2. 'tcSubType' does not generate errors, just postponed constraints, which are checked later in a clean-up phase. Thus, I do not get an error now I could handle, but the error is generated later during the constraint solving phase, where I do not have a handler in
#9582: Associated Type Synonyms do not unfold in InstanceSigs -------------------------------------+------------------------------------- Reporter: | Owner: andreas.abel | Status: patch Type: bug | Milestone: Priority: normal | Version: 7.8.3 Component: Compiler | Keywords: InstanceSigs (Type checker) | TypeFamilies Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: | Related Tickets: None/Unknown | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by simonpj): I'm sorry about being slow to reply -- utterly swamped. place. This is dead right, and I think it's a huge strength of GHC's current type inference mechanism. Essentially all type error messages are generated in `TcErrors`, not in places scattered over the compiler. This is not a "clean-up phase". It is ''the'' place that type errors are generated. For `tcSubType`, the way to control your error messages is through the `CtOrigin` and `UserTypeCtxt` passed to `tcSubType`. The `UserTypeCtx`t is attached (via the ic_info field of `Implication`) to the implication constraint generated by the `tcSubType`. If you want, you can add an extra constructor to `UserTypeCtxt`. The `CtOrigin` is attached to the instantiated constraints generated by the sub-type check. Both are accessible to the error-message generation in `TcErrors`. In short, instead of catching the error(s) ''now'', GHC just attaches enough information to the constraints that you can generate a good error message later. I hope that is of some help. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9582#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler