#15334: (forall x. c x, forall x. d x) is not equivalent to forall x. (c x, d x) -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.5 checker) | Keywords: Resolution: | QuantifiedConstraints Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Read [https://github.com/Gertjan423/ghc-proposals/blob/quantified- constraints/proposals/0000-quantified-constraints.rst the proposal] esp Section 3.2 Would you write {{{ instance (Functor (T a), Traversable (T a), Foldable (T a_)) where ... }}} No! An instance declaration is for a class, and takes the form {{{ instance blah => C t1 .. tn where ... }}} where `C` is a class. Same with quantified constraints, as the syntax (tries to) make clear. I suppose that it's accepted because `(,,)` is a class but really I think it should be rejected. In fact {{{ instance (Eq (T a), Ord (T a)) where {} }}} is not rejected out of hand, but elicits {{{ * No instance for (Ord (T a)) arising from the superclasses of an instance declaration * In the instance declaration for `(Eq (T a), Ord (T a))' }}} which is pretty confusing. I think it's because `(c1, c2)` has superclasses `c1` and `c2`. My conclusion: both in top-level and quantified instances, we should reject a tuple in the head. OK? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15334#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler