Am Sonntag 28 Juni 2009 18:06:52 schrieb Brandon S. Allbery KF8NH:
On Jun 28, 2009, at 12:02 , michael rice wrote:
dec2bin :: Integer -> [Integer] dec2bin n = dec2bin' n [] where dec2bin' n acc
| n == 0 = acc | otherwise = let r = rem n 2
m = div (n - r) 2 in dec2bin' m (r : acc)
is there any way to assign a type signature to the helper function?
Same way you do for a top level binding:
dec2bin :: Integer -> [Integer] dec2bin n = dec2bin' n [] where dec2bin' :: Integer -> [Integer] -> [Integer] dec2bin' n acc
| n == 0 = acc | otherwise = let r = rem n 2
m = div (n - r) 2 in dec2bin' m (r : acc)
But, to mention it before it bites, putting type signatures involving type variables on local helper functions is not entirely straightforward. Consider inBase :: Integral a => a -> a -> [a] 0 `inBase` b = [0] n `inBase` b = local n [] where local 0 acc = acc local m acc = case m `divMod` b of (q,r) -> local q (r:acc) Now try giving a type signature to local. You can't. What is the type of local? It's (type of b) -> [type of b] -> [type of b], but "type of b" isn't available. If you try local :: a -> [a] -> [a] or local :: Integral a => a -> [a] -> [a], you are saying that local works for *every* type a (or for every type a which is an instance of Integral), because the 'a' from local's type signature is a new (implicitly forall'd) type variable. To be able to give local a type signature, you must bring the type variable 'a' into scope: {-# LANGUAGE ScopedTypeVariables #-} inBase :: forall a. Integral a => a -> a -> [a] 0 `inBase` b = [0] n `inBase` b = local n [] where local :: a -> [a] -> [a] -- now this a is the same a as the one above local 0 acc = acc local m acc = case m `divMod` b of (q,r) -> local q (r:acc)