Define EQ Instance-CodePudding

I am trying to create an Eq Instance, but I have some issues creating it.

I have the following data definitions:

data Link = G | S | P deriving (Eq, Show)
data Chain = Empty | Join Link Chain

And Now I want to convert Chain into an Eq Instance (True when it is the same).

Here is my current instance:

instance Eq Chain where
(Join a b) == (Join c d) = (a == c) && (b == d)
Empty      == Empty      = True
_          == _          = False

it marks (a == c) && (b == d)as mistake (Ambiguous occurrence ‘==’)

I am unsure why this is, and would appreciate it if someone could help me.

Thank you in advance!

CodePudding user response：

You failed to indent the method definitions.

instance Eq Chain where
↓↓
  Join a b == Join c d = a==c && b==d
  Empty    == Empty    = True
  _        == _        = False
↑↑

...Or alternatively, but highly dis-recommended, put them in braces

instance Eq Chain where {
Join a b == Join c d = a==c && b==d;
Empty    == Empty    = True;
_        == _        = False
}

Without the indentation or braces, what your code does is declare an empty Eq instance, and a separate, completely new operator that happens to be also called ==. I.e., you actually wrote this:

module Ch where

instance Eq Chain where
  -- missing definitions of `Prelude.==` here

(Ch.==) :: Chain -> Chain -> Bool
Join a b Ch.== Join c d = a==c && b==d
Empty    Ch.== Empty    = True
_        Ch.== _        = False

The compiler error tells you that the a==c and b==d definitions are now ambiguous: is this referring to Prelude.== or to Ch.==?