#12787: Weird type constraint with undecidable instances -------------------------------------+------------------------------------- Reporter: nome | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: Resolution: invalid | UndecidableInstances Operating System: Linux | Architecture: x86_64 Type of failure: GHC rejects | (amd64) valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nome): Thank you for the explanations. For the record: After some more reading (and a lot more thinking) on this topic, I realize that a) type classes are much weirder than I thought, and that b) the language and syntax of contexts is pretty misleading. It looks as though they're something like type-level guards (i.e. a precondition), but they're really constraints on polymorphic types ''resulting'' from applying a type class to a type: {{{#!hs type family constraintPartOfPartialOrd a :: Constraint type instance constraintPartOfPartialOrd a = TotalOrd a type instance constraintPartOfPartialOrd [a] = Eq a }}} ... put like this, it's clear why the deduced constraint is always `TotalOrd a`. So the instance syntax is really the wrong way around, and should instead be something like {{{#!hs instance PartialOrd a = TotalOrd a => tryCompare x y = Just $ tcompare x y }}} or maybe {{{#!hs instance PartialOrd a where type constraint TotalOrd a tryCompare x y = Just $ tcompare x y }}} because constraints are "attached" to the value-level part of a type class instance in pretty much the same way as [https://wiki.haskell.org/GHC/Type_families#An_associated_type_synonym_exampl... associated type synonyms]. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12787#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler