[Haskell-cafe] Re: Problems with Haskell Platform

[Pete Chown]

Ivan Miljenovic wrote:

> Pete Chown <1 at 234.cx> wrote:

>> ... This causes Cabal to install 'foo' (because it
>> is a dependency) and it won't use the distribution's package manager.

> Why won't it?  This, of course, depends on how the distribution ships
> `foo' in regards to static/shared libraries and what the user's
> options to Cabal/cabal-install are.

Is there a way of making Cabal install dependencies using the system 
package manager, then?

For example, I might ask Cabal to install package A.  Package A depends 
on B and C.  A package for B can be downloaded through APT, but there 
are no APT-installable candidates for A and C.  Are you saying that 
Cabal can download and install B using APT, then download A and C from 
Hackage?

> In general, even if some application is built statically then it won't
> work on other machines due to different C library versions (GMP,
> etc.), so I don't think this is such a big deal.

Your program should run on other machines that use the same Linux 
distribution, though.  The problem is that this won't happen if Cabal 
ends up building its own shared libraries.  For simplicity, let's have a 
new example...  The user asks Cabal to install package X, so Cabal goes 
to Hackage and gets the latest version, 1.2.3.5.  It builds shared 
libraries from it.  Meanwhile, the distribution actually packaged X 
version 1.2.3.4.  The user's programs could have been linked against 
1.2.3.4, and then they would have run on all systems using that 
distribution.  As it is, the programs only work on the user's own 
machine, because that's the only machine with the 1.2.3.5 shared library.

One option is to stop Cabal building shared libraries.  That would avoid 
the worst of these problems, but it would still be sub-optimal.  The 
user might want to link against shared libraries, but he won't have the 
chance, because he installed X from the wrong place.

(I believe current versions of ghc insist on linking entirely 
dynamically or entirely statically.  This makes the situation worse, 
because the lack of a shared library for X prevents the use of shared 
libraries for anything else.)

> Of course, the big thing here is whether Linux distributions, etc.
> should ship static or shared libraries by default.

With other languages they tend to ship both, and give the user the 
choice, I suppose.

Pete