#15298: Support spliced function names in type signatures in TH declaration quotes -------------------------------------+------------------------------------- Reporter: ntc2 | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: #11129 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- There doesn't seem to be a way to splice function names into type signatures in Template Haskell declaration quotes `[d|...|]`. For example, `fDecl1` below does not work. According to [https://stackoverflow.com/a/32279198/470844 this StackOverflow answer], the approach in `fDecl2` below used to work, but it doesn't work with recent GHCs and is much less readable than `fDecl1`. {{{#!hs {-# LANGUAGE TemplateHaskell #-} import Language.Haskell.TH fName :: Name fName = mkName "f" fTy :: TypeQ fTy = [t| Int |] fBody :: ExpQ fBody = [e| 3 |] -- | Not allowed: -- -- @ -- error: -- Invalid type signature: $fName :: ... -- Should be of form <variable> :: <type> -- @ -- -- Similarly, using @$(varP fName) :: $fTy@ fails with an analogous -- error. fDecl1 :: DecsQ fDecl1 = [d| $fName :: $fTy $(varP fName) = $fBody |] -- | Not allowed: -- -- @ -- error: -- Splices within declaration brackets not (yet) handled by Template Haskell -- @ fDecl2 :: DecsQ fDecl2 = [d| $((:[]) <$> sigD fName fTy) $(varP fName) = $fBody |] }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15298 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler