#14745: Functional dependency conflicts in givens -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 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: -------------------------------------+------------------------------------- Consider this {{{ {-# LANGUAGE FlexibleContexts, MultiParamTypeClasses, FunctionalDependencies #-} module FunDep where class C a b c | a -> b c instance C Int Bool Char f :: (C Int b c) => a -> c f = undefined }}} When doing the ambiguity check we effectively ask whether this would typecheck {{{ g :: (C Int b c) => a -> c g = f }}} We instantiate `f`'s type, and try to solve from `g`'s type signature. So we end up with {{{ [G] d1: C Int b c [W] d2: C Int beta c }}} Now, from the fundeps we get {{{ Interact d1 with the instance: [D] b ~ Bool, [D] c ~ Char Ineract d2 with the instance: [D] beta ~ Bool, [D] c ~ Char Interact d1 with d2 [D] beta ~ b }}} What is annoying is that if we unify `beta := b`, we can solve the [W] constraint from [G], leaving only [D] constraints which we don't even always report (see discussion on #12466). But if, randomly, we instead unify `beta := Bool`, we get an insoluble constraint `[W] C Int Bool c`, which we report. So whether or not typechecking succeeds is rather random; very unsatisfactory. What is really wrong? Well, that Given constraint `(C Int b c)` is in conflict with the top-level instance decl. Maybe we should fail if that happens? But see #12466... and `Note [Given errors]` in `TcErrors`. The test program in Trac #13651 is just like this, only with type functions rather than type classes. I'm not sure what to do, but I'm leaving this ticket as a record that all is not well. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14745> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler