On Thu, Dec 28, 2006 at 11:56:58PM -0800, pphetra wrote:
data Store a = E a | S [Store a] deriving (Show)
flat :: [Store a] -> [a] flat [] = [] flat ((E x):xs) = [x] ++ flat xs flat ((S x):xs) = flat x ++ flat xs
so *Main> flat [E 1, S[E 2, S[E 3, E 4], E 5]] [1,2,3,4,5]
Since this problem is fundimentally tied to Lisp's dynamic typing, it is no suprise it can be done very easily using Haskell's support for dynamic typing:
import Data.Typeable
data D = forall a. Typeable a => D a deriving(Typeable)
flat :: D -> [D] flat (D x) = maybe [D x] (>>= flat) (cast x)
To use: map (\ (D x) -> cast x) flat (D [D 1, D [D 2, D 3], D 4]) :: [Maybe Integer] The 'D' defines an existantial type, which can hold a value of any type subject to the Typeable constraint. Typeable allows the typesafe cast function, which returns Nothing if the types were different. maybe and >>= are prelude functions used to make the definition shorter; without them:
flat (D x) = case (cast x) of Just xs -> concatMap flat xs Nothing -> [D x]