Proposed ghc-pkg and cabal feature - right list?

Simon Marlow marlowsd at
Mon Mar 15 11:38:04 EDT 2010

On 13/03/2010 20:39, Dan Knapp wrote:
> There doesn't seem to be a mailing list for Cabal itself, so I'm
> posting here.  I came up with an idea for a small feature that I
> believe would make a useful addition to ghc-pkg and Cabal.  I'm
> willing to implement it myself, but I have had some previous
> experiences with other projects where I did some work and then the
> maintainers said "sorry, not interested", so I want to gauge interest
> before I start.  Who should I talk to?
> The feature itself is this:  Arbitrary key-value pairs in Cabal
> package files and the installed-package database.  The use-case is for
> an application supporting plugins to discover installed plugins
> compatible with it, interrogating these fields through the GHC API.
> For example, my content-management system FruitTart could enumerate
> the list of installed packages looking for packages which export a
> field "fruit-tart-plugin-interface-version" with a numeric value
> matching the interface version it's expecting.
> Once again, I'm not asking anyone to do this work for me - I'm eager
> to get my hands dirty and do it myself.  I just want to find out what
> the process would be to get it accepted, once it works.

My first thought was "hmm, there must be another way to do that", but I 
can't think of one, or at least a good one.

Perhaps having arbitrary key-value pairs in the package database would 
be a good thing.  It would help us to avoid breaking things when we need 
to change the format, for one thing.  We could start using key-values 
for new fields rather than adding them to InstalledPackageInfo. 
However, then we have a strange situation where some fields get 
distinguished status in InstalledPackageInfo.  Of course, for some of 
those fields we have richer types (e.g. License), so it makes sense.

So for me, I can't see any serious objections to doing this, but I'd 
also ask on the cabal-devel at list (in particular we should 
hear what Duncan Coutts thinks).


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