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clarify wording on args@ default handling (#8596)
* clarify wording on args@ default handling Most importantly use shorter sentences and emphasize the key point that defaults aren't taken into account Co-authored-by: Robert Hensing <roberth@users.noreply.github.com> Co-authored-by: John Ericson <git@JohnEricson.me>
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@ -208,30 +208,41 @@ three kinds of patterns:
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```nix
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{ x, y, z, ... } @ args: z + y + x + args.a
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```
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Here `args` is bound to the entire argument, which is further
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matched against the pattern `{ x, y, z,
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... }`. `@`-pattern makes mainly sense with an ellipsis(`...`) as
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Here `args` is bound to the argument *as passed*, which is further
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matched against the pattern `{ x, y, z, ... }`.
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The `@`-pattern makes mainly sense with an ellipsis(`...`) as
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you can access attribute names as `a`, using `args.a`, which was
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given as an additional attribute to the function.
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> **Warning**
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>
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> The `args@` expression is bound to the argument passed to the
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> function which means that attributes with defaults that aren't
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> explicitly specified in the function call won't cause an
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> evaluation error, but won't exist in `args`.
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>
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>
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> `args@` binds the name `args` to the attribute set that is passed to the function.
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> In particular, `args` does *not* include any default values specified with `?` in the function's set pattern.
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>
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> For instance
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>
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>
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> ```nix
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> let
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> function = args@{ a ? 23, ... }: args;
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> f = args@{ a ? 23, ... }: [ a args ];
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> in
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> function {}
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> ````
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>
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> will evaluate to an empty attribute set.
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> f {}
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> ```
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>
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> is equivalent to
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>
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> ```nix
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> let
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> f = args @ { ... }: [ (args.a or 23) args ];
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> in
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> f {}
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> ```
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>
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> and both expressions will evaluate to:
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>
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> ```nix
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> [ 23 {} ]
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> ```
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Note that functions do not have names. If you want to give them a name,
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you can bind them to an attribute, e.g.,
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