I'm working on an assignment in Haskell where I need to implement the inits function. So far, I have the following code:
inits [] = [[]]
inits (x:xs) = [x]: inits (xs)
However, when I run this with the input inits [1,2], I get the following output:
[[1], [2], []]
This doesn't seem right, as the expected output for inits [1, 2] should be:
[[], [1], [1, 2]]
I understand that inits should return all initial segments of the list, but my code isn't doing that correctly. Could someone point me in the right direction on how I can adjust my function to achieve the correct output?
I'm still learning Haskell, so any tips or explanations would be greatly appreciated. Thank you!
As an alternative to explicit recursion, an approach to writing tails would be to use scanr, which works much like foldr but saves the intermediate results as a list.
Prelude> scanr (:) [] [1,2,3]
[[1,2,3],[2,3],[3],[]]
Prelude> scanr (:) [] $ tail [1,2,3]
[[2,3],[3],[]]
Prelude> tails = scanr (:) [] . tail
Prelude> tails [1,2,3]
[[2,3],[3],[]]
Implementing inits can similarly be achieved using scanl:
Prelude> scanl (flip (:)) [] [1,2,3]
[[],[1],[2,1],[3,2,1]]
Prelude> map reverse $ scanl (flip (:)) [] [1,2,3]
[[],[1],[1,2],[1,2,3]]
Prelude> inits = map reverse . scanl (flip (:)) []
Prelude> inits [1,2,3]
[[],[1],[1,2],[1,2,3]]