Even without a method you can trigger the overlapping error. Consider the following program. class C a b c instance C [a] [a] [b] instance C [a] [b] [b] f :: C a b c => a -> b -> c -> Bool f _ _ _ = True g = f [True] [True] [True] The use of 'f' in 'g' produces the following message. Overlapping instances for D [Bool] [Bool] [Bool] arising from a use of âfâ Matching instances: instance D [a] [a] [b] instance D [a] [b] [b] ⢠In the expression: f [True] [True] [True] Cheers, Finn On 06/05/2021 23:05, Viktor Dukhovni wrote:
On Thu, May 06, 2021 at 10:51:32PM +0200, Henning Thielemann wrote:
{-# LANGUAGE MultiParamTypeClasses #-} module FlexibleInstance where
class C a b c where
This class has an empty interface, it just builds triples of types that imply a constraint. No matter how many such triples one creates, there can't be any conflict between them.
instance C [a] [a] [b] where instance C [a] [b] [b] where
These don't conflict.