The compiler is now a bit more conservative in solving constraints
#14836: GHC fails to infer implied superclass constraint -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: duplicate | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #14417 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => closed * resolution: => duplicate * related: => #14417 Comment: 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 (Monad m) => RequiresMonad m where }}} Closing as a duplicate of #14417. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14836#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler