forked from lix-project/lix
582 lines
6.1 KiB
Markdown
582 lines
6.1 KiB
Markdown
# Nix Language
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The Nix language is
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- *domain-specific*
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It only exists for the Nix package manager:
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to describe packages and configurations as well as their variants and compositions.
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It is not intended for general purpose use.
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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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Expressions are only evaluated when their value is 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*
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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
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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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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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`"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 (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
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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 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
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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
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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
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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 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. 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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Search path 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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A 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, 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 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 (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 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
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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 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
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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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</td>
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</tr>
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<tr>
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<td>
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*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 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, 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 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 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 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 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 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 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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</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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</td>
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</tr>
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</table>
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