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https://github.com/pjones/nix-hs

Haskell + nixpkgs = nix-hs
https://github.com/pjones/nix-hs

cabal direnv haskell nixpkgs

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Haskell + nixpkgs = nix-hs

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# Haskell + nixpkgs = nix-hs



[![tests](https://github.com/pjones/nix-hs/workflows/tests/badge.svg)](https://github.com/pjones/nix-hs/actions?query=workflow%3Atests)

[![GitHub tag (latest by date)](https://img.shields.io/github/v/tag/pjones/nix-hs?label=release)](https://github.com/pjones/nix-hs/releases)

[![cachix](https://img.shields.io/badge/cachix-nix--hs-green)](https://app.cachix.org/cache/nix-hs)



A thin layer over the existing [Haskell][] infrastructure in

[nixpkgs][] which adds all of the tools needed for interactive

development.  Here are some of the features that you might find the

most useful:



  * An interactive development environment (via `nix-shell`) that

    includes:



      * GHC (8.8.4, 8.10.4, 9.0.1)



      * `cabal-install` 3.4.0



      * `stack` 2.7.1



      * [hlint][] 3.3.1



      * [haskell-language-server][] 1.1.0



      * [ormolu][ormolu] 0.1.4.1



      * [cabal-fmt][cabal-fmt] 0.1.5.1



      * [stan][stan] 0.0.1.0



      * and a Hoogle database for all of your project's dependencies



    **NOTE:** The following tools do not currently support GHC 9.0.1:



      * haskell-language-server



      * ormolu



      * stan



  * Easy to use system for [overriding Haskell packages](#using-a-broken-package) (e.g., use

    a package not on Hackage, fix a package marked as broken, etc.)

    without having to write tons of Nix code.



  * Works seamlessly with [direnv][], [lorri][], and [niv][] if you

    already have those tools installed.



  * Switch GHC versions by passing an argument to `nix-build` or

    `nix-shell`.



  * [Strip dependencies](#access-to-binary-only-packages) from a package so you can deploy just a

    binary without tons of other packages coming along for the ride.



  * Create an interactive development environment [without adding

    nix-hs](#interactive-environments-without-nix-hs) as a project dependency.



  * Fetch pre-built tools from [the binary cache](#using-the-binary-cache).



## Geting Started



Create a `default.nix` file that looks something like this:



```nix

{ pkgs ? import  { }

}:

let

  nix-hs =

    import

      (fetchTarball

        "https://github.com/pjones/nix-hs/archive/release-21.05.tar.gz")

      {

        inherit pkgs;

      };

in

nix-hs {

  cabal = ./test/hello-world/hello-world.cabal;

}

```



And a `shell.nix` that looks like this:



```nix

# Load an interactive environment:

(import ./. {}).interactive

```



That's it!  Now `nix-build` and `nix-shell` just work!



## Configuration



In addition to the `cabal` argument to the `nix-hs` function, there

are other ways to control how your package is built.



### Enable Flags from the Cabal File



If you have a flag defined in your package's cabal file you can enable

it using the `flags` argument:



```nix

nix-hs {

  cabal = ./mypackage.cabal;

  flags = [ "someflagname" ];

}

```



### Using a Broken Package



If one of your package's dependencies can't be built you can try

overriding it:



```nix

nix-hs {

  cabal = ./mypackage.cabal;



  overrides = lib: self: super: with lib; {

    pipes-text = unBreak (dontCheck (doJailbreak super.pipes-text));

  };

}

```



In the example above, the `overrides` function takes three arguments:



  1. `lib`: An attribute set of library functions.  These are the

     functions provided by the `pkgs.haskellPackages.lib` set plus a

     few more that you might find useful such as:



     - `unBreak`: Remove the `broken` flag from a package

     - `compilerName`: The nixpkgs name of the Haskell compiler

       being used (e.g. `ghc884`)

     - `pkgs`: The full nixpkgs package set, after overriding



  2. `self`: The final set of Haskell packages after applying all

     overrides.  This refers to the future version of the package set

     so if you're not careful you can fall into infinite recursion.

     When in doubt use `super` instead.



  3. `super`: The set of Haskell packages that are being modified.

     Use this attribute set to refer to existing Haskell packages.

     You can also use `super` to access useful functions such as

     `callCabal2nix` and `callHackageDirect`.



The `overrides` function should return an attribute set of Haskell

packages.  The set of returned packages will be merged into the final

set used to build your package.



An example package using the `overrides` function can be found in

`test/overrides/default.nix`.  The full list of additional `lib`

functions is in `lib/haskell.nix`.



### Working with Multi-Package Cabal Projects



If you have a project that contains multiple Cabal packages you can

build them all with a single `default.nix`.  The `cabal` argument to

`nix-hs` can either be a path to a Cabal file *or* an attribute set of

Cabal files:



```nix

nix-hs {

  cabal = {

    package1 = ./package1/package1.cabal;

    package2 = ./package1/package2.cabal;

  };

}

```

## Integrating Your Text Editor and Shell



The best way to let your text editor and shell use the environment

created from Nix is to use [direnv][].  Here's an example `.envrc`

file:



```sh

# Use lorri if it's installed, otherwise load shell.nix:

if has lorri; then

  eval "$(lorri direnv)"

else

  use nix

fi



# Reload if these files change:

watch_file $(find . -name '*.cabal' -o -name '*.nix')

```



**NOTE:** Make sure you have a `shell.nix` file that exposes the

`interactive` attribute of the derivation, like the example above.



## Interactive Environments Without nix-hs



If you don't want to use `nix-hs` to control your `default.nix` you

can still use it for building an interactive development environment.

Just clone this repository and use the `shell/default.nix` file.



For example, to drop into an interactive shell:



```

$ nix-shell /path/to/nix-hs/shell

```



Or



```

$ nix-shell --argstr compiler 8.8.3 /path/to/nix-hs/shell

```



Even better, use [direnv][] so your normal shell and text editor can

see all the installed development tools.  Here's an example `.envrc`

file:



```sh

use nix /path/to/nix-hs/shell

```



## Access to Binary-Only Packages



The derivation generated by the `nix-hs` function makes it easy to

access a "binary only" derivation.  This is perfect for deployments or

Docker containers where you don't want to bring along all of your

package's dependencies (including GHC).



The `bin` attribute of the derivation gives you access to this binary

only derivation.  For example, to create a docker container put the

following in `docker.nix`:



```nix

{ pkgs ? import  { }

}:



let

  mypackage = (import ./. { inherit pkgs; }).bin;



in pkgs.dockerTools.buildImage {

  name = "mypackage";

  tag  = "latest";



  config = {

    Cmd = [ "${mypackage}/bin/hello" ];

  };

}

```



## Using the Binary Cache



If you don't want to spend all day compiling the tools needed to build

your Haskell package and its development environment you can use the

`nix-hs` cache [on Cachix](https://app.cachix.org/cache/nix-hs).



The cache is populated after each `git push` via a [GitHub action](https://github.com/pjones/nix-hs/actions).



NOTE: Due to disk space limitations the cache is limited to the

current LTS version of GHC.



[haskell]: https://www.haskell.org/

[nixpkgs]: https://nixos.org/nix/

[direnv]: https://github.com/direnv/direnv

[lorri]: https://github.com/target/lorri

[niv]: https://github.com/nmattia/niv

[ormolu]: https://github.com/tweag/ormolu

[stan]: https://github.com/kowainik/stan

[cabal-fmt]: https://github.com/phadej/cabal-fmt

[haskell-language-server]: https://github.com/haskell/haskell-language-server

[hlint]: https://github.com/ndmitchell/hlint