Pre-Proposal: Introspective Template Haskell

Wed Nov 11 19:55:56 UTC 2015

Well, as of 7.10.2 the GHC AST no longer has bottoms in it, so it can
already be used quite easily

On Wed, Nov 11, 2015 at 9:53 PM, Thomas Bereknyei <tomberek at gmail.com>
wrote:

> 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
>>> _______________________________________________
>>> ghc-devs mailing list
>>> ghc-devs at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>>
>>
>>
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>>
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