#15632: Undependable Dependencies -------------------------------------+------------------------------------- Reporter: AntC | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.6.1-beta1 Keywords: | Operating System: Unknown/Multiple FunctionalDependencies, | OverlappingInstances | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: 10675 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider {{{#!hs {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, GADTs, FlexibleInstances, FlexibleContexts, UndecidableInstances #-} data Hither = Hither deriving (Eq, Show, Read) -- just three data Thither = Thither deriving (Eq, Show, Read) -- distinct data Yon = Yon deriving (Eq, Show, Read) -- types class Whither a b | a -> b where whither :: a -> b -> b -- instance Whither Int Hither where -- rejected: FunDep conflict -- whither _ y = y -- with Thither instance, so instance {-# OVERLAPPING #-} (b ~ Hither) => Whither Int b where whither _ y = y instance {-# OVERLAPS #-} Whither a Thither where whither _ y = y instance {-# OVERLAPPABLE #-} (b ~ Yon) => Whither a b where whither _ y = y f :: Whither Int b => Int -> b -> b f = whither g :: Whither Bool b => Bool -> b -> b g = whither }}} Should those three instances be accepted together? In particular, the published work on FDs (including the FDs through CHRs paper) says the `Thither` instance should behave as if: {{{#!hs instance (beta ~ Thither) => Whither a beta where ... }}} (in which `beta` is a unification variable and the `~` constraint is type improvement under the FD.) But now the instance head is the same as the `Yon` instance, modulo alpha renaming; with the constraints being contrary. That's demonstrated by the inferred/improved type for `g`: {{{#!hs *Whither> :t g ===> g :: Bool -> Thither -> Thither -- and yet *Whither> g True Yon ===> Yon }}} What do I expect here? * At least the `Thither` and `Yon` instances to be rejected as inconsistent under the FunDep. * (The `Hither` instance is also inconsistent, going by the strict interpretation in published work. But GHC's rule here is bogus, as documented in Trac #10675.) Exploiting Overlapping instances is essential to making this go wrong. The published work shies away from considering `FunDeps + Overlap`; and yet specifying the semantics as if the instances used bare unification variables is almost inevitably going give overlaps -- especially if there are multiple FDs. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15632 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler