another variadic uncurry

Olaf.hs

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE IncoherentInstances #-}
{-# LANGUAGE OverlappingInstances #-}
{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
module Olaf where

{- |


>>> eval (+) (1,2) :: Int
3

>>> eval (+) 1 2 :: Int
3

>>> eval (+) 1 (2 :: Int)
3

>>> eval (+) (1 :: Int) 2
3

>>> eval (,,) (1,2) 3
(1,2,3)

>>> eval (,,) 1 (2,3) :: (Int,Int,Int)
(1,2,3)


-- the wrong instance is chosen:

>>> eval (+) (1 :: Int,2)
...
...No instance for (Show ((Int, ...) -> (Int, ...)))
...

>>> eval (,,) 1 (2,3)
...
...No instance for (Show (... -> ...
...

-}


class Uncurry f r where
  eval :: f -> r

instance (a ~ a') => Uncurry a a' where
  eval = id

instance (a ~ a', Uncurry f (b -> c)) => Uncurry (a -> f) ((a',  b) -> c) where
  eval f (a,b) = eval (f a) b

instance (a ~ a', Uncurry f c) => Uncurry (a -> f) (a' -> c) where
  eval f a = eval (f a)