When I've seen concat function in Haskell book, I wonder how I can flatten the list below in Haskell. In Python, I can do that because I can check its type in function. But in Haskell I couldn't. How can I flatten the list below?
input: [[1, 2], [[2, 3], 5], [[[2, 3], [4, 5]], [2, 3]]]
output: [1, 2, 2, 3, 5, 2, 3, 4, 5, 2, 3]
As already pointed out, you can't have arbitrary nested lists in Haskell. The closest thing (without dirty type class hacks using fancy pragmas) would be something like:
data Nested a = L a | B [Nested a]
flatten :: Nested a -> [a]
flatten (L x) = [x]
flatten (B xs) = concatMap flatten xs
print $ flatten $ B[B[L 1,L 2],B[B[L 2,L 3],L 5],B[B[B[L 2,L 3],B[L 4, L 5]],B[L 2,L 3]]]
--[1,2,2,3,5,2,3,4,5,2,3]
You cant create a list with different depth in haskell. It won't t typecheck. [[a]]
are not the same type as [[[a]]]
. This function will solve your question but only on list with the same depth.
flat::[[a]] -> [a]
flat [] = []
flat l:ls = l ++ flat ls
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