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

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 = { ... };
optionsModule optionsThe combination of all options set through the modules argument passed to mkPkgsSet.
configThe result of evaluating and applying the options with Haskell.nix
.hsPkgsAttrset of Haskell PackagesBuildable packages, created from packages
.packagesAttrset of Haskell Package descriptionsConfiguration for each package in hsPkgs

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; };


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


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


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 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:

haskellLibattrsetThe haskellLib utility functions.
pkgsThe Nixpkgs collection.
pkgconfPkgsA mapping of cabal build-depends names to Nixpkgs packages. (TODO: more information about this)

Top-level attributes


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.


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 = ./.;


A function calling callStackToNix with all arguments.

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

Return value:

hsPkgsAttrset of Haskell PackagesBuildable packages, created from packages
stack-nixprojectNix attribute of callStackToNix return value


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:

hsPkgsAttrset of Haskell PackagesBuildable packages, created from packages
plan-nixprojectNix attribute of callCabalProjectToNix return value
index-stateindex-state attribute of callCabalProjectToNix return value
projectCrossAttrsetLike pkgs.pkgsCross.<system> from nixpkgs p.projectCross.<system> returns the project results for cross compilation (where system is a member of nixpkgs lib.systems.examples). So p.projectCross.ghcjs.hsPkgs is the same as hsPkgs but compiled with ghcjs
projectVariantsAttrsetAttribute set of variant for the project, mapped from flake.variants config values
appendModuleFunctionRe-eval the project with an extra module (or module list).
extend and appendOverlaysFunctionModify a project, or add attributes, through overlays: p.extend(final: prev: { }). The overlays are carried-over projectCross and appendModule invocations.

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.


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

mkStackPkgSet =
    { stack-pkgs, pkg-def-extras ? [], modules ? []}: ...
stack-pkgsimport ./pkgs.nix — The imported file generated by stack‑to‑nix.
pkg‑def‑extrasList of ExtrasFor overriding the package set.
modulesList of ModulesFor overriding the package set.

Return value: a pkgSet


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

mkCabalProjectPkgSet =
    { plan-pkgs, pkg-def-extras ? [], modules ? []}: ...
plan-pkgsimport ./pkgs.nix — The imported file generated by plan‑to‑nix.
pkg‑def‑extrasList of ExtrasFor overriding the package set.
modulesList of ModulesFor overriding the package set.

Return value: a pkgSet


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

Return value: a pkgSet


This is an attrset of hsPkgs packages from Stackage.


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.


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


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


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


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


  pkgSet = mkCabalProjectPkgSet {
    plan-pkgs = (importAndFilterProject (callCabalProjectToNix {
      index-state = "2019-04-30T00:00:00Z";
      src = ./.;
nameStringOptional name for better error messages.
srcPathLocation of the cabal project files.
compiler-nix-nameStringThe name of the ghc compiler to use eg. "ghc884"
index-stateTimestampOptional hackage index-state, eg. "2019-10-10T00:00:00Z".
index-sha256Sha256Optional hash of the truncated hackage index-state.
plan-sha256Sha256Optional hash of the plan-to-nix output (makes the plan-to-nix step a fixed output derivation).
cabalProjectStringOptional cabal project file contents (defaults to readFile "${src}/cabal.project").
cabalProjectLocalStringOptional cabal project file contents (defaults to readFile "${src}/cabal.project.local").
cabalProjectFreezeStringOptional cabal project file contents (defaults to readFile "${src}/cabal.project.freeze").
ghcDeprecated. Use compiler-nix-name instead. Optional ghc to use
nix-toolsOptional nix-tools to use
hpackOptional hpack to use
cabal-installOptional cabal-install to use
configureArgsStringOptional extra arguments to pass to cabal new-configure (--enable-tests is included by default, include --disable-tests to override that).


Imports from a derivation created by callStackToNix or callCabalProjectToNix.

The result is an attrset with the following values:

pkgsattrsetthat can be passed to mkStackPkgSet (as stack-pkgs) or mkCabalProjectPkgSet (as plan-pkgs).
nixthis can be built and cached so that the amount built in the evaluation phase is not too great (helps to avoid timeouts on Hydra).





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.


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


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.


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)
groupStringA sub-component type.
packageSelA function Package -> BoolA predicate to filter packages with.
haskellPackagesPackage setAll packages in the build.

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


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.


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

drvDerivationOne of $pkg.components.tests.$test.

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.


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.


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, ...}: ...
packagesFunctionPackage selection function. It takes a list of Haskell packages and returns a subset of these packages.
componentsFunctionSimilar to packages, by default all the components of the selected packages are selected.
additionalFunctionSimilar to packages, but the selected packages are built and included in ghc-pkg list (not just their dependencies).
withHoogleBooleanWhether to build a Hoogle documentation index and provide the hoogle command.
exactDepsBooleanPrevents the Cabal solver from choosing any package dependency other than what are in the package set.
toolsFunctionAttrSet of tools to make available e.g. { cabal = ""; } or { cabal = { version = ""; }; }. If an AttrSet is provided for a tool, the additional arguments will be passed to the function creating the derivation for that tool. So you can provide an index-state or a materialized argument like that { cabal = { version = ""; index-state = "2020-10-30T00:00:00Z"; materialized = ./cabal.materialized; }; } for example. You can specify and materialize the version of hoogle used to construct the hoogle index by including something like { hoogle = { version = ""; index-state = "2020-05-31T00:00:00Z"; materialized = ./hoogle.materialized; }. Uses a default version of hoogle if omitted.
inputsFromListList of other shells to include in this one. The buildInputs and nativeBuildInputs of each will be included using mkShell.
crossPlatformsFunctionPlatform selection function for cross compilation targets to support eg. ps: with ps; [ghcjs mingwW64] (see nixpkgs lib.systems.examples for list of platform names).
{ ... }AttrsetAll the other arguments are passed to mkDerivation.

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.


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

Parameter: a package selection function.

Return value: a derivation


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


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