#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): However, it should be noted that a derived `Lift` instance for `Exp` would not produce the code above. The problem with that instance is that if `x :: Exp`, then `$(lift x)` would not necessarily equal `x` (or even be of type `Exp`!), which is a rule you'd generally expect to hold. (I realize this law isn't started anywhere in the Haddocks at the moment, but it probably should be.) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14296#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#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

#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): Replying to [comment:5 heisenbug]:

You mean {{{#!hs uhOh :: ExpQ uhOh = lift "uhOh" }}}

Oops. That's right.

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?

Sure, GHC will definitely catch the mistake in the form of a (sometimes hard-to-scrutinize) type error. But I would still be frustrated that that happened in the first place, since it would violate my intuition of how `lift` interacts with splicing. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14296#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#14296: Add `Lift Exp` instance to `Language.Haskell.TH.Syntax` -------------------------------------+------------------------------------- Reporter: heisenbug | Owner: (none) Type: feature request | Status: closed Priority: normal | Milestone: Component: Template Haskell | Version: 8.2.1 Resolution: duplicate | 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 Ben Gamari ): In [changeset:"626f0454ef1ca8f40c38064197dba97a36d52dbb/ghc" 626f045/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="626f0454ef1ca8f40c38064197dba97a36d52dbb" Document a law for TH's Lift class Inspired by the discussion in #14296, I've decided to document a law which is usually in the back of my mind when I'm using Template Haskell's `Lift` class, but isn't formally stated anywhere. That is, every `Lift` instance should satisfy (for all `x`): ```lang=haskell $(lift x) == x ``` Test Plan: Read it Reviewers: austin, goldfire, bgamari Subscribers: rwbarton, thomie Differential Revision: https://phabricator.haskell.org/D4050 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14296#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler