How to use two different compilers for different targets in a .cabal file?

Question

When I run cabal build it uses some Haskell compiler to build the executables and/or test-suites in my .cabal file.

Can I control which compiler is used for the different targets? Ideally, I would like to have separate build targets that use ghc and ghcjs in the same .cabal file. It seems to me that someone might want to use ghc and hugs or two version of ghc in the same project. Is this currently possible?

Also, how does cabal decide what compiler to use when running cabal build? I saw there is a compiler option in my ~/.cabal/config file but changing it from ghc to ghcjs and uncommenting it, did not seem to change what cabal build does.

Answer 1

The compiler to use is determined during the configure step (or during an install step's implicit configure step, which does not share configuration options with a previous configure step). It is also determined by the entity building the package and cannot be influenced by the person writing the package. Probably what happened to you is that a previous cabal build implicitly invoked the configure step and chose a compiler; future builds will keep a previous choice of compiler over one stuck in your global configuration file. You can countermand that by simply manually running cabal configure again.

It is possible to cause a build to fail with the wrong implementation, e.g.

library
    if impl(ghc)
        buildable: False

will prevent cabal from trying to build the package using GHC. However, this isn't really useful for building separate parts of a package with separate compilers, as cabal will refuse to install a package unless it can build the whole thing with a single compiler.

Probably the best way forward is to make separate packages for things that should be built by separate compilers.