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。
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.