Haskell :add a type signature that fixes these type variables(s)?

Question

I try to get symmetric difference of two lists:

such as:

  [1,2,3] [2,3,5] 

the result is [1]

This is my code:

getlist _  []=[]
getlist [] _ = []
getlist (x:xs) ys = if isexist x ys
             then x++ getlist xs ys
                   else
                      getlist xs ys
isexist a [] = False
isexist a (x:xs) = if a == x
          then True
          else isexist a xs

There is an error:

The type variable 'a0' is ambiguous

Possible fix: add a type signature that fixes these type variable(s)

What does that mean? How can I fix it?

Answer 1

To fix the error all you have to do as @Joker_vD pointed out is to replace your ++ with :

To understand the difference between the two you should look at the type signature:

(:) :: a -> [a] -> [a]
(++) :: [a] -> [a] -> [a]

As you could see, : takes an element and a list and returns a list back. ++ on the other hand takes two lists.

You should look at Data.Set and Data.List packages if you want to do set intersections / diff more efficiently.

Note that after fixing the error, the program's output will not match what you wanted. You should investigate why that's happening and should be an easy fix.