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

[Erdi, Gergo]

PUBLIC

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

From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Simon Peyton Jones
Sent: Wednesday, July 12, 2023 7:25 PM
To: Gergő Érdi <gergo at erdi.hu>
Cc: GHC Devs <ghc-devs at haskell.org>
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 <gergo at erdi.hu<mailto:gergo at 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<https://clicktime.symantec.com/15tStcGbBJvRFSPaKjQ96?h=rR_XOsXvthSWoiYbkJKbEwF8CkM-pT30ZpS5jSTU3fM=&u=http://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<https://clicktime.symantec.com/15tStcGbBJvRFSPaKjQ96?h=rR_XOsXvthSWoiYbkJKbEwF8CkM-pT30ZpS5jSTU3fM=&u=http://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<https://clicktime.symantec.com/15tT4GfA6YHc5L3RQrCSL?h=MyTYmIJeEpi9THujD4dSHUfucPEFbsgY8PPm_sRRYV0=&u=https://github.com/digital-asset/ghc-lib/blob/ab01fb2b4d1e3a9338390e9c10ccd769bbf37aeb/ghc-lib-gen/src/Ghclibgen.hs%23L419-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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