#10429: GHC fails to import instance -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- The following code compiles in 7.8.4 but fails to compile in 7.10.1. Tagged.hs {{{#!hs module Tagged where import Control.Monad.Trans newtype TaggedT s m b = TagT (m b) instance MonadTrans (TaggedT s) }}} Main.hs {{{#!hs import Control.Monad.Trans.Class import Tagged returnT :: (MonadTrans t) => t m a -> t m a returnT a = undefined f :: TaggedT Int m a -> TaggedT Int m a f = returnT }}} GHC complains {{{ Could not deduce (MonadTrans (TaggedT Int)) arising from a use of ‘returnT’ }}} If I change the import in Main to `Control.Monad.Trans`, 7.10.1 accepts the program. Since the import `Control.Monad.Trans` is actually a folder, the class `MonadTrans` must refer to the definition in `Control.Monad.Trans.Class`, so GHC should find the instance. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10429 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler