Format:

{-# MINIMAL function1 | function2 #-}

Minimal pragmas are boolean expressions. For instance, with | as logical OR, either definition of the above functions must be defined. Comma indicates logical AND where both definitions must be defined.

See also: External Link

{-# LANGUAGE GADTs #-}

module Main where

main :: IO ()
main = 
        do
                print x

data MayBe' a 
        where
                Just' :: Eq a => a -> MayBe' a
                Nothing' :: Eq a => MayBe' a

class Eq' a where
    (===) :: a -> a -> Bool
    (/==) :: a -> a -> Bool
    (===) x y = not (x /== y)
    x /== y = not (x === y)
    {-# MINIMAL (===) | (/==) #-}


instance Eq' (MayBe' a)
        where
                -- (===) (Just' x) (Just' y) = x == y
                -- (/==) (Just' x) (Just' y) = x /= y

x = (Just' 2) === (Just' 3)

…provides the warning…

app\Main.hs:23:10: warning: [-Wmissing-methods]
    * No explicit implementation for
        either `===' or `/=='
    * In the instance declaration for `Eq' (MayBe' a)'
   |
23 | instance Eq' (MayBe' a)
   | 

…whereas this code…

instance Eq' (MayBe' a)
        where
                -- (===) (Just' x) (Just' y) = x == y
                (/==) (Just' x) (Just' y) = x /= y

x = (Just' 2) === (Just' 3)

…leads to the following output:

False