thax – create haskell tags from nix dependencies

This nix expression provides a few functions for the creation of hasktags from a dependency tree of haskell derivations as produced by cabal2nix.

Usage

All functions take a list of haskell derivations. The functions in individual produce lists of derivations, each of which contain the tags file for a package, while those in combined produce a derivation for a merged tag file.

There are three functions in each of those two sets:

• deps generates tags for only the dependencies
• packages generates tags for only the targets
• all combines the above

Ad-hoc from the shell

nix-build -A combined.packages --arg targets '[(import <nixpkgs> {}).haskellPackages.aeson]'

Builds only the tags for aeson without dependencies, while combined.all would build its dependencies as well and merge everything into the file tags in the output store path.

In your project config

let
  pkgs = import <nixpkgs> {};
  tags = import (fetchTarball "https://github.com/tek/thax/tarball/master") { inherit pkgs; };
  packages = [(pkgs.haskellPackages.callCabal2nix "spaceship" ./. {})]; # however your project is set up
in {
  projectTags = tags.combined.all { targets = packages; };
}

For example, in an obelisk project:

let
  pkgs = import <nixpkgs> {};
  obelisk = (import ./.obelisk/impl {}).project ./. ({ ... }: {});
  targets = [obelisk.ghc.frontend obelisk.ghc.backend obelisk.ghc.common];
  tags = import (fetchTarball "https://github.com/tek/thax/tarball/master") { inherit pkgs; };
in
  obelisk // {
    projectTags = tags.combined.all { inherit targets; };
  }

Now you can generate all project dependencies' tags with:

cp $(nix-build --no-link -A projectTags)/tags .tags

Relative paths

The tags of the packages you are developing in your project should not be pointing to the store, but nix will copy them over before running the derivation builder.

Therefore all packages passed into the API functions will be tagged with relative paths by default, while all dependencies will have absolute paths.

You can override this behaviour by passing relative = false; to the functions, as in:

tags.combined.all { inherit targets; relative = false; }

or more granularly by setting the relative attribute on a package, like:

tags.combined.all { targets = [mypackage // { relative = false; }]; }

Directory prefixes

If the relative path isn't enough, because your local packages are located in subdirectories, you can set the package's attribute tagsPrefix like so:

tags.combined.all { targets = [mypackage // { tagsPrefix = "packages/mypack"; }]; }

GHC

Since base et al. aren't regular dependencies, the all function will include the GHC sources. If that is not desired, you can deactivate it:

tags.combined.all { inherit targets; base = false; }