#14296: Add `Lift Exp` instance to `Language.Haskell.TH.Syntax` -------------------------------------+------------------------------------- Reporter: heisenbug | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Template Haskell | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: #14030 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): Let me try and articulate more precisely what this invariant is capturing. Here are some examples of the invariant at work: {{{#!hs {-# LANGUAGE TemplateHaskell #-} import Language.Haskell.TH.Syntax test :: IO () test = mapM_ print [ $(lift True) == True , $(lift "hello") == "hello" ] }}} These should all print `True`, and more generally, you should be able to stick in any other `Bool` or `String` and also have it print all `True`s. But it's easy to subvert this with your proposed `Lift Exp` instance, since you can have, e.g.: {{{#!hs uhOh :: Exp uhOh = lift "uhOh" }}} Now `$(lift uhOh)` won't be an `Exp`, but rather a `String`. This is not at all what we want. Replying to [comment:3 heisenbug]:
What about the derivation mechanism for `Lift` instances? Will it do ''the right thing'' in this case too?
Right. Here is an except of the `-ddump-deriv` output from deriving a `Lift` instance for `Exp`: {{{#!hs deriving instance Lift Exp ===> instance Lift Exp where lift (VarE a) = conE 'VarE `appE` lift a lift (ConE a) = conE 'ConE `appE` lift a lift (AppE f x) = conE 'AppE `appE` lift f `appE` lift x ... }}} Now calling `lift` on an `Exp` will produce an `ExpQ` that represents an `Exp` value. It's quite meta, granted, but that's how it should be :) In any case, I've been meaning to resolve #14030 for a while now, but haven't had much time to look into it recently. One obstacle is that deriving `Lift` instances for data types in `template-haskell` during stage-1 compilation is harder than it sounds because GHC HEAD moved around the locations of functions that are used when deriving `Lift` instances themselves. Therefore, I think I'll have to disable generating `Lift` instances on stage-1 compilers for now to make it work, but I've yet to figure out how to accomplish that. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14296#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler