#10532: Unnecessary constraints when using overlapping instances -------------------------------------+------------------------------------- Reporter: MikeIzbicki | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 7.10.1 checker) | Keywords: Resolution: invalid | Architecture: Operating System: Unknown/Multiple | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by MikeIzbicki): I understand the differences between `-XOverlappingInstances` and `-XIncoherentInstances`, and my question is not about that. My question is just about syntax. Without `-XOverlappingInstances`, GHC interprets the types `a -> Int` and `Foo a => a -> Int` as the same thing. Whenever you can use one, you can also use the other (at least in all the cases I tried). But without overlapping instances, GHC no longer interprets these types the same way. Only the second type is valid for `f`, but I would rather write the first type because I know GHC can find an instance `Foo` for all `a`. The particular instance will depend on the weird interactions caused by overlapping instances, but there's guaranteed to be an instance available. My question is: Why is this syntactic difference useful? Maybe parametricity would be broken without it? ---- Even if this syntactic difference is useful, I still think there is a GHC bug. Consider what happens in the overlapping instances case when you omit the type signature for `f = foo`. GHC infers the type `a -> Int`, but then complains that the class constraint `Foo` can't be found. GHC should instead infer the type `Foo a => a -> Int`, which would let us omit the type signature. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10532#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler