#12882: Unexpected constraint when using ExistentialQuantification -------------------------------------+------------------------------------- Reporter: danilo2 | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: invalid | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by rwbarton): * priority: high => normal * status: new => closed * resolution: => invalid Comment:
The only difference between `tst1` and `tst2` is different pattern binding.
Different forms of pattern binding have different semantics. Here the pattern match is required to ensure that the argument was actually built with the `Elem` constructor, so that it contains a `Typeable a` dictionary. Since `tst2` ignores its argument, it should be fine to write `tst2 undefined`, but then where does the `Typeable` instance come from? In contrast, `tst1 undefined` clearly diverges. For the same reason, it doesn't make sense here to make `data Elem t = Typeable t => Elem Int` into a newtype, since it holds two pieces of data: a `Typeable t` dictionary and an `Int`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12882#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler