Flattening a list...

Flatten.hs

module Flatten where
data NestedList a = Elem a | List [NestedList a]

flatten :: NestedList a -> [a]
flatten (Elem x) = [x]
flatten (List x) = concatMap flatten x

ghci session

Prelude> :l Flatten
[1 of 1] Compiling Flatten          ( Flatten.hs, interpreted )
Ok, modules loaded: Flatten.
*Flatten> flatten [1,2,3]

<interactive>:1:9:
    Couldn't match expected type `NestedList a0'
                with actual type `[t0]'
    In the first argument of `flatten', namely `[1, 2, 3]'
    In the expression: flatten [1, 2, 3]
    In an equation for `it': it = flatten [1, 2, 3]