Shared data type for extension flags

malcolm.wallace malcolm.wallace at
Thu Sep 10 08:28:30 UTC 2015

"ghc-types" sounds like a package for fancy type hackery.  I would never think to find language extension flags in such a place.  How about "ghc-package-db", or "ghc-language-extensions"?

On 10 Sep, 2015,at 03:30 AM, "Edward Z. Yang" <ezyang at> wrote:

I don't think it makes very much sense to reuse bin-package-db; at
least, not without renaming it at the very least (who'd expect
a list of language extension flags to live in a binary package
database?) We could name it something like 'ghc-types'?


Excerpts from Simon Peyton Jones's message of 2015-09-08 05:35:00 -0700:
Yes, we’d have to broaden the description of the package. I defer to Edward Yang and Duncan Coutts who have a clearer idea of the architecture in this area.


From: Michael Smith [mailto:michael at]
Sent: 02 September 2015 17:27
To: Simon Peyton Jones; Matthew Pickering
Cc: GHC developers
Subject: Re: Shared data type for extension flags

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 <simonpj at<mailto:simonpj at>> wrote:
we already have such a shared library, I think: bin-package-db. would that do?


From: ghc-devs [mailto:ghc-devs-bounces at<mailto:ghc-devs-bounces at>] 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 at<mailto:matthewtpickering at>> 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 <michael at<mailto:michael at>> wrote:
> #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]
> [2]
> [3]
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