  or use -fno-monomorphism-restriction
Failed, modules loaded: none.