diff --git a/doc/manual/src/language/constructs.md b/doc/manual/src/language/constructs.md index a3590f55d..bd7bd92a5 100644 --- a/doc/manual/src/language/constructs.md +++ b/doc/manual/src/language/constructs.md @@ -132,6 +132,32 @@ a = src-set.a; b = src-set.b; c = src-set.c; when used while defining local variables in a let-expression or while defining a set. +in a let expression, inherit can be used to selectively bring specific attributes of a set into scope. For example + + +```nix +let + x = { a = 1; b = 2; }; + inherit (builtins) attrNames; +in +{ + names = attrNames x; +} +``` + +is equivalent to + +```nix +let + x = { a = 1; b = 2; }; +in +{ + names = builtins.attrNames x; +} +``` + +both resolve to `{ names = [ "a" "b" ]; }`. + ## Functions Functions have the following form: @@ -146,65 +172,65 @@ three kinds of patterns: - If a pattern is a single identifier, then the function matches any argument. Example: - + ```nix let negate = x: !x; concat = x: y: x + y; in if negate true then concat "foo" "bar" else "" ``` - + Note that `concat` is a function that takes one argument and returns a function that takes another argument. This allows partial parameterisation (i.e., only filling some of the arguments of a function); e.g., - + ```nix map (concat "foo") [ "bar" "bla" "abc" ] ``` - + evaluates to `[ "foobar" "foobla" "fooabc" ]`. - A *set pattern* of the form `{ name1, name2, …, nameN }` matches a set containing the listed attributes, and binds the values of those attributes to variables in the function body. For example, the function - + ```nix { x, y, z }: z + y + x ``` - + can only be called with a set containing exactly the attributes `x`, `y` and `z`. No other attributes are allowed. If you want to allow additional arguments, you can use an ellipsis (`...`): - + ```nix { x, y, z, ... }: z + y + x ``` - + This works on any set that contains at least the three named attributes. - + It is possible to provide *default values* for attributes, in which case they are allowed to be missing. A default value is specified by writing `name ? e`, where *e* is an arbitrary expression. For example, - + ```nix { x, y ? "foo", z ? "bar" }: z + y + x ``` - + specifies a function that only requires an attribute named `x`, but optionally accepts `y` and `z`. - An `@`-pattern provides a means of referring to the whole value being matched: - + ```nix args@{ x, y, z, ... }: z + y + x + args.a ``` - + but can also be written as: - + ```nix { x, y, z, ... } @ args: z + y + x + args.a ```