[arch-haskell] Further to the discussion in pull request for yi

[Peter Simons]

Hi Magnus,

 > What do changes do you propose we make to our tool chain to
 > support patching of packages?

the cabal2arch utility (or rather, the underlying archlinux
library) needs the ability to accept three kind of patches and
bake them into the generated output:

 1) diffs that ought to be applied to the Cabal file before
    processing,

 2) diffs that ought to be applied to the generated PKGBUILD
    after processing, and

 3) diffs that ought to be applied to the extracted source code
    as part of the build.

Type (1) can be used to fix omissions in the Cabal file that
affect the generated PKGBUILD, like the kind of under-specified
dependencies currently exhibited by haskell-{opengl,glut}.

Type (2) can be used to fix incorrectly generated PKGBUILD files
like the one for haskell-platform, which comes out having a
non-existent homepage URL.

Type (3) can be used to perform the kind of build-time fixes that
we currently need for haskell-yesod and/or yi.

A combination of all three of those should allow us to build just
about anything, no matter how broken the package is.

Take care,
Peter