Question

test :: [String] -> [String]

test = foldr step []

  where step x ys

          | elem x ys = x : ys

          | otherwise = ys

I am trying to build a new list consisting of all the distinct strings being input. My test data is:

test ["one", "one", "two", "two", "three"]

expected result:

["one", "two", "three"]

I am new to Haskell, and I am sure that I am missing something very fundamental and obvious, but have run out of ways to explore this. Could you provide pointers to where my thinking is deficient?

The actual response is []. It seems that the first guard condition is never met (if I replace it with True, the original list is replicated), so the output list is never built.

My understanding was that the fold would accumulate the result of step on each item of the list, adding it to the empty list. I anticipated that step would test each item for its inclusion in the output list (the first element tested not being there) and would add anything that was not already included to the output list. Obviously not 🙂

Answer 1

Your reasoning is correct: you just need to switch = x : ys and = ys so that you add the x when it’s not an element of ys. Also, Data.List.nub does this exact thing.