[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](../user-guide.md#using-nix-repl) to
test things.

 * [Types](#types) — the kinds of data that you will encounter working with [Haskell.nix][].
 * [Top-level attributes](#top-level-attributes) — Functions and derivations defined in the Haskell.nix attrset.
 * [Package-set functions](#package-set-functions) — Helper functions defined on the `hsPkgs` package set.

# 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 = { ... };
    };
  };
}
```

| Attribute      | Type | Description                                         |
|----------------|------|-----------------------------------------------------|
| `options` | Module options | The combination of all options set through the `modules` argument passed to `mkPkgsSet`. |
| `config` |  | The result of evaluating and applying the `options` with [Haskell.nix][] |
| `.hsPkgs` | Attrset of [Haskell Packages](#haskell-package) | Buildable packages, created from `packages` |
| `.packages` | Attrset of [Haskell Package descriptions](#haskell-package-descriptions) | Configuration for each package in `hsPkgs` |
| `.compiler` | Attrset | |



## 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](./modules.md).

## 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](./modules.md).


## 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](https://www.haskell.org/cabal/users-guide/developing-packages.html)
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](./options.md), 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:

| Argument         | Type | Description         | 
|------------------|------|---------------------|
| `haskellLib`     | attrset | The [haskellLib](#haskelllib) utility functions. |
| `pkgs` | | The Nixpkgs collection. |
| `pkgconfPkgs` | | A 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](#package-set) based on the `pkgs.nix` output
of `stack-to-nix`.

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

| Argument         | Type | Description         | 
|------------------|------|---------------------|
| `stack-pkgs`     |  | `import ./pkgs.nix` — The imported file generated by `stack‑to‑nix`. |
| `pkg‑def‑extras` | List of [Extras](#extras) | For overriding the package set. |
| `modules` | List of [Modules](#modules) | For overriding the package set. |

**Return value**: a [`pkgSet`](#package-set)

## mkCabalProjectPkgSet

Creates a [package set](#package-set) based on the `pkgs.nix` output
of `plan-to-nix`.

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

| Argument         | Type | Description         | 
|------------------|------|---------------------|
| `plan-pkgs`     |  | `import ./pkgs.nix` — The imported file generated by `plan‑to‑nix`. |
| `pkg‑def‑extras` | List of [Extras](#extras) | For overriding the package set. |
| `modules` | List of [Modules](#modules) | For overriding the package set. |

**Return value**: a [`pkgSet`](#package-set)

## mkPkgSet

This is the base function used by both `mkStackPkgSet` and
`mkCabalProjectPkgSet`.

**Return value**: a [`pkgSet`](#package-set)

## 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`](#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](commands.md).

## callStackToNix

## callCabalProjectToNix

## hackage
## stackage

## fetchExternal

## cleanSourceHaskell

## haskellLib

Assorted functions for operating on [Haskell.nix][] data. This is
distinct from `pkgs.haskell.lib` in the current Nixpkgs Haskell
Infrastructure.

# Package-set functions

These functions exist within the `hsPkgs` package set.

## shellFor

Create a `nix-shell` [development
environment](../user-guide/development.md) for developing one or more
packages.

```
shellFor =
    { packages, withHoogle ? true, ...}: ...
```


| Argument       | Type | Description         | 
|----------------|------|---------------------|
| `packages`     | Function | Package selection function. It takes a list of [Haskell packages](#haskell-package) and returns a subset of these packages. |
| `withHoogle` | Boolean | Whether to build a Hoogle documentation index and provide the `hoogle` command. |
| `{ ... }` | Attrset | All the other arguments are passed to [`mkDerivation`](https://nixos.org/nixpkgs/manual/#sec-using-stdenv). |

**Return value**: a derivation

## ghcWithPackages

Creates a `nix-shell` [development
environment](../user-guide/development.md) 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.

[haskell.nix]: https://github.com/input-output-hk/haskell.nix