#15828: Type family equation foralls allow strange re-quantification of class-bound type variables -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.7 Keywords: TypeFamilies | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code typechecks on GHC HEAD (8.7+): {{{#!hs {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TypeFamilies #-} module Bug where import Data.Kind class C1 a where type T1 a b class C2 a where type T2 a b instance C1 (Maybe a) where type forall a b. T1 (Maybe a) b = b instance C2 (Maybe a) where type forall b. T2 (Maybe a) b = b }}} But ought it to? There is something funny happening in the `C1` instance: it's explicitly quantifying `a`, despite the fact that it had previously been quantified by the class head! Moreover, it appears that you can safely drop the `a` in the explicit `forall`, as the `C2 (Maybe a)` instance witnesses. What does the documentation have to say on this topic? This is all I could find:
When an explicit `forall` is present, all //type// variables mentioned must be bound by the `forall`.
I couldn't find anything on the interaction between this feature and associated type families. We should: 1. Decide which of the two programs above should be accepted, and 2. Update the documentation to reflect this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15828 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler