[Haskell-cafe] Re: two problems with Data.Binary and Data.ByteString

[Duncan Coutts]

On Thu, 2008-08-28 at 21:34 +0200, Ben Franksen wrote:

> Just some raw ideas:
> 
> What if we had a way to express 'optional dependencies' between packages in
> a cabal file. Something like 'if package x is installed (and satisfies
> given version constraints) then add module UseX'.
> 
> One problem with this idea is that I might install the missing (optional)
> package afterwards, and then I still do not have the instance I would like
> (unless I re-build). Could cabal be instructed to re-build a package if an
> optional dependency becomes available (or if one gets removed)?

Right, requiring an order of installation is rather unfortunate. Always
rebuilding things doesn't really fit well with many deployment models.

Duncan