Hi, I'm new to Haskell programming and have the following problem. ----------------------------------------------------- (|>) f g = g f data Stream a where S :: (s -> Step s a) -> s -> Stream a data Step s a = Done | Yield a s | Skip a s toStream :: [a] -> Stream a toStream ax = S step ax where step [] = Done step (a:ax) = Yield a ax fromStream :: Stream a -> [a] fromStream (S step s) = loop s where loop s = case step s of Done -> [] Skip a s' -> loop s' Yield a s' -> a : loop s' filterStream :: (a -> Bool) -> Stream a -> Stream a filterStream p (S step s) = S filter s where filter s = case step s of Done -> Done Skip a s' -> Skip a s' Yield a s' -> if p a then Yield a s' else Skip a s' mapStream :: (a -> b) -> Stream a -> Stream b mapStream f (S step s) = S map s where map s = case step s of Done -> Done Skip a s' -> Skip (f a) s' Yield a s' -> Yield (f a) s' class Streamable a where to :: a -> Stream a instance Streamable [a] where to = toStream -- ERROR: see below s f x = x |> toStream |> f |> fromStream smap x = s (mapStream x) sfilter x = s (filterStream x) (%) a b = mod a b main = do print ([0..20] |> sfilter (\x -> x % 2 == 0)) --------------------------------------------------------- Error 1 Couldn't match expected type `a' (a rigid variable) against inferred type `[a]' `a' is bound by the instance declaration at C:\Users\Sert\Lab\Haskell\HaskellApp1\HaskellApp1\src/Main.hs:63:0 Expected type: [a] -> Stream [a] Inferred type: [[a]] -> Stream [a] In the expression: toStream In the definition of `to': to = toStream C:\Users\Sert\Lab\Haskell\HaskellApp1\HaskellApp1\src\Main.hs 64 8 How can I make the types match so that I can declare lists streamable? Is there something like in-place type annotations as in ML/OCaml/F#? Best Regards, Cetin Sert corsis.de/blog