Haskell.nix contains a library of functions for creating buildable package sets from their Nix expression descriptions. The library is what you get when importing Haskell.nix. It might be helpful to load the library in the Nix REPL to test things.

• Data structures — the kinds of data that you will encounter working with Haskell.nix.
• Top-level attributes — Functions and derivations defined in the Haskell.nix attrset.
• Package-set functions — Helper functions defined on the hsPkgs package set.

Data structures

Package Set

The result of mkPkgSet. This is an application of the NixOS module system.

{
  options = { ... };
  config = {
    hsPkgs = { ... };
    packages = { ... };
    compiler = {
      version = "X.Y.Z";
      nix-name = "ghcXYZ";
      packages = { ... };
    };
  };
}

Haskell Package description

The Haskell package descriptions are values of the pkgSet.config.packages attrset. These are not derivations, but just the configuration for building an individual package. The configuration options are described under packages.<name> in Module options.

Component description

The component descriptions are values of the pkgSet.config.packages.<package>.components attrset. These are not derivations, but just the configuration for building an individual component. The configuration options are described under packages.<name>.components.* in Module options.

Haskell Package

In Haskell.nix, a Haskell package is a derivation which has a components attribute. This derivation is actually just for the package Setup.hs script, and isn't very interesting. To actually use the package, look within the components structure.

components = {
  library = COMPONENT;
  exes = { NAME = COMPONENT; };
  tests = { NAME = COMPONENT; };
  benchmarks = { NAME = COMPONENT; };
}

Component

In Haskell.nix, a component is a derivation corresponding to a Cabal component of a package.

Identifier

A package identifier is an attrset pair of name and version.

Extras

Extras allow adding more packages to the package set. These will be functions taking a single parameter hackage. They should return an attrset of package descriptions.

Modules

Modules are the primary method of configuring building of the package set. They are either:

1. an attrset containing option declarations, or
2. a function that returns an attrset containing option declarations.

If using the function form of a module, the following named parameters will be passed to it:

Top-level attributes

project'

Function that accepts attribute set with a src attribute and looks for stack.yaml file relative to it.

If file exists, it calls stackProject function. Otherwise it will call cabalProject function.

Example:

pkgs.haskell-nix.project' {
  # 'cleanGit' cleans a source directory based on the files known by git
  src = pkgs.haskell-nix.haskellLib.cleanGit {
    name = "haskell-nix-project";
    src = ./.;
  };
}

stackProject'

A function calling callStackToNix with all arguments.

Then feeding its result into mkStackPkgSet passing also pkg-def-extras and modules arguments.

Return value:

cabalProject'

A function calling callCabalProjectToNix with all arguments.

Then feeding its result into mkCabalProjectPkgSet passing also pkg-def-extras, extra-hackages and modules arguments.

Return value:

project, cabalProject and stackProject

These versions of the function are the same as project', cabalProject' and stackProject', but hsPkgs attributes are also included in the return value directly. That way a package can be referenced as (project {...}).foo instead of (project' {...}).hsPkgs.foo.

mkStackPkgSet

Creates a package set based on the pkgs.nix output of stack-to-nix.

mkStackPkgSet =
    { stack-pkgs, pkg-def-extras ? [], modules ? []}: ...

Return value: a pkgSet

mkCabalProjectPkgSet

Creates a package set based on the pkgs.nix output of plan-to-nix.

mkCabalProjectPkgSet =
    { plan-pkgs, pkg-def-extras ? [], modules ? []}: ...

Return value: a pkgSet

mkPkgSet

This is the base function used by both mkStackPkgSet and mkCabalProjectPkgSet.

Return value: a pkgSet

snapshots

This is an attrset of hsPkgs packages from Stackage.

haskellPackages

A hsPkgs package set, which is one of the recent LTS Haskell releases from snapshots.

The chosen LTS is updated occasionally in Haskell.nix, though a manual process.

nix-tools

A derivation containing the nix-tools command-line tools.

callStackToNix

Runs stack-to-nix and produces the output needed for importAndFilterProject.

Example:

  pkgSet = mkStackPkgSet {
    stack-pkgs = (importAndFilterProject (callStackToNix {
      src = ./.;
    })).pkgs;
    pkg-def-extras = [];
    modules = [];
  };

callCabalProjectToNix

Runs cabal new-configure and plan-to-nix and produces the output needed for importAndFilterProject.

Example:

  pkgSet = mkCabalProjectPkgSet {
    plan-pkgs = (importAndFilterProject (callCabalProjectToNix {
      index-state = "2019-04-30T00:00:00Z";
      src = ./.;
    })).pkgs;

importAndFilterProject

Imports from a derivation created by callStackToNix or callCabalProjectToNix.

The result is an attrset with the following values:

hackage

stackage

fetchExternal

cleanSourceHaskell

cleanSourceHaskell = { src, name ? null }: ...

Filters a source tree removing common filenames that are not Haskell build sources.

This can avoid unecessary rebuilds when these files change.

It's recommended to provide name so that the source derivation remains constant regardless of how it was fetched.

Example:

src = pkgs.haskell-nix.cleanSourceHaskell {
  src = ./.;
  name = "myproject-src";
};

haskellSourceFilter

haskellSourceFilter = name: type: ...

This is a source filter function which cleans common build products and files not needed to do a Haskell build from a source directory.

It should be used with pkgs.lib.cleanSourceWith. Alternatively, use the convenience function cleanSourceHaskell.

haskellLib

Assorted functions for operating on Haskell.nix data. This is distinct from pkgs.haskell.lib in the current Nixpkgs Haskell Infrastructure.

collectComponents, collectComponents'

Extracts a selection of components from a Haskell package set.

This can be used to filter out all test suites or benchmarks of your project, so that they can be built in Hydra (see check if you want to run the tests as well as build them).

collectComponents' is an alias of collectComponents without predicate for filtering.

collectComponents =
    group: packageSel: haskellPackages: ...
collectComponents' = group: collectComponents (_: true)

Return value: a recursive attrset mapping package names → component names → components.

Example:

tests = collectComponents "tests" (package: package.identifier.name == "mypackage") hsPkgs;

Will result in moving derivations from hsPkgs.mypackage.components.tests.unit-tests to tests.mypackage.unit-tests.

collectChecks, collectChecks'

These are just like collectComponents and collectComponents', except that they collect the checks attributes of packages (which aren't components, and so can't be collected by the other functions.

check

This function turns a derivation that builds a test into one to run it.

For convenience $pkg.components.tests are mapped with this function to $pkg.checks.

This function is intended for use with tests but it should also work for exes and benchmarks if you just want to run them to make sure they execute.

subComponentTypes

Sub-component types identify components and are one of:

• sublibs
• foreignlibs
• exes
• tests
• benchmarks

Project functions

These functions are included in the project return values. In the past they also existed within project.hsPkgs, but have now been removed from there.

shellFor

Create a nix-shell development environment for developing one or more packages with ghci or cabal v2-build (but not Stack).

shellFor =
    { packages, withHoogle ? true, exactDeps ? false, ...}: ...

Return value: a derivation

⚠️ Warning:

exactDeps = true will set the CABAL_CONFIG environment variable to disable remote package servers. This is a known limitation which we would like to solve. Use exactDeps = false if this is a problem.

ghcWithPackages

Creates a nix-shell development environment including the given packages selected from this package set.

Parameter: a package selection function.

Return value: a derivation

Example:

haskell.haskellPackages.ghcWithPackages (ps: with ps; [ lens conduit ])

ghcWithHoogle

The same as ghcWithPackages, except, a hoogle command with a Hoogle documentation index of the packages will be included in the shell.