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47 lines
6.5 KiB
Markdown
47 lines
6.5 KiB
Markdown
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# Syntax Summary {#sec-nix-syntax-summary}
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Below is a summary of the most important syntactic constructs in the Nix
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expression language. It's not complete. In particular, there are many
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other built-in functions. See the [Nix
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manual](https://nixos.org/nix/manual/#chap-writing-nix-expressions) for
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the rest.
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| Example | Description |
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|-----------------------------------------------|--------------------------------------------------------------------------------------------------------------------|
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| *Basic values* | |
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| `"Hello world"` | A string |
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| `"${pkgs.bash}/bin/sh"` | A string containing an expression (expands to `"/nix/store/hash-bash-version/bin/sh"`) |
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| `true`, `false` | Booleans |
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| `123` | An integer |
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| `./foo.png` | A path (relative to the containing Nix expression) |
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| *Compound values* | |
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| `{ x = 1; y = 2; }` | A set with attributes named `x` and `y` |
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| `{ foo.bar = 1; }` | A nested set, equivalent to `{ foo = { bar = 1; }; }` |
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| `rec { x = "foo"; y = x + "bar"; }` | A recursive set, equivalent to `{ x = "foo"; y = "foobar"; }` |
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| `[ "foo" "bar" ]` | A list with two elements |
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| *Operators* | |
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| `"foo" + "bar"` | String concatenation |
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| `1 + 2` | Integer addition |
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| `"foo" == "f" + "oo"` | Equality test (evaluates to `true`) |
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| `"foo" != "bar"` | Inequality test (evaluates to `true`) |
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| `!true` | Boolean negation |
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| `{ x = 1; y = 2; }.x` | Attribute selection (evaluates to `1`) |
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| `{ x = 1; y = 2; }.z or 3` | Attribute selection with default (evaluates to `3`) |
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| `{ x = 1; y = 2; } // { z = 3; }` | Merge two sets (attributes in the right-hand set taking precedence) |
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| *Control structures* | |
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| `if 1 + 1 == 2 then "yes!" else "no!"` | Conditional expression |
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| `assert 1 + 1 == 2; "yes!"` | Assertion check (evaluates to `"yes!"`). See [](#sec-assertions) for using assertions in modules |
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| `let x = "foo"; y = "bar"; in x + y` | Variable definition |
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| `with pkgs.lib; head [ 1 2 3 ]` | Add all attributes from the given set to the scope (evaluates to `1`) |
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| *Functions (lambdas)* | |
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| `x: x + 1` | A function that expects an integer and returns it increased by 1 |
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| `(x: x + 1) 100` | A function call (evaluates to 101) |
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| `let inc = x: x + 1; in inc (inc (inc 100))` | A function bound to a variable and subsequently called by name (evaluates to 103) |
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| `{ x, y }: x + y` | A function that expects a set with required attributes `x` and `y` and concatenates them |
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| `{ x, y ? "bar" }: x + y` | A function that expects a set with required attribute `x` and optional `y`, using `"bar"` as default value for `y` |
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| `{ x, y, ... }: x + y` | A function that expects a set with required attributes `x` and `y` and ignores any other attributes |
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| `{ x, y } @ args: x + y` | A function that expects a set with required attributes `x` and `y`, and binds the whole set to `args` |
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| *Built-in functions* | |
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| `import ./foo.nix` | Load and return Nix expression in given file |
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| `map (x: x + x) [ 1 2 3 ]` | Apply a function to every element of a list (evaluates to `[ 2 4 6 ]`) |
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