Problem 07

99 Haskell Problems

Problem 7

(**) Flatten a nested list structure.

Transform a list, possibly holding lists as elements into a `flat’ list by replacing each list with its elements (recursively).

Example:

* (my-flatten '(a (b (c d) e)))
(A B C D E)

Example in Haskell:

We have to define a new data type, because lists in Haskell are homogeneous.

data NestedList a = Elem a | List [NestedList a]
*Main> flatten (Elem 5)
[5]
*Main> flatten (List [Elem 1, List [Elem 2, List [Elem 3, Elem 4], Elem 5]])
[1,2,3,4,5]
*Main> flatten (List [])
[]

Solution:

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