Huge hack to support explicit numeric literals in Haskell

NumericLits.hs

{- TODO: The Num and Fractional instances don't implement all methods.
 - In practice this is fine but for completeness they should be done.
 -}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}
module NumericLits where

data I = I
i :: I
i = I

instance (a ~ Int) => Num (I -> a) where
  fromInteger = const . fromInteger

data F = F
f :: F
f = F

instance (a ~ Float) => Num (F -> a) where
  fromInteger = const . fromInteger

instance (a ~ Float) => Fractional (F -> a) where
  fromRational = const . fromRational

data D = D
d :: D
d = D

instance (a ~ Double) => Num (D -> a) where
  fromInteger = const . fromInteger

instance (a ~ Double) => Fractional (D -> a) where
  fromRational = const . fromRational

usage.hs

λ 0i
0
λ :t 0i
0i :: Int
λ :t 0f
0f :: Float
λ :t 0.2f
0.2f :: Float
λ :t 0d
0d :: Double
λ :t 0.4d
0.4d :: Double
λ 0i + 5
5
λ :t 0i + 5
0i + 5 :: Int
λ 7.2f * 8.3
59.76
λ :t 7.2f * 8.3
7.2f * 8.3 :: Float
λ 8.4f * 6.3d

<interactive>:15:8: error:
    • Couldn't match type ‘Float’ with ‘Double’
        arising from the literal ‘6.3’
    • In the expression: 6.3
      In the second argument of ‘(*)’, namely ‘6.3 d’
      In the expression: 8.4 f * 6.3 d