#14292: Coercing between constraints of newtypes -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by AntC): Replying to [comment:5 AntC]:
If the module has `IncoherentInstances`, do both those instances get accepted even without the pragmas? (Or perhaps a more devious scenario where one of the instances is imported.)
Yes, here we go: {{{ {-# LANGUAGE GeneralizedNewtypeDeriving, StandaloneDeriving, FlexibleInstances, IncoherentInstances #-} main = print $ MkN (7 :: Int) + MkN 5 newtype N a = MkN a deriving (Show) instance Num a => Num (N a) where MkN x + MkN y = MkN (x - y) deriving instance Num (N Int) }}} Without `IncoherentInstances` the `MkN 7 + MkN 5` is rejected: overlapping instances [good! that's what I want]. With `IncoherentInstances`, this is accepted and prints `12`. If I switch round the instance decls such that the hand-crafted one is for `Num (N Int)` and the derived is for `Num (N a)`, this prints `2`. Ugh! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14292#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler