Don't try and resolve dependencies for packages that are not buildable

[bos]

(Imported from Trac #566, reported by @batterseapower on 2009-07-03)

I had a component like this:

Executable edit-distance-benchmark
        Main-Is:                Text/EditDistance/Benchmark.hs
        if flag(splitBase)
                Build-Depends:  base >= 3 && < 5, array >= 0.1, random >= 1.0, old-time >= 1.0, process >= 1.0,
                                parallel >= 1.0, unix >= 2.3
        else
                Build-Depends:  base < 3,
                                parallel >= 1.0, unix >= 2.3
        if !flag(benchmark)
                Buildable:      False
        Ghc-Options:            -O2 -fvia-C -Wall

However, I've only just found out that this meant that you couldn't install any parts of my package on Windows, because Cabal tried to resolve the "unix" dependency even if "benchmark" was False (the default)!

I've solved this like so:

Executable edit-distance-benchmark
        Main-Is:                Text/EditDistance/Benchmark.hs
        if !flag(benchmark)
                Buildable:      False
        else
                if flag(splitBase)
                        Build-Depends:  base >= 3 && < 5, array >= 0.1, random >= 1.0, old-time >= 1.0, process >= 1.0,
                                        parallel >= 1.0, unix >= 2.3
                else
                        Build-Depends:  base < 3,
                                        parallel >= 1.0, unix >= 2.3
        Ghc-Options:            -O2 -fvia-C -Wall

But really Cabal should not need to resolve dependencies for things that are not buildable.

[bos]

(Imported comment by @dcoutts on 2009-07-03)

The semantics for "buildable: False" is actually rather tricky. We don't have a precise description of what it should mean. See this discussion:
http://haskell.org/pipermail/glasgow-haskell-users/2009-December/018155.html

[yav]

I've also run into this problem and, having read the above e-mail, I don't really see a difficulty with the semantics of "Buildable". Here's how I'd expect things to work:

1. Figure out the flags for a package (based on user-specified flags and defaults)
2. Using the flags (and other environment variables) resolve all conditionals, eliminating them from the Cabal file
3. Remove all parts where "Buildable: False" appears.
4. Resolve dependencies as usual.

[letmaik]

I second yav's reasoning. I stumbled across this today and find it really confusing.

[ttuegel]

This is a duplicate of #1725.