Very simple Haskell code doesn't work, “Variable not in scope”

Question

I'm new to Haskell and doing some simple exercises. For this exercise I'm writing a function that checks whether one list is a subset of another list using recursion.

Here's the code:

subset [] xs = True

subset (x:xs) ys = if elem x ys == False then False
                   else subset (tail xs) (delete x ys)

I'm getting the following error message:

C:\Functioneel programmeren\week4.hs:9:43: error:
    Variable not in scope: delete :: t1 -> t t1 -> t t1

What does this even mean?

This program only works when I change "(delete x ys)" to "ys", but then it doesn't do what it's supposed to do.

Any help would be great, thanks in advance.

Answer 1

delete needs to be imported. According to https://www.haskell.org/hoogle/?hoogle=delete , you should add import Data.List to the beginning.

Besides, I guess your function subset is wrong. The result of subset [1, 2, 1] [1, 2, 3] is False .