Proposal: liftData for Template Haskell

[Richard Eisenberg]

On May 13, 2015, at 6:13 AM, Merijn Verstraaten <merijn at inconsistent.nl> wrote:

> Is there any reason we can't have GHC derive Lift instance automatically? I know there's already a TH library for this, but I guess I don't see why GHC can't derive them for us. Additionally, the lack of lift instances is really a pain for a lot of compile time evaluation tricks.

GHC *could* do this automatically, but I don't think it *should*. `Lift` is far enough out of the way that I don't think there should be built-in compiler support for it. Doing it in a library is enough.

But, DeriveAnyClass + DefaultSignatures can make it feel like GHC is doing it for you. That is, if we have 

> class Lift a where
>  lift :: a -> Q Exp
>  default lift :: Data a => a -> Q Exp
>  lift = liftData


(as previously proposed) and you have -XDeriveAnyClass turned on, then you can say

> data Foo a = K1 Int Bool | K2 | K3 a
>   deriving Lift

and it should Just Work.

Richard


> 
> Cheers,
> Merijn
> 
>> On 13 May 2015, at 11:40, Edward Kmett <ekmett at gmail.com> wrote:
>> 
>> I'm very much +1 to the addition of liftData, but pretty strongly -1 on the notion of using an overlappable instance for Data a => Lift a. I really don't like encouraging that practice, orphans then start changing behavior in ways that make me deeply uncomfortable.
>> 
>> On the other hand, placing a default signature:
>> 
>> class Lift a where
>>  lift :: a -> Q Exp
>>  default lift :: Data a => a -> Q Exp
>>  lift = liftData
>> 
>> would be something I'd be 100% behind, in addition to adding the explicit liftData export.  It'd rather sharply reduce the pain of defining Lift instances and I was actually surprised it wasn't there.
>> 
>> -Edward
>> 
>> On Wed, May 13, 2015 at 2:28 AM, Gershom B <gershomb at gmail.com> wrote:
>> On May 13, 2015 at 2:15:26 AM, Ganesh Sittampalam (ganesh at earth.li) wrote:
>>> -1: I think that kind of instance (Foo a => Bar a) is generally quite
>>> problematic so there should be a pretty strong case to support it.
>>> 
>>> There can only be one of them - if some other class shows up that can
>>> also provide an equally good implementation of 'lift', there's a conflict.
>>> 
>>> Also won't people get misleading error messages, implying they should
>>> implement Data when Lift would do?
>>> 
>>> "instance Lift X where lift = liftData" doesn't seem too onerous to
>>> write by hand to me, though I guess it may be hard to discover that's an
>>> option.
>> 
>> Isn’t this a case where -XDefaultSignatures and -XDeriveAnyClass can make things at least a bit nicer? Of course we still need to document well how to use them, but such instrumentation of the Lift class would at least make it clear that it is “intended” to be used in such a fashion :-)
>> 
>> -g
>> 
>>> On 05/05/2015 04:36, Edward Z. Yang wrote:
>>>> Hello all,
>>>> 
>>>> It looks like people are opposed to doing with the lift type-class.
>>>> So here is a counterproposal: mark the Lift type class as overlappable,
>>>> and define an instance:
>>>> 
>>>> instance Data a => Lift a where
>>>> ...
>>>> 
>>>> This is fairly desirable, since GHC will sometimes generate a call to
>>>> 'lift', in which case liftData can't be manually filled in. People
>>>> can still define efficient versions of lift.
>>>> 
>>>> Edward
>>>> 
>>>> Excerpts from Edward Z. Yang's message of 2015-04-17 04:21:16 -0700:
>>>>> I propose adding the following function to Language.Haskell.TH:
>>>>> 
>>>>> -- | 'liftData' is a variant of 'lift' in the 'Lift' type class which
>>>>> -- works for any type with a 'Data' instance.
>>>>> liftData :: Data a => a -> Q Exp
>>>>> liftData = dataToExpQ (const Nothing)
>>>>> 
>>>>> I don't really know which submodule this should come from;
>>>>> since it uses 'dataToExpQ', you might put it in Language.Haskell.TH.Quote
>>>>> but arguably 'dataToExpQ' doesn't belong in this module either,
>>>>> and it only lives there because it is a useful function for defining
>>>>> quasiquoters and it was described in the quasiquoting paper.
>>>>> 
>>>>> I might propose getting rid of the 'Lift' class entirely, but you
>>>>> might prefer that class since it doesn't go through SYB (and have
>>>>> the attendant slowdown).
>>>>> 
>>>>> This mode of use of 'dataToExpQ' deserves more attention.
>>>>> 
>>>>> Discussion period: 1 month
>>>>> 
>>>>> Cheers,
>>>>> Edward
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