Re: Usage of Template Haskell quotes in GHC source tree vs. usage of GHC as a library

[Simon Peyton Jones]

Gergo

I'm not close enough to this to have a well-formed opinion, but it looks
like a good question to me, esp if the new dependence on TH is optional.

Would you like to transfer the text into a GHC ticket?

Simon

On Wed, 12 Jul 2023 at 04:40, Gergő Érdi  wrote:

> Hi,
>
>
>
> A recent commit 983ce55815f2dd57f84ee86eee97febf7d80b470 starts using
> TemplateHaskellQuotes in the GHC codebase. It seems this is at odds with
> using GHC as a library, a la ghc-lib.
>
>
>
> The `ghc-lib` approach is to basically take the module hierarchy from the
> `compiler/` subtree, and compile it as a completely vanilla Haskell
> library, with no direct attachment to the host GHC version. This enables
> using e.g. GHC 9.4 to compile a program using the GHC 9.6 API, and so on.
> In particular, it also makes it very easy to apply patches to the version
> of GHC used as a library, since in this setup it doesn’t need to be able to
> bootstrap.
>
>
>
> So what is the problem with using TemplateHaskellQuotes? The problem is
> the dependency on the template-haskell package. When a module inside
> GHC-as-a-library containing TH quotes is compiled, the quotes are
> translated into applications of the constructors defined by the *host*
> GHC’s TH package. But because GHC is tightly coupled to the TH support
> library, GHC-as-a-library needs to ship with its own internal version of
> the library. So the code that tries to process the results of these quotes
> is using the *target* GHC’s TH definitions. And that leads to a conflict:
> code like
>
>
>
> leftName :: Namel
>
> leftName = ‘Left
>
>
> is now a type mismatch between the type of `’Left` being
> template-haskell-2.19.0.0:Language.Haskell.TH.Syntax.Name (example when
> using GHC 9.4.5 as the host) and the type of `leftName` being
> ghc-lib-9.9.20230712:Language.Haskell.TH.Syntax.Name (example when the
> target version is built from recent `master`).
>
>
>
> Currently, `ghc-lib-gen` has a pre-processing step on the GHC source tree
> that replaces these quotations with applications containing direct
> references to the target TH constructors:
> https://github.com/digital-asset/ghc-lib/blob/ab01fb2b4d1e3a9338390e9c10ccd769bbf37aeb/ghc-lib-gen/src/Ghclibgen.hs#L419-L467
> but I am worried that this is very fragile.
>
>
>
> So any ideas on how to tackle this situation better?
>
>
>
> Thanks,
>
> Gergo
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Can't build nofib

[Simon Peyton Jones]

Friends

With a clean HEAD I can't build nofib.  See below.  What should I do?

Thanks

Simon

(cd nofib; cabal v2-run -- nofib-run
--compiler=`pwd`/../_build/stage1/bin/ghc --output=`date -I`)
Resolving dependencies...
Error: cabal: Could not resolve dependencies:
[__0] trying: nofib-0.1.0.0 (user goal)
[__1] next goal: base (dependency of nofib)
[__1] rejecting: base-4.18.0.0/installed-4.18.0.0 (conflict: nofib =>
base>=4.5 && <4.17)
[__1] skipping: base-4.18.0.0, base-4.17.1.0, base-4.17.0.0 (has the same
characteristics that caused the previous version to fail: excluded by
constraint '>=4.5 && <4.17' from 'nofib')
[__1] rejecting: base-4.16.4.0, base-4.16.3.0, base-4.16.2.0, base-4.16.1.0,
base-4.16.0.0, base-4.15.1.0, base-4.15.0.0, base-4.14.3.0, base-4.14.2.0,
base-4.14.1.0, base-4.14.0.0, base-4.13.0.0, base-4.12.0.0, base-4.11.1.0,
base-4.11.0.0, base-4.10.1.0, base-4.10.0.0, base-4.9.1.0, base-4.9.0.0,
base-4.8.2.0, base-4.8.1.0, base-4.8.0.0, base-4.7.0.2, base-4.7.0.1,
base-4.7.0.0, base-4.6.0.1, base-4.6.0.0, base-4.5.1.0, base-4.5.0.0,
base-4.4.1.0, base-4.4.0.0, base-4.3.1.0, base-4.3.0.0, base-4.2.0.2,
base-4.2.0.1, base-4.2.0.0, base-4.1.0.0, base-4.0.0.0, base-3.0.3.2,
base-3.0.3.1 (constraint from non-upgradeable package requires installed
instance)
[__1] fail (backjumping, conflict set: base, nofib)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: base, nofib

make: *** [/home/simonpj/code/Makefile-spj:39: nofib] Error 1
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Re: Can't build nofib

[Andreas Klebinger]

Try adding --allow-newer we probably just haven't updated the bounds yet.

(cd nofib; cabal v2-run --allow-newer -- nofib-run
--compiler=`pwd`/../_build/stage1/bin/ghc --output=`date -I`)

Am 12/07/2023 um 13:38 schrieb Simon Peyton Jones:

Friends

With a clean HEAD I can't build nofib.  See below.  What should I do?

Thanks

Simon

(cd nofib; cabal v2-run -- nofib-run
--compiler=`pwd`/../_build/stage1/bin/ghc --output=`date -I`)
Resolving dependencies...
Error: cabal: Could not resolve dependencies:
[__0] trying: nofib-0.1.0.0 (user goal)
[__1] next goal: base (dependency of nofib)
[__1] rejecting: base-4.18.0.0/installed-4.18.0.0 (conflict: nofib =>
base>=4.5 && <4.17)
[__1] skipping: base-4.18.0.0, base-4.17.1.0, base-4.17.0.0 (has the same
characteristics that caused the previous version to fail: excluded by
constraint '>=4.5 && <4.17' from 'nofib')
[__1] rejecting: base-4.16.4.0, base-4.16.3.0, base-4.16.2.0,
base-4.16.1.0,
base-4.16.0.0, base-4.15.1.0, base-4.15.0.0, base-4.14.3.0, base-4.14.2.0,
base-4.14.1.0, base-4.14.0.0, base-4.13.0.0, base-4.12.0.0, base-4.11.1.0,
base-4.11.0.0, base-4.10.1.0, base-4.10.0.0, base-4.9.1.0, base-4.9.0.0,
base-4.8.2.0, base-4.8.1.0, base-4.8.0.0, base-4.7.0.2, base-4.7.0.1,
base-4.7.0.0, base-4.6.0.1, base-4.6.0.0, base-4.5.1.0, base-4.5.0.0,
base-4.4.1.0, base-4.4.0.0, base-4.3.1.0, base-4.3.0.0, base-4.2.0.2,
base-4.2.0.1, base-4.2.0.0, base-4.1.0.0, base-4.0.0.0, base-3.0.3.2,
base-3.0.3.1 (constraint from non-upgradeable package requires installed
instance)
[__1] fail (backjumping, conflict set: base, nofib)
After searching the rest of the dependency tree exhaustively, these
were the
goals I've had most trouble fulfilling: base, nofib

make: *** [/home/simonpj/code/Makefile-spj:39: nofib] Error 1


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Re: Can't build nofib

[Rodrigo Mesquita]

From the error message it looks like you’re using ghc-9.6(and base 4.18) while 
nofib requires base < 4.17.
I’d say as a temporary workaround you can likely run your invocation 
additionally with —allow-newer, and hope that doesn’t break. Otherwise you 
could downgrade to 9.4 or bump the version manually in the cabal file of nofib?

Rodrigo

> On 12 Jul 2023, at 12:38, Simon Peyton Jones  
> wrote:
> 
> Friends
> 
> With a clean HEAD I can't build nofib.  See below.  What should I do?
> 
> Thanks
> 
> Simon
> 
> (cd nofib; cabal v2-run -- nofib-run 
> --compiler=`pwd`/../_build/stage1/bin/ghc --output=`date -I`)
> Resolving dependencies...
> Error: cabal: Could not resolve dependencies:
> [__0] trying: nofib-0.1.0.0 (user goal)
> [__1] next goal: base (dependency of nofib)
> [__1] rejecting: base-4.18.0.0/installed-4.18.0.0 (conflict: nofib =>
> base>=4.5 && <4.17)
> [__1] skipping: base-4.18.0.0, base-4.17.1.0, base-4.17.0.0 (has the same
> characteristics that caused the previous version to fail: excluded by
> constraint '>=4.5 && <4.17' from 'nofib')
> [__1] rejecting: base-4.16.4.0, base-4.16.3.0, base-4.16.2.0, base-4.16.1.0,
> base-4.16.0.0, base-4.15.1.0, base-4.15.0.0, base-4.14.3.0, base-4.14.2.0,
> base-4.14.1.0, base-4.14.0.0, base-4.13.0.0, base-4.12.0.0, base-4.11.1.0,
> base-4.11.0.0, base-4.10.1.0, base-4.10.0.0, base-4.9.1.0, base-4.9.0.0,
> base-4.8.2.0, base-4.8.1.0, base-4.8.0.0, base-4.7.0.2, base-4.7.0.1,
> base-4.7.0.0, base-4.6.0.1, base-4.6.0.0, base-4.5.1.0, base-4.5.0.0,
> base-4.4.1.0, base-4.4.0.0, base-4.3.1.0, base-4.3.0.0, base-4.2.0.2,
> base-4.2.0.1, base-4.2.0.0, base-4.1.0.0, base-4.0.0.0, base-3.0.3.2,
> base-3.0.3.1 (constraint from non-upgradeable package requires installed
> instance)
> [__1] fail (backjumping, conflict set: base, nofib)
> After searching the rest of the dependency tree exhaustively, these were the
> goals I've had most trouble fulfilling: base, nofib
> 
> make: *** [/home/simonpj/code/Makefile-spj:39: nofib] Error 1
> 
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Re: Can't build nofib

[Simon Peyton Jones]

Thanks.  That is very unfortunate: ./configure does not issue any complaint.

I upgraded from 9.2 because GHC won't compile with 9.2 any more.  But now
you are saying that nofib won't build with 9.6?  So that leaves 9.4 only.

Well I can install 9.4 and rebuild everything.  But really, it would be
good if configure complained if you are using a boot compiler that won't
work.  That's what configure is for!

Simon

On Wed, 12 Jul 2023 at 12:41, Rodrigo Mesquita 
wrote:

> From the error message it looks like you’re using ghc-9.6(and base 4.18)
> while nofib requires base < 4.17.
> I’d say as a temporary workaround you can likely run your invocation
> additionally with —allow-newer, and hope that doesn’t break. Otherwise you
> could downgrade to 9.4 or bump the version manually in the cabal file of
> nofib?
>
> Rodrigo
>
> On 12 Jul 2023, at 12:38, Simon Peyton Jones 
> wrote:
>
> Friends
>
> With a clean HEAD I can't build nofib.  See below.  What should I do?
>
> Thanks
>
> Simon
>
> (cd nofib; cabal v2-run -- nofib-run
> --compiler=`pwd`/../_build/stage1/bin/ghc --output=`date -I`)
> Resolving dependencies...
> Error: cabal: Could not resolve dependencies:
> [__0] trying: nofib-0.1.0.0 (user goal)
> [__1] next goal: base (dependency of nofib)
> [__1] rejecting: base-4.18.0.0/installed-4.18.0.0 (conflict: nofib =>
> base>=4.5 && <4.17)
> [__1] skipping: base-4.18.0.0, base-4.17.1.0, base-4.17.0.0 (has the same
> characteristics that caused the previous version to fail: excluded by
> constraint '>=4.5 && <4.17' from 'nofib')
> [__1] rejecting: base-4.16.4.0, base-4.16.3.0, base-4.16.2.0,
> base-4.16.1.0,
> base-4.16.0.0, base-4.15.1.0, base-4.15.0.0, base-4.14.3.0, base-4.14.2.0,
> base-4.14.1.0, base-4.14.0.0, base-4.13.0.0, base-4.12.0.0, base-4.11.1.0,
> base-4.11.0.0, base-4.10.1.0, base-4.10.0.0, base-4.9.1.0, base-4.9.0.0,
> base-4.8.2.0, base-4.8.1.0, base-4.8.0.0, base-4.7.0.2, base-4.7.0.1,
> base-4.7.0.0, base-4.6.0.1, base-4.6.0.0, base-4.5.1.0, base-4.5.0.0,
> base-4.4.1.0, base-4.4.0.0, base-4.3.1.0, base-4.3.0.0, base-4.2.0.2,
> base-4.2.0.1, base-4.2.0.0, base-4.1.0.0, base-4.0.0.0, base-3.0.3.2,
> base-3.0.3.1 (constraint from non-upgradeable package requires installed
> instance)
> [__1] fail (backjumping, conflict set: base, nofib)
> After searching the rest of the dependency tree exhaustively, these were
> the
> goals I've had most trouble fulfilling: base, nofib
>
> make: *** [/home/simonpj/code/Makefile-spj:39: nofib] Error 1
>
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Re: Can't build nofib

[Rodrigo Mesquita]

I would recommend —allow-newer rather than rebuilding with 9.4. In retrospect, 
9.4 implies base == 4.17, but nofib seems to only allow < 4.17, which would 
leave 9.4 out.

Rodrigo

> On 12 Jul 2023, at 12:48, Simon Peyton Jones  
> wrote:
> 
> Thanks.  That is very unfortunate: ./configure does not issue any complaint.
> 
> I upgraded from 9.2 because GHC won't compile with 9.2 any more.  But now you 
> are saying that nofib won't build with 9.6?  So that leaves 9.4 only.
> 
> Well I can install 9.4 and rebuild everything.  But really, it would be good 
> if configure complained if you are using a boot compiler that won't work.  
> That's what configure is for!
> 
> Simon
> 
> On Wed, 12 Jul 2023 at 12:41, Rodrigo Mesquita  > wrote:
>> From the error message it looks like you’re using ghc-9.6(and base 4.18) 
>> while nofib requires base < 4.17.
>> I’d say as a temporary workaround you can likely run your invocation 
>> additionally with —allow-newer, and hope that doesn’t break. Otherwise you 
>> could downgrade to 9.4 or bump the version manually in the cabal file of 
>> nofib?
>> 
>> Rodrigo
>> 
>>> On 12 Jul 2023, at 12:38, Simon Peyton Jones >> > wrote:
>>> 
>>> Friends
>>> 
>>> With a clean HEAD I can't build nofib.  See below.  What should I do?
>>> 
>>> Thanks
>>> 
>>> Simon
>>> 
>>> (cd nofib; cabal v2-run -- nofib-run 
>>> --compiler=`pwd`/../_build/stage1/bin/ghc --output=`date -I`)
>>> Resolving dependencies...
>>> Error: cabal: Could not resolve dependencies:
>>> [__0] trying: nofib-0.1.0.0 (user goal)
>>> [__1] next goal: base (dependency of nofib)
>>> [__1] rejecting: base-4.18.0.0/installed-4.18.0.0 (conflict: nofib =>
>>> base>=4.5 && <4.17)
>>> [__1] skipping: base-4.18.0.0, base-4.17.1.0, base-4.17.0.0 (has the same
>>> characteristics that caused the previous version to fail: excluded by
>>> constraint '>=4.5 && <4.17' from 'nofib')
>>> [__1] rejecting: base-4.16.4.0, base-4.16.3.0, base-4.16.2.0, base-4.16.1.0,
>>> base-4.16.0.0, base-4.15.1.0, base-4.15.0.0, base-4.14.3.0, base-4.14.2.0,
>>> base-4.14.1.0, base-4.14.0.0, base-4.13.0.0, base-4.12.0.0, base-4.11.1.0,
>>> base-4.11.0.0, base-4.10.1.0, base-4.10.0.0, base-4.9.1.0, base-4.9.0.0,
>>> base-4.8.2.0, base-4.8.1.0, base-4.8.0.0, base-4.7.0.2, base-4.7.0.1,
>>> base-4.7.0.0, base-4.6.0.1, base-4.6.0.0, base-4.5.1.0, base-4.5.0.0,
>>> base-4.4.1.0, base-4.4.0.0, base-4.3.1.0, base-4.3.0.0, base-4.2.0.2,
>>> base-4.2.0.1, base-4.2.0.0, base-4.1.0.0, base-4.0.0.0, base-3.0.3.2,
>>> base-3.0.3.1 (constraint from non-upgradeable package requires installed
>>> instance)
>>> [__1] fail (backjumping, conflict set: base, nofib)
>>> After searching the rest of the dependency tree exhaustively, these were the
>>> goals I've had most trouble fulfilling: base, nofib
>>> 
>>> make: *** [/home/simonpj/code/Makefile-spj:39: nofib] Error 1
>>> 
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>> 

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Re: Can't build nofib

[Andreas Klebinger]

The nofib master branch has updated bounds. It's just ghcs submodule
that's lagging behind.

Am 12/07/2023 um 13:49 schrieb Rodrigo Mesquita:

I would recommend —allow-newer rather than rebuilding with 9.4. In
retrospect, 9.4 implies base == 4.17, but nofib seems to only allow <
4.17, which would leave 9.4 out.

Rodrigo


On 12 Jul 2023, at 12:48, Simon Peyton Jones
 wrote:

Thanks.  That is very unfortunate: ./configure does not issue any
complaint.

I upgraded from 9.2 because GHC won't compile with 9.2 any more.  But
now you are saying that nofib won't build with 9.6? So that leaves
9.4 only.

Well I can install 9.4 and rebuild everything.  But really, it would
be good if configure complained if you are using a boot compiler that
won't work.  That's what configure is for!

Simon

On Wed, 12 Jul 2023 at 12:41, Rodrigo Mesquita
 wrote:

From the error message it looks like you’re using ghc-9.6(and
base 4.18) while nofib requires base < 4.17.
I’d say as a temporary workaround you can likely run your
invocation additionally with —allow-newer, and hope that doesn’t
break. Otherwise you could downgrade to 9.4 or bump the version
manually in the cabal file of nofib?

Rodrigo


On 12 Jul 2023, at 12:38, Simon Peyton Jones
 wrote:

Friends

With a clean HEAD I can't build nofib.  See below.  What should
I do?

Thanks

Simon

(cd nofib; cabal v2-run -- nofib-run
--compiler=`pwd`/../_build/stage1/bin/ghc --output=`date -I`)
Resolving dependencies...
Error: cabal: Could not resolve dependencies:
[__0] trying: nofib-0.1.0.0 (user goal)
[__1] next goal: base (dependency of nofib)
[__1] rejecting: base-4.18.0.0/installed-4.18.0.0 (conflict:
nofib =>
base>=4.5 && <4.17)
[__1] skipping: base-4.18.0.0, base-4.17.1.0, base-4.17.0.0 (has
the same
characteristics that caused the previous version to fail:
excluded by
constraint '>=4.5 && <4.17' from 'nofib')
[__1] rejecting: base-4.16.4.0, base-4.16.3.0, base-4.16.2.0,
base-4.16.1.0,
base-4.16.0.0, base-4.15.1.0, base-4.15.0.0, base-4.14.3.0,
base-4.14.2.0,
base-4.14.1.0, base-4.14.0.0, base-4.13.0.0, base-4.12.0.0,
base-4.11.1.0,
base-4.11.0.0, base-4.10.1.0, base-4.10.0.0, base-4.9.1.0,
base-4.9.0.0,
base-4.8.2.0, base-4.8.1.0, base-4.8.0.0, base-4.7.0.2,
base-4.7.0.1,
base-4.7.0.0, base-4.6.0.1, base-4.6.0.0, base-4.5.1.0,
base-4.5.0.0,
base-4.4.1.0, base-4.4.0.0, base-4.3.1.0, base-4.3.0.0,
base-4.2.0.2,
base-4.2.0.1, base-4.2.0.0, base-4.1.0.0, base-4.0.0.0,
base-3.0.3.2,
base-3.0.3.1 (constraint from non-upgradeable package requires
installed
instance)
[__1] fail (backjumping, conflict set: base, nofib)
After searching the rest of the dependency tree exhaustively,
these were the
goals I've had most trouble fulfilling: base, nofib

make: *** [/home/simonpj/code/Makefile-spj:39: nofib] Error 1

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Re: Can't build nofib

[Simon Peyton Jones]

>
> I would recommend —allow-newer rather than rebuilding with 9.4. In
> retrospect, 9.4 implies base == 4.17, but nofib seems to only allow < 4.17,
> which would leave 9.4 out.


Oh wow -- so there is *no* version of GHC that works as a bootstrap GHC.
That seems bad.  Very bad.

I see a patch from Andreas -- maybe that fixes it?

Meanwhile, --allow-newer I guess.

Simon

On Wed, 12 Jul 2023 at 12:50, Rodrigo Mesquita 
wrote:

> I would recommend —allow-newer rather than rebuilding with 9.4. In
> retrospect, 9.4 implies base == 4.17, but nofib seems to only allow < 4.17,
> which would leave 9.4 out.
>
> Rodrigo
>
> On 12 Jul 2023, at 12:48, Simon Peyton Jones 
> wrote:
>
> Thanks.  That is very unfortunate: ./configure does not issue any
> complaint.
>
> I upgraded from 9.2 because GHC won't compile with 9.2 any more.  But now
> you are saying that nofib won't build with 9.6?  So that leaves 9.4 only.
>
> Well I can install 9.4 and rebuild everything.  But really, it would be
> good if configure complained if you are using a boot compiler that won't
> work.  That's what configure is for!
>
> Simon
>
> On Wed, 12 Jul 2023 at 12:41, Rodrigo Mesquita <
> rodrigo.m.mesqu...@gmail.com> wrote:
>
>> From the error message it looks like you’re using ghc-9.6(and base 4.18)
>> while nofib requires base < 4.17.
>> I’d say as a temporary workaround you can likely run your invocation
>> additionally with —allow-newer, and hope that doesn’t break. Otherwise you
>> could downgrade to 9.4 or bump the version manually in the cabal file of
>> nofib?
>>
>> Rodrigo
>>
>> On 12 Jul 2023, at 12:38, Simon Peyton Jones 
>> wrote:
>>
>> Friends
>>
>> With a clean HEAD I can't build nofib.  See below.  What should I do?
>>
>> Thanks
>>
>> Simon
>>
>> (cd nofib; cabal v2-run -- nofib-run
>> --compiler=`pwd`/../_build/stage1/bin/ghc --output=`date -I`)
>> Resolving dependencies...
>> Error: cabal: Could not resolve dependencies:
>> [__0] trying: nofib-0.1.0.0 (user goal)
>> [__1] next goal: base (dependency of nofib)
>> [__1] rejecting: base-4.18.0.0/installed-4.18.0.0 (conflict: nofib =>
>> base>=4.5 && <4.17)
>> [__1] skipping: base-4.18.0.0, base-4.17.1.0, base-4.17.0.0 (has the same
>> characteristics that caused the previous version to fail: excluded by
>> constraint '>=4.5 && <4.17' from 'nofib')
>> [__1] rejecting: base-4.16.4.0, base-4.16.3.0, base-4.16.2.0,
>> base-4.16.1.0,
>> base-4.16.0.0, base-4.15.1.0, base-4.15.0.0, base-4.14.3.0, base-4.14.2.0,
>> base-4.14.1.0, base-4.14.0.0, base-4.13.0.0, base-4.12.0.0, base-4.11.1.0,
>> base-4.11.0.0, base-4.10.1.0, base-4.10.0.0, base-4.9.1.0, base-4.9.0.0,
>> base-4.8.2.0, base-4.8.1.0, base-4.8.0.0, base-4.7.0.2, base-4.7.0.1,
>> base-4.7.0.0, base-4.6.0.1, base-4.6.0.0, base-4.5.1.0, base-4.5.0.0,
>> base-4.4.1.0, base-4.4.0.0, base-4.3.1.0, base-4.3.0.0, base-4.2.0.2,
>> base-4.2.0.1, base-4.2.0.0, base-4.1.0.0, base-4.0.0.0, base-3.0.3.2,
>> base-3.0.3.1 (constraint from non-upgradeable package requires installed
>> instance)
>> [__1] fail (backjumping, conflict set: base, nofib)
>> After searching the rest of the dependency tree exhaustively, these were
>> the
>> goals I've had most trouble fulfilling: base, nofib
>>
>> make: *** [/home/simonpj/code/Makefile-spj:39: nofib] Error 1
>>
>> ___
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>>
>>
>>
>
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RE: [External] Re: Usage of Template Haskell quotes in GHC source tree vs. usage of GHC as a library

[Erdi, Gergo via ghc-devs]

PUBLIC

https://gitlab.haskell.org/ghc/ghc/-/issues/23647

From: ghc-devs  On Behalf Of Simon Peyton Jones
Sent: Wednesday, July 12, 2023 7:25 PM
To: Gergő Érdi 
Cc: GHC Devs 
Subject: [External] Re: Usage of Template Haskell quotes in GHC source tree vs. 
usage of GHC as a library

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Gergo

I'm not close enough to this to have a well-formed opinion, but it looks like a 
good question to me, esp if the new dependence on TH is optional.

Would you like to transfer the text into a GHC ticket?

Simon

On Wed, 12 Jul 2023 at 04:40, Gergő Érdi mailto:ge...@erdi.hu>> 
wrote:
Hi,

A recent commit 983ce55815f2dd57f84ee86eee97febf7d80b470 starts using 
TemplateHaskellQuotes in the GHC codebase. It seems this is at odds with using 
GHC as a library, a la ghc-lib.

The `ghc-lib` approach is to basically take the module hierarchy from the 
`compiler/` subtree, and compile it as a completely vanilla Haskell library, 
with no direct attachment to the host GHC version. This enables using e.g. GHC 
9.4 to compile a program using the GHC 9.6 API, and so on. In particular, it 
also makes it very easy to apply patches to the version of GHC used as a 
library, since in this setup it doesn't need to be able to bootstrap.

So what is the problem with using TemplateHaskellQuotes? The problem is the 
dependency on the template-haskell package. When a module inside 
GHC-as-a-library containing TH quotes is compiled, the quotes are translated 
into applications of the constructors defined by the *host* GHC's TH package. 
But because GHC is tightly coupled to the TH support library, GHC-as-a-library 
needs to ship with its own internal version of the library. So the code that 
tries to process the results of these quotes is using the *target* GHC's TH 
definitions. And that leads to a conflict: code like

leftName :: Namel
leftName = 'Left

is now a type mismatch between the type of `'Left` being 
template-haskell-2.19.0.0:Language.Haskell.TH.Syntax.Name
 (example when using GHC 9.4.5 as the host) and the type of `leftName` being 
ghc-lib-9.9.20230712:Language.Haskell.TH.Syntax.Name
 (example when the target version is built from recent `master`).

Currently, `ghc-lib-gen` has a pre-processing step on the GHC source tree that 
replaces these quotations with applications containing direct references to the 
target TH constructors: 
https://github.com/digital-asset/ghc-lib/blob/ab01fb2b4d1e3a9338390e9c10ccd769bbf37aeb/ghc-lib-gen/src/Ghclibgen.hs#L419-L467
 but I am worried that this is very fragile.

So any ideas on how to tackle this situation better?

Thanks,
Gergo
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