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This is needed to avoid this https://github.com/mesonbuild/meson/issues/13774 when we go back to making our subproject directory `src`.
632 lines
8.7 KiB
Markdown
632 lines
8.7 KiB
Markdown
# Nix Language
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The Nix language is designed for conveniently creating and composing *derivations* – precise descriptions of how contents of existing files are used to derive new files.
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> **Tip**
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>
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> These pages are written as a reference.
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> If you are learning Nix, nix.dev has a good [introduction to the Nix language](https://nix.dev/tutorials/nix-language).
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The language is:
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- *domain-specific*
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It comes with [built-in functions](@docroot@/language/builtins.md) to integrate with the Nix store, which manages files and performs the derivations declared in the Nix language.
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- *declarative*
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There is no notion of executing sequential steps.
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Dependencies between operations are established only through data.
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- *pure*
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Values cannot change during computation.
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Functions always produce the same output if their input does not change.
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- *functional*
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Functions are like any other value.
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Functions can be assigned to names, taken as arguments, or returned by functions.
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- *lazy*
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Values are only computed when they are needed.
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- *dynamically typed*
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Type errors are only detected when expressions are evaluated.
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# Overview
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This is an incomplete overview of language features, by example.
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<table>
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<tr>
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<th>
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Example
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</th>
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<th>
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Description
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</th>
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</tr>
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<tr>
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<td>
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*Basic values ([primitives](@docroot@/language/types.md#primitives))*
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</td>
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<td>
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</td>
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</tr>
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<tr>
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<td>
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`"hello world"`
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</td>
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<td>
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A [string](@docroot@/language/types.md#type-string)
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</td>
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</tr>
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<tr>
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<td>
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```
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''
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multi
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line
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string
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''
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```
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</td>
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<td>
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<!-- FIXME: using two no-break spaces, because apparently mdBook swallows the second regular space! -->
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A multi-line string. Strips common prefixed whitespace. Evaluates to `"multi\n line\n string"`.
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</td>
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</tr>
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<tr>
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<td>
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`# Explanation`
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</td>
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<td>
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A [comment](@docroot@/language/syntax.md#comments).
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</td>
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</tr>
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<tr>
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<td>
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`"hello ${ { a = "world"; }.a }"`
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`"1 2 ${toString 3}"`
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`"${pkgs.bash}/bin/sh"`
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</td>
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<td>
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[String interpolation](@docroot@/language/string-interpolation.md) (expands to `"hello world"`, `"1 2 3"`, `"/nix/store/<hash>-bash-<version>/bin/sh"`)
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</td>
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</tr>
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<tr>
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<td>
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`true`, `false`
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</td>
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<td>
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[Booleans](@docroot@/language/types.md#type-boolean)
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</td>
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</tr>
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<tr>
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<td>
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`null`
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</td>
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<td>
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[Null](@docroot@/language/types.md#type-null) value
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</td>
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</tr>
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<tr>
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<td>
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`123`
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</td>
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<td>
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An [integer](@docroot@/language/types.md#type-int)
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</td>
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</tr>
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<tr>
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<td>
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`3.141`
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</td>
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<td>
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A [floating point number](@docroot@/language/types.md#type-float)
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</td>
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</tr>
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<tr>
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<td>
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`/etc`
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</td>
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<td>
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An absolute [path](@docroot@/language/types.md#type-path)
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</td>
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</tr>
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<tr>
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<td>
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`./foo.png`
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</td>
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<td>
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A [path](@docroot@/language/types.md#type-path) relative to the file containing this Nix expression
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</td>
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</tr>
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<tr>
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<td>
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`~/.config`
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</td>
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<td>
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A home [path](@docroot@/language/types.md#type-path). Evaluates to the `"<user's home directory>/.config"`.
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</td>
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</tr>
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<tr>
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<td>
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`<nixpkgs>`
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</td>
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<td>
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A [lookup path](@docroot@/language/constructs/lookup-path.md) for Nix files. Value determined by [`$NIX_PATH` environment variable](../command-ref/env-common.md#env-NIX_PATH).
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</td>
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</tr>
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<tr>
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<td>
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*Compound values*
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</td>
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<td>
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</td>
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</tr>
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<tr>
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<td>
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`{ x = 1; y = 2; }`
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</td>
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<td>
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An [attribute set](@docroot@/language/types.md#attribute-set) with attributes named `x` and `y`
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</td>
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</tr>
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<tr>
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<td>
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`{ foo.bar = 1; }`
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</td>
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<td>
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A nested set, equivalent to `{ foo = { bar = 1; }; }`
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</td>
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</tr>
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<tr>
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<td>
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`rec { x = "foo"; y = x + "bar"; }`
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</td>
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<td>
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A [recursive set](@docroot@/language/syntax.md#recursive-sets), equivalent to `{ x = "foo"; y = "foobar"; }`.
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</td>
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</tr>
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<tr>
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<td>
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`[ "foo" "bar" "baz" ]`
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`[ 1 2 3 ]`
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`[ (f 1) { a = 1; b = 2; } [ "c" ] ]`
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</td>
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<td>
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[Lists](@docroot@/language/types.md#list) with three elements.
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</td>
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</tr>
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<tr>
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<td>
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*Operators*
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</td>
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<td>
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</td>
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</tr>
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<tr>
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<td>
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`"foo" + "bar"`
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</td>
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<td>
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String concatenation
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</td>
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</tr>
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<tr>
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<td>
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`1 + 2`
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</td>
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<td>
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Integer addition
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</td>
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</tr>
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<tr>
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<td>
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`"foo" == "f" + "oo"`
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</td>
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<td>
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Equality test (evaluates to `true`)
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</td>
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</tr>
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<tr>
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<td>
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`"foo" != "bar"`
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</td>
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<td>
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Inequality test (evaluates to `true`)
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</td>
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</tr>
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<tr>
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<td>
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`!true`
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</td>
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<td>
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Boolean negation
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</td>
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</tr>
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<tr>
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<td>
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`{ x = 1; y = 2; }.x`
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</td>
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<td>
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[Attribute selection](@docroot@/language/types.md#attribute-set) (evaluates to `1`)
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</td>
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</tr>
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<tr>
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<td>
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`{ x = 1; y = 2; }.z or 3`
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</td>
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<td>
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[Attribute selection](@docroot@/language/types.md#attribute-set) with default (evaluates to `3`)
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</td>
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</tr>
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<tr>
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<td>
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`{ x = 1; y = 2; } // { z = 3; }`
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</td>
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<td>
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Merge two sets (attributes in the right-hand set taking precedence)
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</td>
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</tr>
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<tr>
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<td>
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*Control structures*
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</td>
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<td>
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</td>
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</tr>
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<tr>
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<td>
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`if 1 + 1 == 2 then "yes!" else "no!"`
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</td>
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<td>
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[Conditional expression](@docroot@/language/syntax.md#conditionals).
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</td>
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</tr>
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<tr>
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<td>
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`assert 1 + 1 == 2; "yes!"`
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</td>
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<td>
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[Assertion](@docroot@/language/syntax.md#assertions) check (evaluates to `"yes!"`).
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</td>
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</tr>
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<tr>
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<td>
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`let x = "foo"; y = "bar"; in x + y`
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</td>
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<td>
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Variable definition. See [`let`-expressions](@docroot@/language/syntax.md#let-expressions).
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</td>
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</tr>
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<tr>
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<td>
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`with builtins; head [ 1 2 3 ]`
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</td>
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<td>
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Add all attributes from the given set to the scope (evaluates to `1`).
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See [`with`-expressions](@docroot@/language/syntax.md#with-expressions) for details and shadowing caveats.
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</td>
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</tr>
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<tr>
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<td>
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`inherit pkgs src;`
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</td>
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<td>
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Adds the variables to the current scope (attribute set or `let` binding).
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Desugars to `pkgs = pkgs; src = src;`.
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See [Inheriting attributes](@docroot@/language/syntax.md#inheriting-attributes).
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</td>
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</tr>
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<tr>
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<td>
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`inherit (pkgs) lib stdenv;`
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</td>
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<td>
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Adds the attributes, from the attribute set in parentheses, to the current scope (attribute set or `let` binding).
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Desugars to `lib = pkgs.lib; stdenv = pkgs.stdenv;`.
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See [Inheriting attributes](@docroot@/language/syntax.md#inheriting-attributes).
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</td>
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</tr>
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<tr>
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<td>
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*[Functions](@docroot@/language/syntax.md#functions) (lambdas)*
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</td>
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<td>
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</td>
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</tr>
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<tr>
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<td>
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`x: x + 1`
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</td>
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<td>
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A [function](@docroot@/language/syntax.md#functions) that expects an integer and returns it increased by 1.
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</td>
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</tr>
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<tr>
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<td>
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`x: y: x + y`
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</td>
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<td>
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Curried [function](@docroot@/language/syntax.md#functions), equivalent to `x: (y: x + y)`. Can be used like a function that takes two arguments and returns their sum.
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</td>
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</tr>
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<tr>
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<td>
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`(x: x + 1) 100`
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</td>
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<td>
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A [function](@docroot@/language/syntax.md#functions) call (evaluates to 101)
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</td>
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</tr>
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<tr>
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<td>
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`let inc = x: x + 1; in inc (inc (inc 100))`
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</td>
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<td>
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A [function](@docroot@/language/syntax.md#functions) bound to a variable and subsequently called by name (evaluates to 103)
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</td>
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</tr>
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<tr>
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<td>
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`{ x, y }: x + y`
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</td>
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<td>
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A [function](@docroot@/language/syntax.md#functions) that expects a set with required attributes `x` and `y` and concatenates them
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</td>
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</tr>
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<tr>
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<td>
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`{ x, y ? "bar" }: x + y`
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</td>
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<td>
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A [function](@docroot@/language/syntax.md#functions) that expects a set with required attribute `x` and optional `y`, using `"bar"` as default value for `y`
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</td>
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</tr>
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<tr>
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<td>
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`{ x, y, ... }: x + y`
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</td>
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<td>
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A [function](@docroot@/language/syntax.md#functions) that expects a set with required attributes `x` and `y` and ignores any other attributes
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</td>
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</tr>
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<tr>
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<td>
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`{ x, y } @ args: x + y`
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`args @ { x, y }: x + y`
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</td>
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<td>
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A [function](@docroot@/language/syntax.md#functions) that expects a set with required attributes `x` and `y`, and binds the whole set to `args`
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</td>
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</tr>
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<tr>
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<td>
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*Built-in functions*
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</td>
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<td>
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</td>
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</tr>
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<tr>
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<td>
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`import ./foo.nix`
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</td>
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<td>
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Load and return Nix expression in given file.
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See [import](@docroot@/language/builtins.md#builtins-import).
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</td>
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</tr>
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<tr>
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<td>
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`map (x: x + x) [ 1 2 3 ]`
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</td>
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<td>
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Apply a function to every element of a list (evaluates to `[ 2 4 6 ]`).
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See [`map`](@docroot@/language/builtins.md#builtins-map).
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</td>
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</tr>
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</table>
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