#13608: Expose the type of quasiquotes -------------------------------------+------------------------------------- Reporter: | Owner: facundo.dominguez | facundo.dominguez Type: bug | Status: patch Priority: normal | Milestone: Component: Template Haskell | Version: 8.0.1 Resolution: | Keywords: QuasiQuotes Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: 12778 | Differential Rev(s): Phab:D3610 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
Looks like typed splices and quasiquotes will pose some gotchas.
Ah, yes, I understand what's happening here, and (once again) it's awkward. GHC has to compile ''and run'' the term inside the splice, here `$$(q)`. But since `q :: forall a. C a => Q (TExp a)`, looking at `$$(q)` in isolation we just see that `q` has type `Q (TExp alpha)` with constraint `C alpha`, but we don't know what `alpha` is. It'll ultimately be fixed by the `asTypeOf (0::Int)` part, but not yet. If you change it to `$$(q) :: Int` then it does work because the information about the `Int` type is pushed inwards from the type signature. That is horribly delicate, and I had not realised it before. The robust way to do it would be `$$(q :: Q (TExp Int))`, putting all the type info inside the splice. This doesn't happen for untyped splices because they don't expect to get any type info from the context. I think I should probably ''stop'' pushing type info from the context into a typed splice, so that it would fail reliably. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13608#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler