159 lines
6 KiB
Markdown
159 lines
6 KiB
Markdown
# Nixpkgs lib
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This directory contains the implementation, documentation and tests for the Nixpkgs `lib` library.
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## Overview
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The evaluation entry point for `lib` is [`default.nix`](default.nix).
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This file evaluates to an attribute set containing two separate kinds of attributes:
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- Sub-libraries:
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Attribute sets grouping together similar functionality.
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Each sub-library is defined in a separate file usually matching its attribute name.
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Example: `lib.lists` is a sub-library containing list-related functionality such as `lib.lists.take` and `lib.lists.imap0`.
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These are defined in the file [`lists.nix`](lists.nix).
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- Aliases:
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Attributes that point to an attribute of the same name in some sub-library.
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Example: `lib.take` is an alias for `lib.lists.take`.
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Most files in this directory are definitions of sub-libraries, but there are a few others:
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- [`minver.nix`](minver.nix): A string of the minimum version of Nix that is required to evaluate Nixpkgs.
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- [`tests`](tests): Tests, see [Running tests](#running-tests)
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- [`release.nix`](tests/release.nix): A derivation aggregating all tests
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- [`misc.nix`](tests/misc.nix): Evaluation unit tests for most sub-libraries
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- `*.sh`: Bash scripts that run tests for specific sub-libraries
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- All other files in this directory exist to support the tests
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- [`systems`](systems): The `lib.systems` sub-library, structured into a directory instead of a file due to its complexity
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- [`path`](path): The `lib.path` sub-library, which includes tests as well as a document describing the design goals of `lib.path`
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- All other files in this directory are sub-libraries
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### Module system
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The [module system](https://nixos.org/manual/nixpkgs/#module-system) spans multiple sub-libraries:
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- [`modules.nix`](modules.nix): `lib.modules` for the core functions and anything not relating to option definitions
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- [`options.nix`](options.nix): `lib.options` for anything relating to option definitions
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- [`types.nix`](types.nix): `lib.types` for module system types
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## PR Guidelines
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Follow these guidelines for proposing a change to the interface of `lib`.
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### Provide a Motivation
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Clearly describe why the change is necessary and its use cases.
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Make sure that the change benefits the user more than the added mental effort of looking it up and keeping track of its definition.
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If the same can reasonably be done with the existing interface,
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consider just updating the documentation with more examples and links.
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This is also known as the [Fairbairn Threshold](https://wiki.haskell.org/Fairbairn_threshold).
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Through this principle we avoid the human cost of duplicated functionality in an overly large library.
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### Make one PR for each change
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Don't have multiple changes in one PR, instead split it up into multiple ones.
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This keeps the conversation focused and has a higher chance of getting merged.
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### Name the interface appropriately
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When introducing new names to the interface, such as new function, or new function attributes,
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make sure to name it appropriately.
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Names should be self-explanatory and consistent with the rest of `lib`.
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If there's no obvious best name, include the alternatives you considered.
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### Write documentation
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Update the [reference documentation](#reference-documentation) to reflect the change.
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Be generous with links to related functionality.
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### Write tests
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Add good test coverage for the change, including:
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- Tests for edge cases, such as empty values or lists.
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- Tests for tricky inputs, such as a string with string context or a path that doesn't exist.
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- Test all code paths, such as `if-then-else` branches and returned attributes.
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- If the tests for the sub-library are written in bash,
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test messages of custom errors, such as `throw` or `abortMsg`,
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At the time this is only not necessary for sub-libraries tested with [`tests/misc.nix`](./tests/misc.nix).
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See [running tests](#running-tests) for more details on the test suites.
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### Write tidy code
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Name variables well, even if they're internal.
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The code should be as self-explanatory as possible.
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Be generous with code comments when appropriate.
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As a baseline, follow the [Nixpkgs code conventions](https://github.com/NixOS/nixpkgs/blob/master/CONTRIBUTING.md#code-conventions).
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### Write efficient code
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Nix generally does not have free abstractions.
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Be aware that seemingly straightforward changes can cause more allocations and a decrease in performance.
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That said, don't optimise prematurely, especially in new code.
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## Reference documentation
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Reference documentation for library functions is written above each function as a multi-line comment.
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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).
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The nixdoc README describes the [comment format](https://github.com/nix-community/nixdoc#comment-format).
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See [doc/README.md](../doc/README.md) for how to build the manual.
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## Running tests
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All library tests can be run by building the derivation in [`tests/release.nix`](tests/release.nix):
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```bash
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nix-build tests/release.nix
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```
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Some commands for quicker iteration over parts of the test suite are also available:
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```bash
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# Run all evaluation unit tests in tests/misc.nix
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# if the resulting list is empty, all tests passed
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nix-instantiate --eval --strict tests/misc.nix
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# Run the module system tests
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tests/modules.sh
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# Run the lib.sources tests
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tests/sources.sh
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# Run the lib.filesystem tests
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tests/filesystem.sh
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# Run the lib.path property tests
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path/tests/prop.sh
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# Run the lib.fileset tests
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fileset/tests.sh
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```
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## Commit conventions
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- Make sure you read about the [commit conventions](../CONTRIBUTING.md#commit-conventions) common to Nixpkgs as a whole.
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- Format the commit messages in the following way:
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```
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lib.(section): (init | add additional argument | refactor | etc)
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(Motivation for change. Additional information.)
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```
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Examples:
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* lib.getExe': check arguments
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* lib.fileset: Add an additional argument in the design docs
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Closes #264537
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