Pre-Proposal: Introspective Template Haskell

Wed Nov 11 19:53:25 UTC 2015

If I understand this correctly it would also subsume haskell-src-exts. If
so I would definitely support this.
On Nov 11, 2015 2:21 PM, "Edward Kmett" <ekmett at gmail.com> wrote:

> In practice I find that almost every piece of template-haskell code I've
> written gets broken by something every other release of GHC, so it hasn't
> exactly been a shining beacon of backwards compatibility thus far.
>
> Invariably it is always missing _something_ that I need, and anything that
> ties it to a more canonical form like this would be a very good thing.
>
> I'd strongly support this move.
>
> A sample just from my current working directory:
>
> haskell> grep -r MIN_VERSION_template_haskell */src
>
> bifunctors/src/Data/Bifunctor/TH/Internal.hs:#if
> MIN_VERSION_template_haskell(2,10,0)
>
> bifunctors/src/Data/Bifunctor/TH/Internal.hs:#if
> MIN_VERSION_template_haskell(2,7,0)
>
> bifunctors/src/Data/Bifunctor/TH/Internal.hs:#if
> MIN_VERSION_template_haskell(2,10,0)
>
> bifunctors/src/Data/Bifunctor/TH/Internal.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> bifunctors/src/Data/Bifunctor/TH/Internal.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> bifunctors/src/Data/Bifunctor/TH/Internal.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> bifunctors/src/Data/Bifunctor/TH.hs:#ifndef MIN_VERSION_template_haskell
>
> bifunctors/src/Data/Bifunctor/TH.hs:#if __GLASGOW_HASKELL__ < 710 &&
> MIN_VERSION_template_haskell(2,8,0)
>
> bifunctors/src/Data/Bifunctor/TH.hs:#if MIN_VERSION_template_haskell(2,7,0)
>
> bifunctors/src/Data/Bifunctor/TH.hs:#if MIN_VERSION_template_haskell(2,7,0)
>
> bifunctors/src/Data/Bifunctor/TH.hs:#if MIN_VERSION_template_haskell(2,7,0)
>
> bifunctors/src/Data/Bifunctor/TH.hs:#if MIN_VERSION_template_haskell(2,7,0)
>
> bifunctors/src/Data/Bifunctor/TH.hs:#if MIN_VERSION_template_haskell(2,7,0)
>
> bifunctors/src/Data/Bifunctor/TH.hs:# if __GLASGOW_HASKELL__ >= 710 ||
> !(MIN_VERSION_template_haskell(2,8,0))
>
> bifunctors/src/Data/Bifunctor/TH.hs:#if MIN_VERSION_template_haskell(2,7,0)
>
> free/src/Control/Monad/Free/TH.hs:#if MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Control/Lens/Internal/FieldTH.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Control/Lens/Internal/TH.hs:#ifndef MIN_VERSION_template_haskell
>
> lens/src/Control/Lens/Internal/TH.hs:#define
> MIN_VERSION_template_haskell(x,y,z) (defined(__GLASGOW_HASKELL__) &&
> __GLASGOW_HASKELL__ >= 706)
>
> lens/src/Control/Lens/Internal/TH.hs:#if
> MIN_VERSION_template_haskell(2,9,0)
>
> lens/src/Control/Lens/Plated.hs:#if !(MIN_VERSION_template_haskell(2,8,0))
>
> lens/src/Control/Lens/TH.hs:#ifndef MIN_VERSION_template_haskell
>
> lens/src/Control/Lens/TH.hs:#define MIN_VERSION_template_haskell(x,y,z)
> (defined(__GLASGOW_HASKELL__) && __GLASGOW_HASKELL__ >= 706)
>
> lens/src/Control/Lens/TH.hs:#if !(MIN_VERSION_template_haskell(2,7,0))
>
> lens/src/Control/Lens/TH.hs:#if MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Control/Lens/TH.hs:#if !(MIN_VERSION_template_haskell(2,7,0))
>
> lens/src/Language/Haskell/TH/Lens.hs:#ifndef MIN_VERSION_template_haskell
>
> lens/src/Language/Haskell/TH/Lens.hs:#define
> MIN_VERSION_template_haskell(x,y,z) 1
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,9,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,9,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,9,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,9,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> !MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,9,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> !MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> !MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,9,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,9,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,9,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,9,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,8,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> !MIN_VERSION_template_haskell(2,10,0)
>
> lens/src/Language/Haskell/TH/Lens.hs:#if
> MIN_VERSION_template_haskell(2,9,0)
>
> linear/src/Linear/V.hs:#ifdef MIN_VERSION_template_haskell
>
> linear/src/Linear/V.hs:#if !(MIN_VERSION_reflection(1,3,0)) &&
> defined(MIN_VERSION_template_haskell)
>
> linear/src/Linear/V.hs:#if !(MIN_VERSION_reflection(1,3,0)) &&
> defined(MIN_VERSION_template_haskell)
>
> nats/src/Numeric/Natural.hs:#if defined(MIN_VERSION_hashable) ||
> defined(MIN_VERSION_template_haskell)
>
> nats/src/Numeric/Natural.hs:#ifdef MIN_VERSION_template_haskell
>
> nats/src/Numeric/Natural.hs:#ifdef MIN_VERSION_template_haskell
>
> tables/src/Data/Table.hs:#if MIN_VERSION_template_haskell(2,10,0)
>
> tables/src/Data/Table.hs:#if MIN_VERSION_template_haskell(2,9,0)
>
> tagged/src/Data/Proxy/TH.hs:#if MIN_VERSION_template_haskell(2,8,0)
>
> tagged/src/Data/Proxy/TH.hs:#if MIN_VERSION_template_haskell(2,8,0)
>
> tagged/src/Data/Proxy/TH.hs:#if MIN_VERSION_template_haskell(2,8,0)
>
> tagged/src/Data/Proxy/TH.hs:#if MIN_VERSION_template_haskell(2,8,0)
>
> On Wed, Nov 11, 2015 at 12:46 PM, Eric Seidel <eric at seidel.io> wrote:
>
>> I think backwards-compatibility is still a potential issue, not because
>> the pattern/type synonym layer seems implausible, but because I suspect
>> people will begin to sidestep the compatibility layer and just use the
>> GHC API (I certainly would). GHC is not shy about breaking
>> backwards-compatibility between major releases, so it seems possible
>> that this could extend to breaking TH. Missing features is not nearly as
>> bad as breaking most clients of TH.
>>
>> But perhaps this isn't a very likely scenario. TH mostly exports
>> datatypes for haskell syntax, smart constructors, and a few functions
>> for looking up metadata. I doubt these pieces of GHC change very often,
>> and when they do it's probably an extension rather than a breaking
>> change. Someone with more historical knowledge of GHC could comment :)
>>
>> All in all, I like this idea a lot!
>>
>> Eric
>>
>> On Wed, Nov 11, 2015, at 08:26, Richard Eisenberg wrote:
>> > Hi devs,
>> >
>> > There's a bunch of open tickets around Template Haskell. A great many of
>> > them are attempts to make TH more like what's already in GHC. Of course,
>> > when someone improves GHC, then TH also has to be updated. But this
>> > doesn't always happen, leading to several of these tickets.
>> >
>> > I believe I have a solution to the problem: just eliminate Template
>> > Haskell and provide direct access to GHC's internal structures. The idea
>> > (still very sketchy; hence pre-proposal) is like this (numbered for easy
>> > reference, but no order is implied):
>> >
>> > 1. TH quotes would remain. DsMeta would desugar quotes into Core code
>> > that produces HsExprs. For example, [| 1 |] would have type (Q (LHsExpr
>> > Name)). (Or perhaps (Q (LHsExpr RdrName)) if that works out better for
>> > clients.)
>> >
>> > 2. TH splices would remain, working much as they do now. The expression
>> > inside, say, an expression splice would have type (Q exp) where we can
>> > satisfy the constraint (SpliceExpr exp). There would be instances for
>> > (SpliceExpr (LHsExpr Name)) and (SpliceExpr (LHsExpr RdrName)) as well
>> as
>> > the non-located variants. Generalizing the type of expressions here
>> > allows users not to worry about un-renaming when roundtripping between
>> > quotes and splices.
>> >
>> > 3. Reification would remain, using an Info structure much like we have
>> > now. Would we expose the real GHC TyCons as the result of reification?
>> Or
>> > is it better to give the users HsDecls? This would need to be fleshed
>> > out.
>> >
>> > 4. Lifting would remain, doing the obvious thing.
>> >
>> > 5. The template-haskell package could even remain, as a
>> > backward-compatibility shim. It would declare gobs of pattern synonyms
>> > and type synonyms to wrap the new underlying interface. This re-imagined
>> > template-haskell package would not need to be a boot library, and could
>> > be upgraded separately from GHC. We could even maintain multiple
>> versions
>> > of the library so that TH clients wouldn't have to change their code
>> when
>> > GHC upgrades. Perhaps someday we could think about deprecating, if
>> that's
>> > the way the wind blows.
>> >
>> > So, the end result is a completely re-engineered TH, but I believe we
>> > could keep full backward compatibility. (I have not considered Typed TH
>> > in any depth yet. But my hope is that it's not too different from the
>> > above.) And, we're left with a significantly lower maintenance burden,
>> > especially if we eliminate template-haskell someday.
>> >
>> > And, tantalizingly, the flexibility in splices might allow us to splice
>> > in *Core* code someday. Perhaps we could also reify Core code. Then
>> > clients could write their own custom, domain-aware optimizations. Like
>> > RULES on steroids. But that's all for the next phase. (Giving due
>> credit,
>> > this last bit is inspired by work David Christiansen is doing in Idris.)
>> >
>> > What's wrong with this idea? I feel like *something* has to be more
>> > complicated than I've made it seem!
>> >
>> > Richard
>> > _______________________________________________
>> > ghc-devs mailing list
>> > ghc-devs at haskell.org
>> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>> _______________________________________________
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>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>
>
>
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