ghcWithPackages wrapper removal

The current Nixpkgs Haskell infrastructure and haskell.nix both provide a ghcWithPackages derivation which contains shell script wrappers that wrap ghc and ghc-pkg.

In the Nixpkgs Haskell infrastructure, the wrapper scripts are used for building Haskell packages. However, in haskell.nix, the wrappers are only used for development environments.

The wrapper scripts provide a ghc command that "knows" about the package set and has all Haskell package dependencies available to it.

We would like to remove the wrapper scripts, but it's currently not possible to configure all build tools using environment variables alone.

Plain ghc

When using ghc or ghci by itself, the GHC_ENVIRONMENT variable can point to a configuration file containing an exact package set. This works quite well.

ghc-pkg

The package tool ghc-pkg does not recognize GHC_ENVIRONMENT, but does recognize a GHC_PACKAGE_PATH pointing to a package.conf.d.

This works well. However, the cabal command will refuse to start if GHC_PACKAGE_PATH is set.

Setup.hs

When invoking Setup.hs configure, the package database is provided with the --package-db argument and exact dependencies in the package set can be provided as --dependency arguments.

The haskell.nix component builder uses Setup.hs with these command-line options to build Haskell packages.

cabal new-build

Cabal-install will observe the CABAL_CONFIG environment variable, which points to a cabal config file. This config file can provide a package-db value, but it can't specify exact versions of packages.

Cabal is designed to solve dependencies, not simply take the package set which is given to it.

Therefore, cabal does not use GHC_ENVIRONMENT, but instead creates its own environment file. It will not accept --dependency arguments.

As far as I know, the best way to force cabal to take a pre-computed package set is to use a new-freeze file. However there is no environment variable (or config file entry) which can specify a path to a freeze file.

Specifying a package-db path in the cabal config file is not enough for it to successfully resolve dependencies.

As mentioned before, cabal does not work when GHC_PACKAGE_PATH is set. The best way to work around this is to wrap ghc and ghc-pkg in shell scripts.