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---
title: User's Guide for Haskell in Nixpkgs
author: Peter Simons
date: 2015-06-01
---
# Haskell
## How to install Haskell packages
Nixpkgs distributes build instructions for all Haskell packages registered on
[Hackage](http://hackage.haskell.org/), but strangely enough normal Nix package
lookups don't seem to discover any of them, except for the default version of ghc, cabal-install, and stack:
```
$ nix-env -i alex
error: selector ‘alex’ matches no derivations
$ nix-env -qa ghc
ghc-7.10.2
```
The Haskell package set is not registered in the top-level namespace because it
is *huge*. If all Haskell packages were visible to these commands, then
name-based search/install operations would be much slower than they are now. We
avoided that by keeping all Haskell-related packages in a separate attribute
set called `haskellPackages`, which the following command will list:
```
$ nix-env -f "<nixpkgs>" -qaP -A haskellPackages
haskellPackages.a50 a50-0.5
haskellPackages.AAI AAI-0.2.0.1
haskellPackages.abacate abacate-0.0.0.0
haskellPackages.abc-puzzle abc-puzzle-0.2.1
haskellPackages.abcBridge abcBridge-0.15
haskellPackages.abcnotation abcnotation-1.9.0
haskellPackages.abeson abeson-0.1.0.1
[... some 14000 entries omitted ...]
```
To install any of those packages into your profile, refer to them by their
attribute path (first column):
```shell
nix-env -f "<nixpkgs>" -iA haskellPackages.Allure ...
```
The attribute path of any Haskell packages corresponds to the name of that
particular package on Hackage: the package `cabal-install` has the attribute
`haskellPackages.cabal-install`, and so on. (Actually, this convention causes
trouble with packages like `3dmodels` and `4Blocks`, because these names are
invalid identifiers in the Nix language. The issue of how to deal with these
rare corner cases is currently unresolved.)
Haskell packages whose Nix name (second column) begins with a `haskell-` prefix
are packages that provide a library whereas packages without that prefix
provide just executables. Libraries may provide executables too, though: the
package `haskell-pandoc`, for example, installs both a library and an
application. You can install and use Haskell executables just like any other
program in Nixpkgs, but using Haskell libraries for development is a bit
trickier and we'll address that subject in great detail in section [How to
create a development environment](#how-to-create-a-development-environment).
Attribute paths are deterministic inside of Nixpkgs, but the path necessary to
reach Nixpkgs varies from system to system. We dodged that problem by giving
`nix-env` an explicit `-f "<nixpkgs>"` parameter, but if you call `nix-env`
without that flag, then chances are the invocation fails:
```
$ nix-env -iA haskellPackages.cabal-install
error: attribute ‘haskellPackages’ in selection path
‘haskellPackages.cabal-install’ not found
```
On NixOS, for example, Nixpkgs does *not* exist in the top-level namespace by
default. To figure out the proper attribute path, it's easiest to query for the
path of a well-known Nixpkgs package, i.e.:
```
$ nix-env -qaP coreutils
nixos.coreutils coreutils-8.23
```
If your system responds like that (most NixOS installations will), then the
attribute path to `haskellPackages` is `nixos.haskellPackages`. Thus, if you
want to use `nix-env` without giving an explicit `-f` flag, then that's the way
to do it:
```shell
nix-env -qaP -A nixos.haskellPackages
nix-env -iA nixos.haskellPackages.cabal-install
```
Our current default compiler is GHC 8.8.x and the `haskellPackages` set
contains packages built with that particular version. Nixpkgs contains the last
three major releases of GHC and there is a whole family of package sets
available that defines Hackage packages built with each of those compilers,
too:
```shell
nix-env -f "<nixpkgs>" -qaP -A haskell.packages.ghc865
nix-env -f "<nixpkgs>" -qaP -A haskell.packages.ghc8101
```
The name `haskellPackages` is really just a synonym for
`haskell.packages.ghc882`, because we prefer that package set internally and
recommend it to our users as their default choice, but ultimately you are free
to compile your Haskell packages with any GHC version you please. The following
command displays the complete list of available compilers:
```
$ nix-env -f "<nixpkgs>" -qaP -A haskell.compiler
haskell.compiler.ghc8101 ghc-8.10.1
haskell.compiler.integer-simple.ghc8101 ghc-8.10.1
haskell.compiler.ghcHEAD ghc-8.11.20200505
haskell.compiler.integer-simple.ghcHEAD ghc-8.11.20200505
haskell.compiler.ghc822Binary ghc-8.2.2-binary
haskell.compiler.ghc844 ghc-8.4.4
haskell.compiler.ghc863Binary ghc-8.6.3-binary
haskell.compiler.ghc865 ghc-8.6.5
haskell.compiler.integer-simple.ghc865 ghc-8.6.5
haskell.compiler.ghc882 ghc-8.8.2
haskell.compiler.integer-simple.ghc882 ghc-8.8.2
haskell.compiler.ghc883 ghc-8.8.3
haskell.compiler.integer-simple.ghc883 ghc-8.8.3
haskell.compiler.ghcjs ghcjs-8.6.0.1
```
We have no package sets for `jhc` or `uhc` yet, unfortunately, but for every
version of GHC listed above, there exists a package set based on that compiler.
Also, the attributes `haskell.compiler.ghcXYC` and
`haskell.packages.ghcXYC.ghc` are synonymous for the sake of convenience.
## How to create a development environment
### How to install a compiler
A simple development environment consists of a Haskell compiler and one or both
of the tools `cabal-install` and `stack`. We saw in section
[How to install Haskell packages](#how-to-install-haskell-packages) how you can install those programs into your
user profile:
```shell
nix-env -f "<nixpkgs>" -iA haskellPackages.ghc haskellPackages.cabal-install
```
Instead of the default package set `haskellPackages`, you can also use the more
precise name `haskell.compiler.ghc7102`, which has the advantage that it refers
to the same GHC version regardless of what Nixpkgs considers "default" at any
given time.
Once you've made those tools available in `$PATH`, it's possible to build
Hackage packages the same way people without access to Nix do it all the time:
```shell
cabal get lens-4.11 && cd lens-4.11
cabal install -j --dependencies-only
cabal configure
cabal build
```
If you enjoy working with Cabal sandboxes, then that's entirely possible too:
just execute the command
```shell
cabal sandbox init
```
before installing the required dependencies.
The `nix-shell` utility makes it easy to switch to a different compiler
version; just enter the Nix shell environment with the command
```shell
nix-shell -p haskell.compiler.ghc784
```
to bring GHC 7.8.4 into `$PATH`. Alternatively, you can use Stack instead of
`nix-shell` directly to select compiler versions and other build tools
per-project. It uses `nix-shell` under the hood when Nix support is turned on.
See [How to build a Haskell project using Stack](#how-to-build-a-haskell-project-using-stack).
If you're using `cabal-install`, re-running `cabal configure` inside the spawned
shell switches your build to use that compiler instead. If you're working on
a project that doesn't depend on any additional system libraries outside of GHC,
then it's even sufficient to just run the `cabal configure` command inside of
the shell:
```shell
nix-shell -p haskell.compiler.ghc784 --command "cabal configure"
```
Afterwards, all other commands like `cabal build` work just fine in any shell
environment, because the configure phase recorded the absolute paths to all
required tools like GHC in its build configuration inside of the `dist/`
directory. Please note, however, that `nix-collect-garbage` can break such an
environment because the Nix store paths created by `nix-shell` aren't "alive"
anymore once `nix-shell` has terminated. If you find that your Haskell builds
no longer work after garbage collection, then you'll have to re-run `cabal
configure` inside of a new `nix-shell` environment.
### How to install a compiler with libraries
GHC expects to find all installed libraries inside of its own `lib` directory.
This approach works fine on traditional Unix systems, but it doesn't work for
Nix, because GHC's store path is immutable once it's built. We cannot install
additional libraries into that location. As a consequence, our copies of GHC
don't know any packages except their own core libraries, like `base`,
`containers`, `Cabal`, etc.
We can register additional libraries to GHC, however, using a special build
function called `ghcWithPackages`. That function expects one argument: a
function that maps from an attribute set of Haskell packages to a list of
packages, which determines the libraries known to that particular version of
GHC. For example, the Nix expression `ghcWithPackages (pkgs: [pkgs.mtl])`
generates a copy of GHC that has the `mtl` library registered in addition to
its normal core packages:
```
$ nix-shell -p "haskellPackages.ghcWithPackages (pkgs: [pkgs.mtl])"
[nix-shell:~]$ ghc-pkg list mtl
/nix/store/zy79...-ghc-7.10.2/lib/ghc-7.10.2/package.conf.d:
mtl-2.2.1
```
This function allows users to define their own development environment by means
of an override. After adding the following snippet to `~/.config/nixpkgs/config.nix`,
```nix
{
packageOverrides = super: let self = super.pkgs; in
{
myHaskellEnv = self.haskell.packages.ghc7102.ghcWithPackages
(haskellPackages: with haskellPackages; [
# libraries
arrows async cgi criterion
# tools
cabal-install haskintex
]);
};
}
```
it's possible to install that compiler with `nix-env -f "<nixpkgs>
|