#11115: Indicate missing associated type instances -------------------------------------+------------------------------------- Reporter: rimmington | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If there is no matching instance for a typeclass that defines an associated type, GHC cannot further reduce an application of that associated type. For nested type family applications, this produces rather large error messages, eg: {{{ <interactive>:98:20: Couldn't match type ‘units-2.3:Data.Metrology.Factor.Normalize (Data.Metrology.Suspicious.Poly.SurfaceUnitFactorsOf (Data.Singletons.Prelude.Tuple.Snd (Data.Metrology.Suspicious.Poly.SplitPrefix (PrefixExp Deci) Deci Core)) units-2.3:Data.Metrology.Factor.@- '['units-2.3:Data.Metrology.Factor.F Core ('Data.Metrology.Z.S 'Data.Metrology.Z.Zero)])’ with ‘'[]’ In the expression: (25 %> centi Core) `ceilToUnit` deci Core In an equation for ‘it’: it = (25 %> centi Core) `ceilToUnit` deci Core }}} The cause of the above error message is that `Deci` is not a member of the typeclass `ExpUnitPrefix` that defines the associated type `PrefixExp`. Unfortunately this is not clear at all from the error message, and adding additional class constraints to the appropriate functions doesn't improve things. It would be nice if GHC pointed out that there is no `ExpUnitPrefix Deci` instance, or at least that `PrefixExp Deci` cannot be further reduced. I imagine this would be useful for open type families as well, but I can't comment personally. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11115 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler