Hi, Am Dienstag, den 23.07.2013, 18:58 +0000 schrieb Simon Peyton-Jones:
If you add -XIncoherentInstances *just to the module that has instance IsNT a a*, then it'll work fine I think. This says "pick this instance even though an instantiation of the constraint might match a more specific instance". You don't need the flag in importing modules.
That does not seem to be the case: Enabling the flag just for the special "IsNT a a" instance does not work: Prelude GHC.NT> :t castNT :: (Either Int a) -> (Either Age a) <interactive>:1:1: Overlapping instances for IsNT a1 a1 arising from a use of ‛castNT’ Matching instances: instance [incoherent] IsNT a a -- Defined at <interactive>:12:10 instance (IsNT a a', IsNT b b') => IsNT (Either a b) (Either a' b') -- Defined at <interactive>:7:1 (The choice depends on the instantiation of ‛a1’ To pick the first instance above, use -XIncoherentInstances when compiling the other instance declarations) In the expression: castNT :: (Either Int a) -> (Either Age a) (as correctly specified by the error message.) But now the solution is easy to see: When deriving IsNT, simply set the incoherent flag for every instance, independent of any active pragmas. Greetings, Joachim -- Joachim “nomeata” Breitner mail@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org