#12540: RFC: Allow not quantifying every top-level quantifiee -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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: -------------------------------------+------------------------------------- Minor nuisance, does this happen to anyone else? Currently Haskell has an all-nothing policy on quantified type variables, either you quantify none or all (fine, let's ignore non-prenex quantification `foo :: a -> forall b. b -> a`). Can we have a top-level quantification of only a subset of the free variables? Say I'm working on a function {{{#!hs reflected :: forall s m a. (Applicative m, Reifies s a) => TaggedT s m a }}} and I need to make ''s'' a scoped type variable, I always accidentally write {{{#!hs reflected :: forall s. (Applicative m, Reifies s a) => TaggedT s m a reflected = TagT . pure . reflect $ (Proxy :: Proxy s) }}} This causes GHC to complain that the other types — ''m'', ''a'' — are not in scope so and I have to add the remaining quantifiees I don't really care about (may have long names as well). It could be worse (''dramatization'') {{{#!hs ipartsOf :: forall i p f s t a. (Indexable [i] p, Functor f) => Traversing (Indexed i) f s t a a -> Over p f s t [a] [a] ipartsOf l = conjoined (\f s -> let b = inline l sell s in outs b <$> f (wins b)) (\f s -> let b = inline l sell s; (is, as) = unzip (pins b) in outs b <$> indexed f (is :: [i]) as) appendAssocAxiom :: forall p q r as bs cs. p as -> q bs -> r cs -> Dict ((as ++ (bs ++ cs)) ~ ((as ++ bs) ++ cs)) appendAssocAxiom _ _ _ = unsafeCoerce (Dict :: Dict (as ~ as)) }}} It would be nice to be nice to only have to specify the type one is interested in: {{{#!hs ipartsOf :: forall i. (Indexable [i] p, Functor f) => Traversing (Indexed i) f s t a a -> Over p f s t [a] [a] appendAssocAxiom :: forall as. p as -> q bs -> r cs -> Dict ((as ++ (bs ++ cs)) ~ ((as ++ bs) ++ cs)) }}} and have the others chosen in some way. This is not just useful for writing, but it makes it easier to read: If I see a type `forall i p f s t a. ...` any of them may appear in the function body, if I see a type `forall i. ...` I know only one is. Thoughts? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12540 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler