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doc: note that function bindings are accessible in default values

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Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
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Valentin Gagarin 2024-10-10 22:40:37 +02:00 • committed by John Ericson
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41
doc/manual/source/language/syntax.md
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@ -443,7 +443,7 @@ three kinds of patterns:
This works on any set that contains at least the three named
attributes.
It is possible to provide *default values* for attributes, in
- 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,
@ -503,6 +503,45 @@ three kinds of patterns:
> [ 23 {} ]
> ```
- All bindings introduced by the function are in scope in the entire function expression; not just in the body.
It can therefore be used in default values.
> **Example**
>
> A parameter (`x`), is used in the default value for another parameter (`y`):
>
> ```nix
> let
> f = { x, y ? [x] }: { inherit y; };
> in
> f { x = 3; }
> ```
>
> This evaluates to:
>
> ```nix
> {
> y = [ 3 ];
> }
> ```
> **Example**
>
> The binding of an `@` pattern, `args`, is used in the default value for a parameter, `x`:
>
> ```nix
> let
> f = args@{ x ? args.a, ... }: x;
> in
> f { a = 1; }
> ```
>
> This evaluates to:
>
> ```nix
> 1
> ```
Note that functions do not have names. If you want to give them a name,
you can bind them to an attribute, e.g.,