#10086: Disambiguate type variables using dictionaries -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.11 checker) | Keywords: Resolution: | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: Other | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by rwbarton):
In this example we call idTF with a Maybe Char argument, which means we are using Id (Maybe a) instance.
But you can't infer that because there could be another instance `TF (Either Int a) = Maybe a` that you can't see. Or put differently, adding a new (compatible) instance for a type family is not supposed to break any uses of that type family. If `TF` was a closed type family, then perhaps GHC could make this sort of inference. It amounts to checking injectivity "locally" for particular types as needed: i.e. check not that `TF x = y` has at most one solution for every `y`, but just for the particular `y` that is needed. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10086#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler