---
title: Nix and Haskell
date: 2018-09-20
---

The simplest way you can use haskell and nix together is by calling
`callCabal2nix` on the cabal file and using the resulting artifacts.

I usually create two files `release.nix` and `default.nix`. At least when
developing, I avoid using the `cabal2nix` tool and instead let nix handle
calling it for me. If I ever published an actual package, I would probably
provide the autogenerated `default.nix` and nothing else.

**Build instructions** are managed by `default.nix`. At its simplest it's just
a call to `callCabal2nix`, but might contain for example some extra cabal flags
or adding patches.

```nix
# default.nix
{ haskellPackages }:

haskellPackages.callCabal2nix "foo" ./. {}
```

**Pinning** of haskell packages and system libraries are managed by this file.
Nixpkgs requires a bootstrap `<nixpkgs>` in scope so that we can pin to a
specific commit hash.

```nix
# release.nix
{ pkgs ? import <nixpkgs> {} }:

# No pinning yet
pkgs.callPackage ./default.nix {}
```