Question

I’m having a problem with getting and dropping the first value from list in the recursive function.

Here is the code:

removeDuplicates a u = [if a == [] then u else removeDuplicates newA newU
                       | let newU = (head a):u 
                       | let newA = tail a]

And error:

Illegal parallel list comprehension: use -XParallelListComp

And another idea:

removeDuplicates a u = [if a == [] then u else removeDuplicates (tail a) newU
                       | let newU = (head a):u]

And another error:

Occurs check: cannot construct the infinite type: a0 = [a0]
    Expected type: [a0]
      Actual type: [[a0]]
    In the return type of a call of `removeDuplicates'
    In the expression: removeDuplicates (tail a) newU

Thanks in advance.

Edit: At the moment only thing I’m trying to do with this function is moving all items from first list to second with recursive function one by one. After that I will add few more things to remove duplicate values from list.

Answer 1

It looks like you’re trying to do:

removeDuplicates [] u = u
removeDuplicates (x:xs) u = removeDuplicates xs (x:u)

But this is basically reverse:

reverse = foldl (flip (:)) []