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.

I was under the impression that that was a built-in function in Haskell: hackage.haskell.org/package/base-4.9.1.0/docs/… — Meurth
– Meurth, Commented May 30, 2017 at 14:17
By the way, your if can be rewritten in a more readable form as elem x ys && subset .... Usually, we tend to avoid ==False and if..then False in Haskell. Also tail xs looks wrong, are you sure you don't want xs instead ? — chi
– chi, Commented May 30, 2017 at 15:03

crvv · Accepted Answer · 2017-05-30 14:50:36Z

4

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.

edited May 30, 2017 at 14:50

answered May 30, 2017 at 14:20

crvv

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4 Comments

Meurth Over a year ago

Thanks, I didn't know I had to import it. I think the result of that should be false though, since there is only a single 1 in the second list in your example

crvv Over a year ago

I don't know what should subset do, so I just guess it. I guessed wrong.

Polygnome Over a year ago

@Meurth A set can, mathematically speaking, not contain duplicates. In that case, sublist would probably the better name for your function, since lists/sequences can contain duplicates. from a mathematical point of view, {1, 1, 1} equals {1, 1} equals {1}.

Meurth Over a year ago

@Polygnome Thanks for the reply, I suppose my teacher could learn something from that! :D

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