The compiler is now a bit more conservative in solving constraints
#14417: Overly specific instances may not be resolved -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler (Type | Version: 8.3 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): This is expected behavior. From the [https://downloads.haskell.org/~ghc/8.0.1/docs/html/users_guide/8.0.1-notes.h... GHC 8.0.1 release notes]: previously provided by superclasses (see Trac #11762). For instance, consider this program,:
{{{#!hs {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-}
class Super a class (Super a) => Left a class (Super a) => Right a instance (Left a) => Right a -- this is now an error }}}
GHC now rejects this instance, claiming it cannot deduce the `Super a`
superclass constraint of the `Right` typeclass. This stands in contrast to previous releases, which would accept this declaration, using the `Super a` constraint implied by the `Left a` constraint. To fix this simply add the needed superclass constraint explicitly,
{{{#!hs instance (Left a, Super a) => Right a }}}
In other words, it's a limitation that was imposed after the introduction of `UndecidableSuperClasses`. So from GHC's perspective, the correct thing to do here is to use this instance declaration instead: {{{#!hs instance A x => C x }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14417#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler