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| -rw-r--r-- | doc/manual/src/language/constructs.md | 45 |
1 files changed, 28 insertions, 17 deletions
diff --git a/doc/manual/src/language/constructs.md b/doc/manual/src/language/constructs.md index 2482f2d17..a3590f55d 100644 --- a/doc/manual/src/language/constructs.md +++ b/doc/manual/src/language/constructs.md @@ -208,30 +208,41 @@ three kinds of patterns: ```nix { x, y, z, ... } @ args: z + y + x + args.a ``` - - Here `args` is bound to the entire argument, which is further - matched against the pattern `{ x, y, z, - ... }`. `@`-pattern makes mainly sense with an ellipsis(`...`) as + + Here `args` is bound to the argument *as passed*, which is further + matched against the pattern `{ x, y, z, ... }`. + The `@`-pattern makes mainly sense with an ellipsis(`...`) as you can access attribute names as `a`, using `args.a`, which was given as an additional attribute to the function. - + > **Warning** - > - > The `args@` expression is bound to the argument passed to the - > function which means that attributes with defaults that aren't - > explicitly specified in the function call won't cause an - > evaluation error, but won't exist in `args`. - > + > + > `args@` binds the name `args` to the attribute set that is passed to the function. + > In particular, `args` does *not* include any default values specified with `?` in the function's set pattern. + > > For instance - > + > > ```nix > let - > function = args@{ a ? 23, ... }: args; + > f = args@{ a ? 23, ... }: [ a args ]; > in - > function {} - > ```` - > - > will evaluate to an empty attribute set. + > f {} + > ``` + > + > is equivalent to + > + > ```nix + > let + > f = args @ { ... }: [ (args.a or 23) args ]; + > in + > f {} + > ``` + > + > and both expressions will evaluate to: + > + > ```nix + > [ 23 {} ] + > ``` Note that functions do not have names. If you want to give them a name, you can bind them to an attribute, e.g., |