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Rodney Lorrimar
b4b18edeec
- Dev note about nixpkgs pin - More information about selecting a GHC version for cabal projects - A little bit more information about cross compiling, but this is still incomplete.
67 lines
2.4 KiB
Markdown
67 lines
2.4 KiB
Markdown
# `ghcWithPackages` wrapper removal
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The current [Nixpkgs Haskell infrastructure][nixpkgs-haskell] and `haskell.nix` both
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provide a `ghcWithPackages` derivation which contains shell script
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wrappers that wrap `ghc` and `ghc-pkg`.
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In the Nixpkgs Haskell infrastructure, the wrapper scripts are used
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for building Haskell packages. However, in `haskell.nix`, the wrappers
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are only used for development environments.
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The wrapper scripts provide a `ghc` command that "knows" about the
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package set and has all Haskell package dependencies available to it.
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We would like to remove the wrapper scripts, but it's currently not
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possible to configure all build tools using environment variables
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alone.
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## Plain `ghc`
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When using `ghc` or `ghci` by itself, the `GHC_ENVIRONMENT` variable
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can point to a configuration file containing an exact package
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set. This works quite well.
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## `ghc-pkg`
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The package tool `ghc-pkg` does not recognize `GHC_ENVIRONMENT`, but
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does recognize a `GHC_PACKAGE_PATH` pointing to a `package.conf.d`.
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This works well. However, the `cabal` command will refuse to start if
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`GHC_PACKAGE_PATH` is set.
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## `Setup.hs`
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When invoking `Setup.hs configure`, the package database is provided
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with the `--package-db` argument and exact dependencies in the package
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set can be provided as `--dependency` arguments.
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The `haskell.nix` component builder uses `Setup.hs` with these
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command-line options to build Haskell packages.
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## `cabal new-build`
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Cabal-install will observe the `CABAL_CONFIG` environment variable,
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which points to a cabal config file. This config file can provide a
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`package-db` value, but it can't specify exact versions of packages.
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Cabal is designed to solve dependencies, not simply take the package
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set which is given to it.
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Therefore, `cabal` does not use `GHC_ENVIRONMENT`, but instead creates
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its own environment file. It will not accept `--dependency` arguments.
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As far as I know, the best way to force `cabal` to take a pre-computed
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package set is to use a `new-freeze` file. However there is no
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environment variable (or config file entry) which can specify a path
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to a freeze file.
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Specifying a `package-db` path in the cabal config file is not enough
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for it to successfully resolve dependencies.
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As mentioned before, `cabal` does not work when `GHC_PACKAGE_PATH` is
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set. The best way to work around this is to wrap `ghc` and `ghc-pkg`
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in shell scripts.
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[nixpkgs-haskell]: https://nixos.org/nixpkgs/manual/#users-guide-to-the-haskell-infrastructure
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