[Numeric] Getting blas-ffi to build under nix

[Henning Thielemann]

On Wed, 28 Aug 2019, dominic at steinitz.org wrote:

> I thought I would broaden out the discussion Henning and I have been having.
> I have managed to get http://hackage.haskell.org/package/blas-ffi to build under nix. Ideally we would like
> cabal2nix to do this automatically for us. We can do this if
>
>  1. We use openblas rather than blas.

I suggest this anyway.

>  2. Replace `PkgConfig-Depends: blas` with `PkgConfig-Depends: lapack (as the lapack derivation in nix generates
>     a blas.pc) in the cabal file.

This sounds like a hack. blas-ffi depends only on the BLAS library and 
thus should need blas.pc only. OpenBLAS has its own blas.pc and otherwise 
plain BLAS library should provide blas.pc. In Debian you can even install 
both of them and select between them using Debian's alternatives system.

>  3. Fix nix to generate .pc files for blas - I don’t know the consequences of doing this but I can’t see any
>     downside.

This should be the right way.