#1830: Automatic derivation of Lift -------------------------------------+------------------------------------- Reporter: guest | Owner: | RyanGlScott Type: feature request | Status: new Priority: normal | Milestone: ⊥ Component: Template Haskell | Version: 6.8.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: Phab:D1168 -------------------------------------+------------------------------------- Changes (by RyanGlScott): * owner: => RyanGlScott * differential: => Phab:D1168 Comment: I have a differential ( Phab:D1168 ) submitted which implements this feature, but I would like some feedback. I'll copy over my questions from the Phabricator page: 1. What should we do if someone attempts to derive `Lift` on a datatype with no constructors? I (somewhat arbitrarily) chose it to generate code like this: {{{#!hs data Void deriving Lift instance Lift Void where lift _ = appE (varE 'error) (litE (stringL "Void lift")) }}} but {{{lift _ = error "Void lift"}}} would also work. 2. How should primitive data types be handled? I adapted the approach `Show` takes, which has special cases for `Char#`, `Float#`, `Double#`, `Int#`, and `Word#`. Note that `template-haskell` also has a `StringPrimL` constructor, so we could handle `Addr#` as well, but there's not currently a function of type `String -> [Word8]` in `base` that I could use in conjunction with it ([http://haddock.stackage.org/lts-3.2/ghc-7.10.2/SMRep.html#v:stringToWord8s there is in] `ghc`, but I'm not sure if it's verboten to put GHC internals in derived code). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/1830#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler