NumberDispatch.hs

{-# LANGUAGE ConstraintKinds #-}
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE TypeApplications #-}

module NumberDispath where

data Number
    = DoubleNum !Double
    | IntegerNum !Integer
    | IntNum !Int
    | RationalNum !Rational

withNumber :: (c Double, c Int, c Integer, c Rational) => (forall a. (c a) => a -> r) -> Number -> r
withNumber f (DoubleNum x) = f x
withNumber f (IntegerNum x) = f x
withNumber f (IntNum x) = f x
withNumber f (RationalNum x) = f x

-- that one works (compiles and runs)
a = withNumber' @Show show (RationalNum 3)

-- that one does not compiles
b = withNumber' @Integral toInteger (IntegerNum 3)