CODEWITHSUNDEEP
classhaskellambiguous
user3210273
user3210273

Reputation: 1

Ambiguous class occurence in Haskell

Using Trees and defining a new "Eq" I'm getting an "Ambiguous class occurrence 'Eq' *** Could refer to: Hugs.Prelude.Eq Main.Eq".

I'm aware that I'm trying to add a new definition for the existing Eq-Class of the prelude, but I don't want to use import prelude hiding (Eq), because my new equality is using the "==" for numerical types. The Operator is called "=+", because it is not a real equality (which I think is already 'loaded' via deriving), but only a structural one.

data Tree = Nil | Node Int Tree Tree deriving (Eq, Ord, Show)
instance Eq Tree where
    Nil =+ Nil      =  true
    (Node a tl1 tr1) =+ (Node b tl2 tr2) = (a==b) && (tl1==tl2) && (tl1==tl2)

I'd be thankful for any suggestions.

Upvotes: 0

Views: 423

Answers (3)

MathematicalOrchid
MathematicalOrchid

Reputation: 62818

Why are you deriving Eq if you want to write a custom one? Just drop Eq from the deriving clause, and this should all work.

Upvotes: 0

leftaroundabout
leftaroundabout

Reputation: 120711

If you want a new class to coexist with a predefined one, you should obviously give it a properly distinguishable name, e.g.

class PseudoEq x where
  (=+) :: x -> x -> Bool

instance PseudoEq Tree where
    Nil            =+ Nil            =  true
    Node a tl1 tr1 =+ Node b tl2 tr2 
          = a==b && tl1==tl2 && tl1==tl2

Simply qualifying the instance with Main.Eq does pretty much the same thing, but it's confusing to call your class Eq at all if it's "not real equality".

(Though == also doesn't need to be a proper equality; for many types this only means "equivalent in all regards the user can observe".)

Upvotes: 3

Zeta
Zeta

Reputation: 105886

Just use fully qualified class names:

module Main where

data Tree = Nil | Node Int Tree Tree deriving (Prelude.Eq, Ord, Show)

class Eq a where
  (=+) :: a -> a -> Bool

instance Main.Eq Tree where
    Nil =+ Nil      =  True
    (Node a tl1 tr1) =+ (Node b tl2 tr2) = (a==b) && (tl1==tl2) && (tl1==tl2)

But don't do that

You shouldn't create such ambiguity in first place. For example, your Eq should be called StructEq instead, if it's "only a structural one".

Upvotes: 2

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