Merge pull request #244044 from tweag/lib-readme

# Nixpkgs lib

This directory contains the implementation, documentation and tests for the Nixpkgs `lib` library.

## Overview

The evaluation entry point for `lib` is [`default.nix`](default.nix).

This file evaluates to an attribute set containing two separate kinds of attributes:

- Sub-libraries:

  Attribute sets grouping together similar functionality.

  Each sub-library is defined in a separate file usually matching its attribute name.

  Example: `lib.lists` is a sub-library containing list-related functionality such as `lib.lists.take` and `lib.lists.imap0`.

  These are defined in the file [`lists.nix`](lists.nix).

- Aliases:

  Attributes that point to an attribute of the same name in some sub-library.

  Example: `lib.take` is an alias for `lib.lists.take`.

Most files in this directory are definitions of sub-libraries, but there are a few others:

- [`minver.nix`](minver.nix): A string of the minimum version of Nix that is required to evaluate Nixpkgs.

- [`tests`](tests): Tests, see [Running tests](#running-tests)

  - [`release.nix`](tests/release.nix): A derivation aggregating all tests

  - [`misc.nix`](tests/misc.nix): Evaluation unit tests for most sub-libraries

  - `*.sh`: Bash scripts that run tests for specific sub-libraries

  - All other files in this directory exist to support the tests

- [`systems`](systems): The `lib.systems` sub-library, structured into a directory instead of a file due to its complexity

- [`path`](path): The `lib.path` sub-library, which includes tests as well as a document describing the design goals of `lib.path`

- All other files in this directory are sub-libraries

### Module system

The [module system](https://nixos.org/manual/nixpkgs/#module-system) spans multiple sub-libraries:

- [`modules.nix`](modules.nix): `lib.modules` for the core functions and anything not relating to option definitions

- [`options.nix`](options.nix): `lib.options` for anything relating to option definitions

- [`types.nix`](types.nix): `lib.types` for module system types

## Reference documentation

Reference documentation for library functions is written above each function as a multi-line comment.

These comments are processed using [nixdoc](https://github.com/nix-community/nixdoc) and [rendered in the Nixpkgs manual](https://nixos.org/manual/nixpkgs/stable/#chap-functions).

The nixdoc README describes the [comment format](https://github.com/nix-community/nixdoc#comment-format).

See the [chapter on contributing to the Nixpkgs manual](https://nixos.org/manual/nixpkgs/#chap-contributing) for how to build the manual.

## Running tests

All library tests can be run by building the derivation in [`tests/release.nix`](tests/release.nix):

```bash

nix-build tests/release.nix

```

Some commands for quicker iteration over parts of the test suite are also available:

```bash

# Run all evaluation unit tests in tests/misc.nix

# if the resulting list is empty, all tests passed

nix-instantiate --eval --strict tests/misc.nix

# Run the module system tests

tests/modules.sh

# Run the lib.sources tests

tests/sources.sh

# Run the lib.filesystem tests

tests/filesystem.sh

# Run the lib.path property tests

path/tests/prop.sh

```

#!/usr/bin/env bash

# Property tests for the `lib.path` library

#

# Property tests for lib/path/default.nix

# It generates random path-like strings and runs the functions on

# them, checking that the expected laws of the functions hold

# Run:

# [nixpkgs]$ lib/path/tests/prop.sh

# or:

# [nixpkgs]$ nix-build lib/tests/release.nix

set -euo pipefail

shopt -s inherit_errexit

# to run these tests:

# nix-instantiate --eval --strict nixpkgs/lib/tests/misc.nix

# if the resulting list is empty, all tests passed

/*

Nix evaluation tests for various lib functions.

Since these tests are implemented with Nix evaluation, error checking is limited to what `builtins.tryEval` can detect, which is `throw`'s and `abort`'s, without error messages.

If you need to test error messages or more complex evaluations, see ./modules.sh, ./sources.sh or ./filesystem.sh as examples.

To run these tests:

  [nixpkgs]$ nix-instantiate --eval --strict lib/tests/misc.nix

If the resulting list is empty, all tests passed.

Alternatively, to run all `lib` tests:

  [nixpkgs]$ nix-build lib/tests/release.nix

*/

with import ../default.nix;

let

#!/usr/bin/env bash

#

# This script is used to test that the module system is working as expected.

# Executing it runs tests for `lib.modules`, `lib.options` and `lib.types`.

# By default it test the version of nixpkgs which is defined in the NIX_PATH.

#

# Run:

# [nixpkgs]$ lib/tests/modules.sh

# or:

# [nixpkgs]$ nix-build lib/tests/release.nix

set -o errexit -o noclobber -o nounset -o pipefail

shopt -s failglob inherit_errexit

#!/usr/bin/env bash

# Tests lib/sources.nix

# Run:

# [nixpkgs]$ lib/tests/sources.sh

# or:

# [nixpkgs]$ nix-build lib/tests/release.nix

set -euo pipefail

shopt -s inherit_errexit