#16070: Better inferred signatures -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 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 (taken from [https://github.com/ghc-proposals/ghc- proposals/pull/158#issuecomment-448549030 here]) {{{ class FieldType x r ~ a => HasField (x :: k) r a where type FieldType x r getField :: r -> a setField :: r -> a -> r f r = setField @"bar" r (getField @"foo" r) }}} We get this type inferred for `f`: {{{ f :: forall r. (HasField "bar" r (FieldType "bar" r), HasField "foo" r (FieldType "bar" r)) => r -> r }}} But actually this signature is equivalent {{{ f :: forall r a. (HasField "bar" r a, HasField "foo" r a) => r -> r }}} and it's quite a bit shorter. Similarly, from this definition {{{ g r = setField r }}} We get this inferred type {{{ g :: forall r p. HasField "bar" r (FieldType "bar" r) => p -> r -> FieldType "bar" r -> r }}} where again we might prefer {{{ g :: forall r a. HasField "bar" r a => r -> a -> r }}} '''Question''': could we make GHC infer the briefer types? After all, it does some similar abbreviation with type classes. Where we could infer `(Ord a, Eq a)` we collapse out the `Eq a` and just infer the context `(Ord a)`. There is nothing record-specific about this ticket. It's just about using inferred equality constraints to simplify the inferred signature. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16070 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler