{-# Language FunctionalDependencies #-}
{-# Language UndecidableInstances #-}

module Prova where

-- This is taken from the GHC manual on UndecidableInstances,
-- (“Coverage condition”) on the risks of turning UndecidableInstances
-- on.

class Mul a b c | a b -> c where
  (.*.) :: a -> b -> c

instance Mul Int Int Int where (.*.) = undefined
instance Mul Int Float Float where (.*.) = undefined
instance Mul a b c => Mul a [b] [c] where (.*.) = undefined

f b x y = if b then x .*. [y] else y

{-

prova.hs:17:23-25: error:
    • Reduction stack overflow; size = 201
      When simplifying the following type: Mul a0 [[c]] [c]
      Use -freduction-depth=0 to disable this check
      (any upper bound you could choose might fail unpredictably with
       minor updates to GHC, so disabling the check is recommended if
       you're sure that type checking should terminate)
    • In the expression: x .*. [y]
      In the expression: if b then x .*. [y] else y
      In an equation for ‘f’: f b x y = if b then x .*. [y] else y
-}

-- If I uncomment the next line, the example works.
-- f :: Mul a [b] b => Bool -> a -> b -> b

-- How is is that GHC is not able to infer the required `Mul a [b] b`
-- constraint?