#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 heisenbug): Replying to [comment:4 RyanGlScott]:
<SNIP>
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" }}}
You mean {{{#!hs uhOh :: ExpQ uhOh = lift "uhOh" }}}
Now `$(lift uhOh)` won't be an `Exp`, but rather a `String`. This is not at all what we want.
Totally agree. My question is when this happens in a `[p|($uhOh -> binding)|]` and this ends up in a splice, ultimately being compiled by GHC, then I'll get a type error "Expected `Exp`, got: `String`", correct?
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 :) That's good. Thanks! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14296#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler