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52 lines
2.6 KiB
Markdown
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<!-- TODO: Render this document in front of function documentation in case https://github.com/nix-community/nixdoc/issues/19 is ever supported -->
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# File sets {#sec-fileset}
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The [`lib.fileset`](#sec-functions-library-fileset) library allows you to work with _file sets_.
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A file set is a mathematical set of local files that can be added to the Nix store for use in Nix derivations.
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File sets are easy and safe to use, providing obvious and composable semantics with good error messages to prevent mistakes.
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These sections apply to the entire library.
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See the [function reference](#sec-functions-library-fileset) for function-specific documentation.
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The file set library is currently very limited but is being expanded to include more functions over time.
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## Implicit coercion from paths to file sets {#sec-fileset-path-coercion}
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All functions accepting file sets as arguments can also accept [paths](https://nixos.org/manual/nix/stable/language/values.html#type-path) as arguments.
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Such path arguments are implicitly coerced to file sets containing all files under that path:
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- A path to a file turns into a file set containing that single file.
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- A path to a directory turns into a file set containing all files _recursively_ in that directory.
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If the path points to a non-existent location, an error is thrown.
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::: {.note}
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Just like in Git, file sets cannot represent empty directories.
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Because of this, a path to a directory that contains no files (recursively) will turn into a file set containing no files.
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:::
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:::{.note}
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File set coercion does _not_ add any of the files under the coerced paths to the store.
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Only the [`toSource`](#function-library-lib.fileset.toSource) function adds files to the Nix store, and only those files contained in the `fileset` argument.
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This is in contrast to using [paths in string interpolation](https://nixos.org/manual/nix/stable/language/values.html#type-path), which does add the entire referenced path to the store.
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:::
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### Example {#sec-fileset-path-coercion-example}
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Assume we are in a local directory with a file hierarchy like this:
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```
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├─ a/
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│ ├─ x (file)
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│ └─ b/
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│ └─ y (file)
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└─ c/
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└─ d/
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```
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Here's a listing of which files get included when different path expressions get coerced to file sets:
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- `./.` as a file set contains both `a/x` and `a/b/y` (`c/` does not contain any files and is therefore omitted).
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- `./a` as a file set contains both `a/x` and `a/b/y`.
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- `./a/x` as a file set contains only `a/x`.
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- `./a/b` as a file set contains only `a/b/y`.
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- `./c` as a file set is empty, since neither `c` nor `c/d` contain any files.
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