CODEWITHSUNDEEP
haskell
Connor Mills
Connor Mills

Reputation: 23

Eq on abstract data type giving error (haskell)

trying to remove an element from an abstract data type I made called Map in Haskell, I am using equality to see what key matches up in order to remove the element from the Map

data Map k v = Map {pairs::[(k, v)]}  deriving (Show)

removeElement :: Eq k -> Map k v -> Map k v
removeElement key (Map pairs) = Map [x |x <- pairs,not(fst x == key)]

gives me the error :

main.hs:33:18: error:
    * Expected a type, but `Eq k' has kind `Constraint'
    * In the type signature:
        removeElement :: Eq k -> Map k v -> Map k v
   |
33 | removeElement :: Eq k -> Map k v -> Map k v
   |                  ^^^^
<interactive>:3:1: error:
    * Variable not in scope: main
    * Perhaps you meant `min' (imported from Prelude)

from my understanding once establishing the Eq k, I don't have to anywhere else and I can't really comprehend what the compiler is trying to tell me, would appreciate help.

Upvotes: 2

Views: 58

Answers (1)

Owen
Owen

Reputation: 39356

In Haskell, Eq is not a type but is a typeclass. For typeclasses, you should use => rather than ->.

You also need to add in a k to stand for the argument you are calling key:

removeElement :: Eq k => k -> Map k v -> Map k v
removeElement key (Map pairs) = Map [x |x <- pairs,not(fst x == key)]

Note that there is no argument that corresponds to Eq k.

The compiler message, "Expected a type, but 'Eq k' has kind 'Constraint'", is basically saying that Eq k is not a type, but you're trying to use it like a type.

Upvotes: 6

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