I have an assigment and im kinda stuck:
xs = [4, 1]
ys = [3, 2]
the goal is to create a list [1, 2, 3, 4]
I am allowed to use the following commands:
head, tail, ++, !!, reverse, take, drop, init
I know that tail xs ++ (reverse ys) results in [1, 2, 3]
but how do i get the 4 at the end?
My "solution" would have been something along the lines of:
zs = tail xs ++ (reverse ys +++ (head xs)
zs = [1, 2, 3, 4]
but instead im getting the following error:
<interactive>:39:1: error:
* Non type-variable argument in the constraint: Num [a]
(Use FlexibleContexts to permit this)
* When checking the inferred type
it :: forall a. (Num a, Num [a]) => [a]
The problem is i have no clue what that means It would be nice if someone could explain me how the brackets work in haskell and how i can solve the problem.
You have unbalanced parentheses in your demonstrated attempt.
tail xs ++ (reverse ys +++ (head xs)
Assuming you meant:
tail xs ++ (reverse ys ++ (head xs))
head xs is not a list, so using it with ++ will yield an error. But we can make it a list with a single element quite easily.
tail xs ++ (reverse ys ++ [head xs])
This now yields [1, 2, 3, 4], but you can further remove the parentheses because of how ++ associates.
tail xs ++ reverse ys ++ [head xs]