typeclasses1.hs

data EitherOr a b =
      Hello a
    | Goodbye b

instance (Eq a, Eq b) => Eq (EitherOr a b) where
    (==) (Hello a) (Hello a') =
        a == a'
    (==) (Goodbye b) (Goodbye b') =
        b == b'
    (==) _ _ = False

λ> Hello "a" == Hello "a"

<interactive>:38:1: warning: [-Wtype-defaults]
    • Defaulting the following constraint to type ‘()’
        Eq b0 arising from a use of ‘==’
    • In the expression: Hello "a" == Hello "a"
      In an equation for ‘it’: it = Hello "a" == Hello "a"

<interactive>:38:1: warning: [-Wtype-defaults]
    • Defaulting the following constraint to type ‘()’
        Eq b0 arising from a use of ‘==’
    • In the expression: Hello "a" == Hello "a"
      In an equation for ‘it’: it = Hello "a" == Hello "a"
True
*TypeClasses1
λ> (Hello "a" :: EitherOr String ()) == (Hello "a" :: EitherOr String ())
True