[Haskell-cafe] Re: Listing package dependencies from GHC

[Simon Marlow]

Duncan Coutts wrote:
> On Thu, 2007-05-03 at 16:49 +0200, Thomas Schilling wrote:
>>>  By dependencies I
>>> meant, library packages that GHC knows about.
>>>
>>> For example, if I load something simple like
>>>
>>>> import System.Posix.Files
>>>> main = touchFile "example"
>>> into GHCi and execute ":main" it prints
>>>
>>>> Loading package unix-1.0 ... linking ... done.
>> Oh, right.  Well, then please file it as a bug report on Cabal.  I'll  
>> be working on Cabal configs as my Summer of Code project; this is  
>> clearly related.  With a bit of luck it'll be done before end of August.
> 
> This is not a Cabal bug. By design, Cabal does not just pick up any
> packages from the environment like --make does. One of the main points
> of Cabal is to be able to explicitly track dependencies of a package, so
> we do require that they all be listed explicitly. Cabal then tells ghc
> to *only* look in those listed packages and ignore all others even if
> they happen to be installed.
> 
> Now certainly it would be nicer if ghc+cabal could be more helpful and
> tell you all the missing packages that you need to list in the
> "build-depends" field in the .cabal file, rather than currently where
> you have to do it one by one.
> 
> This feature could be implemented in GHC, or it could actually be
> implemented in Cabal. To do it in cabal requires that cabal gain an
> infrastructure for doing dependency analysis of modules. We don't have
> that yet. We do need this infrastructure anyway though, since for
> example some pre-processors (notably c2hs) need to be called in
> dependency order. It would also allow other nice things like having
> cabal work out for you exactly which pre-processors and build tools are
> required, and make partial and parallel rebuilds possible.

This doesn't do exactly what you want, but it's useful nonetheless:

 > ghc --show-iface Main.hi | grep 'package dependencies'
package dependencies: base haskell98

Cheers,
	Simon