#10087: DefaultSignatures: error message mentions internal name -------------------------------------+------------------------------------- Reporter: andreas.abel | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.8.4 checker) | Resolution: | Keywords: Generics Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Other | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: infoneeded => new Comment: Thanks, Simon! [https://ghc.haskell.org/trac/ghc/ticket/10087?replyto=8#comment:7 This comment] helps tremendously in figuring out how things currently work. Replying to [comment:8 simonpj]:
So we need something like {{{ No instance for (Eq D) arising from the generic default method for `reflexive` In the instance declaration for ‘C D’ }}} Would that be about right?
I agree completely! I was confused because after reading [https://ghc.haskell.org/trac/ghc/ticket/10087?replyto=8#comment:1 Pedros' comment], I was under the impression he was implying that code should be legal. I probably misinterpreted it wildly and came to a very wrong conclusion.
Better perhaps to do what happens for non-generic default methods, which is to generate typechecked code directly (and emit some constraints). Compare what we do for the `Nothing` case of `DefMethInfo` in `tc_default` in `tcMethods` in `TcInstDcls`.
Does that make sense?
That seems sensible. Clearly, the mechanism we use for `VanillaDM` works well, because I've never seen an error message mention a name that begins with `$dm` :) If we modify the code that handles `GenericDM` to use the same tricks, that would probably make the error message much more palatable. We could also insert the phrase `"the generic default method for"` to make the origin of the issue clearer. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10087#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler