Re: [PATCH] build-system: Add haskell-build-system.

[Federico Beffa <beffa@ieee.org>]

On Sun, Mar 29, 2015 at 4:00 PM, Ricardo Wurmus <rekado@elephly.net> wrote:
>
> Ludovic Courtès writes:
>
>> Isn’t there any environment variable akin to LD_LIBRARY_PATH or
>> GUILE_LOAD_PATH?  That would greatly simplify things.
>
> There is the GHC_PACKAGE_PATH environment variable which is supposed to
> be a colon-separated list of package databases.
>
> See section 4.9.5.1 in the latest GHC user guide.
> https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/packages.html

There are a couple of considerations about this:

* All Hackage packages are based on Cabal, not the GHC specific tools.
If GHC_PACKAGE_PATH is set, running the configure phase "runhaskell
Setup.hs configure" will stop with a message saying that Cabal is not
compatible with the use of this variable.

NixOS does the same: in the build phase they explicitly unset
GHC_PACKAGE_PATH and build a temporary database:

https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/haskell-modules/generic-builder.nix

* AFAIU the directories listed in GHC_PACKAGE_PATH must still include
a database cache named 'package.cache' (section 4.9.5 of the GHC
manual). Therefore, the fix file name would clash in profiles and
prevent the use of libraries in profiles.

Having never used nixpkgs, I'm not sure how they handle the database
in profiles. Would you have a pointer to a "standard" mechanism (like
how they handle 'dir' files, icon caches, ...).

Regards,
Fede