[Haskell-cafe] Re: Configuring cabal dependencies at install-time

[Martijn van Steenbergen]

John Lato wrote:
>> From: Jeff Heard <jefferson.r.heard at gmail.com>
>>
>> Is there a way to do something like autoconf and configure
>> dependencies at install time?  Building buster, I keep adding
>> dependencies and I'd like to keep that down to a minimum without the
>> annoyance of littering Hackage with dozens of packages.  For instance,
>> today I developed an HTTP behaviour and that of course requires
>> network and http, which were previously not required.  I'm about to
>> put together a haxr XML-RPC behaviour as well, and that of course
>> would add that much more to the dependency list.  HaXml, haxr, and
>> haxr-th most likely.
>>
>> so... any way to do that short of making a bunch of separate packages
>> with one or two modules apiece?  Otherwise I'm thinking of breaking
>> things up into buster, buster-ui, buster-network, buster-console, and
>> buster-graphics to start and adding more as I continue along.
>>
> 
> I'd be interested in hearing answers to this as well.  I'm not a fan
> of configure-style compile-time conditional compilation, at least for
> libraries.  It makes it much harder to specify dependencies.  With
> this, if package Foo depends on buster and the HTTP behavior, it's no
> longer enough to specify "build-depends: buster" because that will
> only work if buster was configured properly on any given system.
> 
> I think that the proper solution is to break up libraries into
> separate packages as Jeff suggests (buster, buster-ui, etc.), but then
> the total packages on hackage would explode.  I don't feel great about
> doing that with my own packages either; is it a problem?  If so, maybe
> there could be just one extra package, e.g. buster and buster-extras.
> Is there a better solution I'm missing?

Cabal's flag system sounds like a nice solution for this, except I don't 
know if it's possible to add specific flags to your build dependencies, i.e.

build-depends: buster -fhttp

Martijn.