#8240: Better error messages for type family constraints ------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- Consider this constraint {{{ F Int ~ Bool -- Arising (say) on line 5 F Int ~ Char -- Arising (say) on line 10 }}} At the moment we'll combine the two to {{{ F Int ~ Bool -- Arising on line 5 Bool ~ Char -- Arising on line 10 }}} The second of these errors is "insoluble" and is reported first. But it's totally misleading: nowhere in the program did we directly need `Int~Char`. Another variant of this came up in Trac #8227. There we get the constraint {{{ Scalar (V a) ~ t0 Scalar (V a) ~ t0 -> t0 }}} which leads to {{{ Scalar (V a) ~ t0 -> t0 t0 ~ t0 -> t0 -- Occurs check }}} which gives the confusing error {{{ T8227.hs:16:44: Occurs check: cannot construct the infinite type: t0 ~ t0 -> t0 Expected type: Scalar (V (t0 -> t0)) Actual type: Scalar (V a) }}} I think the solution is * Do not combine two Wanteds {{{ [W] F tys ~ t1 [W] F tys ~ t2 }}} unless t1 and t2 are both touchable unification variables. * Instead (perhaps) generate a Derived constraint {{{ [D] t1 ~ t2 }}} to express the fact that t1 and t2 had better end up equal. * To do this the inert set would need to contain many `CTyFunEq` constraints with the same LHS. I'm not very sure about this, and no time now, so I'm just making this ticket to record the issue. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8240 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler