#14897: QuantifiedConstraints: Can't print type of quantified constraint -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.5 Resolution: invalid | Keywords: | QuantifiedConstraints, wipT2893 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 simonpj): * status: new => closed * resolution: => invalid Comment: What `:t e` does is to infer the type of the expression `e` and print it. So with `a :: (forall xx. Monoid (f xx)) => f a`, it's as if you'd typed the top level decl {{{ foo = a }}} What happens? We instantiate the type of `a` to get a Wanted constraint {{{ [W] forall x. Monoid (f1 x) }}} and the type `f1 a1`, where `f1` and `a1` are unification variables. Now we try to simplify the constraints and infer the most general type for `foo`. Unsurprisingly, we fail with the reported error message. If, rather than inferring the type of an arbitrary ''expression'', you want to ask for the type (and other info) about an ''identifier'', use `:info a`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14897#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler