Package dependences for building versus linking

[Duncan Coutts]

On Fri, 2009-10-02 at 19:14 +0000, C Rodrigues wrote:
> Cabal introduces unnecessary package dependences, which in one case
> causes a Cabal build to fail whereas ghc --make does not fail.  I'm
> not sure whether this should be considered a bug in Cabal.
> I'm trying to Cabalize a program.  The executable depends on a package
> that can be compiled (the GHC path), but not interpreted (the GHCi
> path).  Some of my source files use this compile-only package, and
> other files use Template Haskell.  TH code is passed to the
> interpreter.
> If I build using ghc --make, the TH interpreter loads only the
> packages needed by the current file.  This works because the
> interpreter only runs for source files that use TH, and no file uses
> both TH and the compile-only package.  If I build using Cabal,
> however, the TH interpreter attempts to load every package needed by
> the project, apparently because Cabal passes every package on the
> command line.  Compilation fails because the compile-only package
> cannot be loaded.
> Should I submit a bug report?

This is not a bug we can fix in Cabal, at least not before replacing the
build mechanism (which is to call ghc --make).

It might be worth reporting against ghc. One presumes that if ghc can
work out which packages are needed by a single module and load just
those ones for the TH stuff, then it can do the same when Cabal tells it
all the packages it is allowed to use.

Duncan