Re: Shared data type for extension flags

I don't know about the entire AST. GHC's AST contains a lot of complexity that one wouldn't want to expose at the TH level. And the separation allows GHC to change the internal AST around while maintaining a stable interface for packages depending on TH. That said, there are some bits that I could see being shared. Fixity and Strict from TH come to mind. On Wed, Sep 2, 2015, 11:39 Alan & Kim Zimmerman wrote:

Would this be a feasible approach for harmonising the AST between GHC and TH too?

Alan On 2 Sep 2015 09:27, "Michael Smith" wrote:

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The package description for that is "The GHC compiler's view of the GHC package database format", and this doesn't really have to do with the package database format. Would it be okay to put this in there anyway?

On Wed, Sep 2, 2015, 07:33 Simon Peyton Jones wrote:

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we already have such a shared library, I think: bin-package-db. would that do?

Simon

*From:* ghc-devs [mailto:ghc-devs-bounces@haskell.org] *On Behalf Of *Michael Smith *Sent:* 02 September 2015 09:21 *To:* Matthew Pickering *Cc:* GHC developers *Subject:* Re: Shared data type for extension flags

That sounds like a good approach. Are there other things that would go nicely in a shared package like this, in addition to the extension data type?

On Wed, Sep 2, 2015 at 1:00 AM, Matthew Pickering < matthewtpickering@gmail.com> wrote:

Surely the easiest way here (including for other tooling - ie haskell-src-exts) is to create a package which just provides this enumeration. GHC, cabal, th, haskell-src-exts and so on then all depend on this package rather than creating their own enumeration.

On Wed, Sep 2, 2015 at 9:47 AM, Michael Smith wrote:

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#10820 on Trac [1] and D1200 on Phabricator [2] discuss adding the capababilty to Template Haskell to detect which language extensions enabled. Unfortunately, since template-haskell can't depend on ghc (as ghc depends on template-haskell), it can't simply re-export the ExtensionFlag type from DynFlags to the user.

There is a second data type encoding the list of possible language extensions in the Cabal package, in Language.Haskell.Extension [3]. But template-haskell doesn't already depend on Cabal, and doing so seems like it would cause difficulties, as the two packages can be upgraded separately.

So adding this new feature to Template Haskell requires introducing a *third* data type for language extensions. It also requires enumerating this full list in two more places, to convert back and forth between the TH Extension data type and GHC's internal ExtensionFlag data type.

Is there another way here? Can there be one single shared data type for this somehow?

[1] https://ghc.haskell.org/trac/ghc/ticket/10820 [2] https://phabricator.haskell.org/D1200 [3]

https://hackage.haskell.org/package/Cabal-1.22.4.0/docs/Language-Haskell-Ext...

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