Proposal: Automatic derivation of Lift

[Matthew Pickering]

Continuing my support of the generics route. Is there a fundamental
reason why it couldn't handle unlifted types? Given their relative
paucity, it seems like a fair compromise to generically define lift
instances for all normal data types but require TH for unlifted types.
This approach seems much smoother from a maintenance perspective.

On Mon, Sep 7, 2015 at 5:26 PM, Ryan Scott <ryan.gl.scott at gmail.com> wrote:
> There is a Lift typeclass defined in template-haskell [1] which, when
> a data type is an instance, permits it to be directly used in a TH
> quotation, like so
>
>     data Example = Example
>
>     instance Lift Example where
>       lift Example = conE (mkNameG_d "<package-name>" "<module-name>" "Example")
>
>     e :: Example
>     e = [| Example |]
>
> Making Lift instances for most data types is straightforward and
> mechanical, so the proposal is to allow automatic derivation of Lift
> via a -XDeriveLift extension:
>
>     data Example = Example deriving Lift
>
> This is actually a pretty a pretty old proposal [2], dating back to
> 2007. I wanted to have this feature for my needs, so I submitted a
> proof-of-concept at the GHC Trac issue page [3].
>
> The question now is: do we really want to bake this feature into GHC?
> Since not many people opined on the Trac page, I wanted to submit this
> here for wider visibility and to have a discussion.
>
> Here are some arguments I have heard against this feature (please tell
> me if I am misrepresenting your opinion):
>
> * We already have a th-lift package [4] on Hackage which allows
> derivation of Lift via Template Haskell functions. In addition, if
> you're using Lift, chances are you're also using the -XTemplateHaskell
> extension in the first place, so th-lift should be suitable.
> * The same functionality could be added via GHC generics (as of GHC
> 7.12/8.0, which adds the ability to reify a datatype's package name
> [5]), if -XTemplateHaskell can't be used.
> * Adding another -XDerive- extension places a burden on GHC devs to
> maintain it in the future in response to further Template Haskell
> changes.
>
> Here are my (opinionated) responses to each of these:
>
> * th-lift isn't as fully-featured as a -XDerive- extension at the
> moment, since it can't do sophisticated type inference [6] or derive
> for data families. This is something that could be addressed with a
> patch to th-lift, though.
> * GHC generics wouldn't be enough to handle unlifted types like Int#,
> Char#, or Double# (which other -XDerive- extensions do).
> * This is a subjective measurement, but in terms of the amount of code
> I had to add, -XDeriveLift was substantially simpler than other
> -XDerive extensions, because there are fewer weird corner cases. Plus,
> I'd volunteer to maintain it :)
>
> Simon PJ wanted to know if other Template Haskell programmers would
> find -XDeriveLift useful. Would you be able to use it? Would you like
> to see a solution other than putting it into GHC? I'd love to hear
> feedback so we can bring some closure to this 8-year-old feature
> request.
>
> Ryan S.
>
> -----
> [1] http://hackage.haskell.org/package/template-haskell-2.10.0.0/docs/Language-Haskell-TH-Syntax.html#t:Lift
> [2] https://mail.haskell.org/pipermail/template-haskell/2007-October/000635.html
> [3] https://ghc.haskell.org/trac/ghc/ticket/1830
> [4] http://hackage.haskell.org/package/th-lift
> [5] https://ghc.haskell.org/trac/ghc/ticket/10030
> [6] https://ghc.haskell.org/trac/ghc/ticket/1830#comment:11
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