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diff --git a/doc/manual/src/language/index.md b/doc/manual/src/language/index.md new file mode 100644 index 000000000..db34fde75 --- /dev/null +++ b/doc/manual/src/language/index.md @@ -0,0 +1,581 @@ +# Nix Language + +The Nix language is + +- *domain-specific* + + It only exists for the Nix package manager: + to describe packages and configurations as well as their variants and compositions. + It is not intended for general purpose use. + +- *declarative* + + There is no notion of executing sequential steps. + Dependencies between operations are established only through data. + +- *pure* + + Values cannot change during computation. + Functions always produce the same output if their input does not change. + +- *functional* + + Functions are like any other value. + Functions can be assigned to names, taken as arguments, or returned by functions. + +- *lazy* + + Expressions are only evaluated when their value is needed. + +- *dynamically typed* + + Type errors are only detected when expressions are evaluated. + +# Overview + +This is an incomplete overview of language features, by example. + +<table> + <tr> + <th> + Example + </th> + <th> + Description + </th> + </tr> + <tr> + <td> + + + *Basic values* + + + </td> + <td> + + + + </td> + </tr> + <tr> + <td> + + `"hello world"` + + </td> + <td> + + A string + + </td> + </tr> + <tr> + <td> + + ``` + '' + multi + line + string + '' + ``` + + </td> + <td> + + A multi-line string. Strips common prefixed whitespace. Evaluates to `"multi\n line\n string"`. + + </td> + </tr> + <tr> + <td> + + `"hello ${ { a = "world" }.a }"` + + `"1 2 ${toString 3}"` + + `"${pkgs.bash}/bin/sh"` + + </td> + <td> + + String interpolation (expands to `"hello world"`, `"1 2 3"`, `"/nix/store/<hash>-bash-<version>/bin/sh"`) + + </td> + </tr> + <tr> + <td> + + `true`, `false` + + </td> + <td> + + Booleans + + </td> + </tr> + <tr> + <td> + + `null` + + </td> + <td> + + Null value + + </td> + </tr> + <tr> + <td> + + `123` + + </td> + <td> + + An integer + + </td> + </tr> + <tr> + <td> + + `3.141` + + </td> + <td> + + A floating point number + + </td> + </tr> + <tr> + <td> + + `/etc` + + </td> + <td> + + An absolute path + + </td> + </tr> + <tr> + <td> + + `./foo.png` + + </td> + <td> + + A path relative to the file containing this Nix expression + + </td> + </tr> + <tr> + <td> + + `~/.config` + + </td> + <td> + + A home path. Evaluates to the `"<user's home directory>/.config"`. + + </td> + </tr> + <tr> + <td> + + <nixpkgs> + + </td> + <td> + + Search path. Value determined by [`$NIX_PATH` environment variable](../command-ref/env-common.md#env-NIX_PATH). + + </td> + </tr> + <tr> + <td> + + *Compound values* + + </td> + <td> + + + + </td> + </tr> + <tr> + <td> + + `{ x = 1; y = 2; }` + + </td> + <td> + + A set with attributes named `x` and `y` + + </td> + </tr> + <tr> + <td> + + `{ foo.bar = 1; }` + + </td> + <td> + + A nested set, equivalent to `{ foo = { bar = 1; }; }` + + </td> + </tr> + <tr> + <td> + + `rec { x = "foo"; y = x + "bar"; }` + + </td> + <td> + + A recursive set, equivalent to `{ x = "foo"; y = "foobar"; }` + + </td> + </tr> + <tr> + <td> + + `[ "foo" "bar" "baz" ]` + + `[ 1 2 3 ]` + + `[ (f 1) { a = 1; b = 2; } [ "c" ] ]` + + </td> + <td> + + Lists with three elements. + + </td> + </tr> + <tr> + <td> + + *Operators* + + </td> + <td> + + + + </td> + </tr> + <tr> + <td> + + `"foo" + "bar"` + + </td> + <td> + + String concatenation + + </td> + </tr> + <tr> + <td> + + `1 + 2` + + </td> + <td> + + Integer addition + + </td> + </tr> + <tr> + <td> + + `"foo" == "f" + "oo"` + + </td> + <td> + + Equality test (evaluates to `true`) + + </td> + </tr> + <tr> + <td> + + `"foo" != "bar"` + + </td> + <td> + + Inequality test (evaluates to `true`) + + </td> + </tr> + <tr> + <td> + + `!true` + + </td> + <td> + + Boolean negation + + </td> + </tr> + <tr> + <td> + + `{ x = 1; y = 2; }.x` + + </td> + <td> + + Attribute selection (evaluates to `1`) + + </td> + </tr> + <tr> + <td> + + `{ x = 1; y = 2; }.z or 3` + + </td> + <td> + + Attribute selection with default (evaluates to `3`) + + </td> + </tr> + <tr> + <td> + + `{ x = 1; y = 2; } // { z = 3; }` + + </td> + <td> + + Merge two sets (attributes in the right-hand set taking precedence) + + </td> + </tr> + <tr> + <td> + + *Control structures* + + </td> + <td> + + + + </td> + </tr> + <tr> + <td> + + `if 1 + 1 == 2 then "yes!" else "no!"` + + </td> + <td> + + Conditional expression + + </td> + </tr> + <tr> + <td> + + `assert 1 + 1 == 2; "yes!"` + + </td> + <td> + + Assertion check (evaluates to `"yes!"`). + + </td> + </tr> + <tr> + <td> + + `let x = "foo"; y = "bar"; in x + y` + + </td> + <td> + + Variable definition + + </td> + </tr> + <tr> + <td> + + `with builtins; head [ 1 2 3 ]` + + </td> + <td> + + Add all attributes from the given set to the scope (evaluates to `1`) + + </td> + </tr> + <tr> + <td> + + *Functions (lambdas)* + + </td> + <td> + + + + </td> + </tr> + <tr> + <td> + + `x: x + 1` + + </td> + <td> + + A function that expects an integer and returns it increased by 1 + + </td> + </tr> + <tr> + <td> + + `x: y: x + y` + + </td> + <td> + + Curried function, equivalent to `x: (y: x + y)`. Can be used like a function that takes two arguments and returns their sum. + + </td> + </tr> + <tr> + <td> + + `(x: x + 1) 100` + + </td> + <td> + + A function call (evaluates to 101) + + </td> + </tr> + <tr> + <td> + + `let inc = x: x + 1; in inc (inc (inc 100))` + + </td> + <td> + + A function bound to a variable and subsequently called by name (evaluates to 103) + + </td> + </tr> + <tr> + <td> + + `{ x, y }: x + y` + + </td> + <td> + + A function that expects a set with required attributes `x` and `y` and concatenates them + + </td> + </tr> + <tr> + <td> + + `{ x, y ? "bar" }: x + y` + + </td> + <td> + + A function that expects a set with required attribute `x` and optional `y`, using `"bar"` as default value for `y` + + </td> + </tr> + <tr> + <td> + + `{ x, y, ... }: x + y` + + </td> + <td> + + A function that expects a set with required attributes `x` and `y` and ignores any other attributes + + </td> + </tr> + <tr> + <td> + + `{ x, y } @ args: x + y` + + `args @ { x, y }: x + y` + + </td> + <td> + + A function that expects a set with required attributes `x` and `y`, and binds the whole set to `args` + + </td> + </tr> + <tr> + <td> + + *Built-in functions* + + </td> + <td> + + + + </td> + </tr> + <tr> + <td> + + `import ./foo.nix` + + </td> + <td> + + Load and return Nix expression in given file + + </td> + </tr> + <tr> + <td> + + `map (x: x + x) [ 1 2 3 ]` + + </td> + <td> + + Apply a function to every element of a list (evaluates to `[ 2 4 6 ]`) + + </td> + </tr> +</table> |