#15621: Error message involving type families points to wrong location -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.4.3 checker) | Resolution: | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
I don't agree with the details in comment:3.
Yes, I was abbreviating! What we get is {{{ -- From 'a' [W] fsk1 ~ alpha (CTyEqCan) [W] F Int ~ fsk1 (CFunEqCan) [W] fsk1 ~ alpha (CTyEqCan) -- From 'b' [W] fsk1 ~ beta (CTyEqCan) [W] fsk2 ~ beta (CTyEqCan) [W] F Bool ~ fsk2 (CFunEqCan) }}} Then from `[W] fsk1 ~ alpha` and `[W] fsk1 ~ beta` we get `[D] alpha ~ beta`. And that leads to `alpha := beta`. The `beta := F Bool` comes during unflattening. The purpose of the Derived constraints is precisely to lead to unifications, exactly as we do here. So we probalbly don't want to stop that happening. Why do we get `fsk1 ~ alpha` rather than `alpha ~ fsk1` followed by unification? See `Note [Canonical orientation for tyvar/tyvar equality constraints]` in `TcCanonical`, and `Note [Eliminate flat-skols]` in `TcType`. I think this ticket is closely related to Trac #14185. If we put the constraints from the two definitions into separate implication constraints (with no skolems and no givens), I think the Right Thing would happen. You'll see some half written code, commented out, around `alwaysBuildImplication` in `TcUnify`. I think that'd fix this. But I didn't have time to review all the error message wibbles -- probably many are improvements. Does anyone else want to have a go? The code is there! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15621#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler