#12164: Type signatures in patterns not (yet) handled by Template Haskell -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: feature | Status: new request | Priority: normal | Milestone: Component: Template | Version: 8.0.1 Haskell | 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: -------------------------------------+------------------------------------- This bit me just now, so I figured I should report a feature request bug. {{{ ezyang@sabre:~/Dev/whisk$ ghc-8.0 --interactive -XTemplateHaskell GHCi, version 8.0.0.20160204: http://www.haskell.org/ghc/ :? for help Prelude> Language.Haskell.TH.runQ [| \(x :: ()) -> x |] <interactive>:1:29: error: Type signatures in patterns not (yet) handled by Template Haskell x :: () }}} In the code, there is a bit of mumbling about scoped type variables. I believe it is referring to code like this: {{{ Prelude> :t (\(x :: a) -> (x :: a)) (\(x :: a) -> (x :: a)) :: a -> a }}} Described in the manual here: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts... #pattern-type-signatures This means that TH has to somehow bring the type into scope. I don't actually know how to do it. It seems a bit awful for this to be the reason why these quotes are not accepted; maybe without scoped type variables we should be able to do something cheap and cheerful. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12164 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler