# Haskell {#haskell}

The Haskell infrastructure in Nixpkgs has two main purposes: The primary purpose
is to provide a Haskell compiler and build tools as well as infrastructure for
packaging Haskell-based packages.

The secondary purpose is to provide support for Haskell development environments
including prebuilt Haskell libraries. However, in this area sacrifices have been
made due to self-imposed restrictions in Nixpkgs, to lessen the maintenance
effort and to improve performance. (More details in the subsection
[Limitations.](#haskell-limitations))

## Available packages {#haskell-available-packages}

The compiler and most build tools are exposed at the top level:

* `ghc` is the default version of GHC
* Language specific tools: `cabal-install`, `stack`, `hpack`, …

Many “normal” user facing packages written in Haskell, like `niv` or `cachix`,
are also exposed at the top level, and there is nothing Haskell specific to
installing and using them.

All of these packages are originally defined in the `haskellPackages` package set.
The same packages are re-exposed with a reduced dependency closure for convenience (see `justStaticExecutables` or `separateBinOutput` below).

:::{.note}
See [](#chap-language-support) for techniques to explore package sets.
:::

The `haskellPackages` set includes at least one version of every package from [Hackage](https://hackage.haskell.org/) as well as some manually injected packages.

The attribute names in `haskellPackages` always correspond with their name on
Hackage. Since Hackage allows names that are not valid Nix without escaping,
you need to take care when handling attribute names like `3dmodels`.

For packages that are part of [Stackage] (a curated set of known to be
compatible packages), we use the version prescribed by a Stackage snapshot
(usually the current LTS one) as the default version. For all other packages we
use the latest version from [Hackage](https://hackage.org) (the repository of
basically all open source Haskell packages). See [below](#haskell-available-versions) for a few more details on this.

Roughly half of the 16K packages contained in `haskellPackages` don’t actually
build and are [marked as broken semi-automatically](https://github.com/NixOS/nixpkgs/blob/haskell-updates/pkgs/development/haskell-modules/configuration-hackage2nix/broken.yaml).
Most of those packages are deprecated or unmaintained, but sometimes packages
that should build, do not build. Very often fixing them is not a lot of work.

<!--
TODO(@sternenseemann):
How you can help with that is
described in [Fixing a broken package](#haskell-fixing-a-broken-package).
-->

`haskellPackages` is built with our default compiler, but we also provide other releases of GHC and package sets built with them.
Available compilers are collected under `haskell.compiler`.

Each of those compiler versions has a corresponding attribute set `packages` built with
it. However, the non-standard package sets are not tested regularly and, as a
result, contain fewer working packages. The corresponding package set for GHC
9.4.8 is `haskell.packages.ghc948`. In fact `haskellPackages` (at the time of writing) is just an alias
for `haskell.packages.ghc984`:

Every package set also re-exposes the GHC used to build its packages as `haskell.packages.*.ghc`.

### Available package versions {#haskell-available-versions}

We aim for a “blessed” package set which only contains one version of each
package, like [Stackage], which is a curated set of known to be compatible
packages. We use the version information from Stackage snapshots and extend it
with more packages. Normally in Nixpkgs the number of building Haskell packages
is roughly two to three times the size of Stackage. For choosing the version to
use for a certain package we use the following rules:

1. By default, for `haskellPackages.foo` is the newest version of the package
`foo` found on [Hackage](https://hackage.org), which is the central registry
of all open source Haskell packages. Nixpkgs contains a reference to a pinned