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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.

Types

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 = { ... };
    };
  };
}
AttributeTypeDescription
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
.compilerAttrset

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; };
  all = COMPONENT;
}

Component

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

Haskell.nix also defines a special all component, which is the union of all components in the 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:

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

Top-level attributes

mkStackPkgSet

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

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

mkCabalProjectPkgSet

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

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

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;
ArgumentTypeDescription
nameStringOptional name for better error messages.
srcPathLocation of the cabal project files.
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).
cabalProjectPathOptional cabal project file (defaults to “${src}/cabal.project”).
ghcOptional 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).

importAndFilterProject

Imports from a derivation created by callStackToNix or callCabalProjectToNix.

The result is an attrset with the following values:

AttributeTypeDescription
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).

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)
ArgumentTypeDescription
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.

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.

ArgumentTypeDescription
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.

subComponentTypes

Sub-component types identify components and are one of:

  • sublibs
  • foreignlibs
  • exes
  • tests
  • benchmarks

Package-set functions

These functions exist within the hsPkgs package set.

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, ...}: ...
ArgumentTypeDescription
packagesFunctionPackage selection function. It takes a list of Haskell packages and returns a subset of these packages.
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.
{ ... }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](../dev/removing-with-package-wrapper.md)
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.