#13258: GHC head 8.1.20170209 does not compile package 'mono-traversable' -------------------------------------+------------------------------------- Reporter: clinton | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #13249 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => closed * resolution: => invalid * related: => #13249 Comment: This is expected behavior. GHC 8.2 tightened up validity checking for default type signatures. This catches many things which are just plain type incorrect (e.g., see #12918). A consequence of this is that you can't write default type signatures quite as liberally as before, but luckily, it's an extremely easy fix: {{{#!hs class MonoFunctor mono where omap :: (Element mono -> Element mono) -> mono -> mono default omap :: (Functor f, Element (f a) ~ a, f a ~ mono) => (Element mono -> Element mono) -> mono -> mono omap = fmap {-# INLINE omap #-} }}} The benefit is now the connection between the default and non-default type signatures is much clearer. That is, the default type signature is exactly the same as the non-default one, just with more constraints. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13258#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler