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document identifier syntax for attribute sets

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it's more likely for readers to find it right there.

this also slightly rewords examples to make them stand out better.
in the long run there probably needs to be a dedicated section on formal syntax, and better highlighting of examples.
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Valentin Gagarin 2023-05-11 19:23:13 +02:00
parent dfc393ffd3
commit d8bfeda164
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47
doc/manual/src/language/constructs.md
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@ -2,8 +2,11 @@
## Recursive sets
Recursive sets are just normal sets, but the attributes can refer to
each other. For example,
Recursive sets are like normal [attribute sets](./values.md#attribute-set), but the attributes can refer to each other.
> *rec-attrset* = `rec {` [ *name* `=` *expr* `;` `]`... `}`
Example:
```nix
rec {
@ -12,7 +15,9 @@ rec {
}.x
```
evaluates to `123`. Note that without `rec` the binding `x = y;` would
This evaluates to `123`.
Note that without `rec` the binding `x = y;` would
refer to the variable `y` in the surrounding scope, if one exists, and
would be invalid if no such variable exists. That is, in a normal
(non-recursive) set, attributes are not added to the lexical scope; in a
@ -33,7 +38,10 @@ will crash with an `infinite recursion encountered` error message.
## Let-expressions
A let-expression allows you to define local variables for an expression.
For instance,
> *let-in* = `let` [ *identifier* = *expr* ]... `in` *expr*
Example:
```nix
let
@ -42,18 +50,19 @@ let
in x + y
```
evaluates to `"foobar"`.
This evaluates to `"foobar"`.
## Inheriting attributes
When defining a set or in a let-expression it is often convenient to
copy variables from the surrounding lexical scope (e.g., when you want
to propagate attributes). This can be shortened using the `inherit`
keyword. For instance,
When defining an [attribute set](./values.md#attribute-set) or in a [let-expression](#let-expressions) it is often convenient to copy variables from the surrounding lexical scope (e.g., when you want to propagate attributes).
This can be shortened using the `inherit` keyword.
Example:
```nix
let x = 123; in
{ inherit x;
{
inherit x;
y = 456;
}
```
@ -62,15 +71,23 @@ is equivalent to
```nix
let x = 123; in
{ x = x;
{
x = x;
y = 456;
}
```
and both evaluate to `{ x = 123; y = 456; }`. (Note that this works
because `x` is added to the lexical scope by the `let` construct.) It is
also possible to inherit attributes from another set. For instance, in
this fragment from `all-packages.nix`,
and both evaluate to `{ x = 123; y = 456; }`.
> **Note**
>
> This works because `x` is added to the lexical scope by the `let` construct.
It is also possible to inherit attributes from another attribute set.
Example:
In this fragment from `all-packages.nix`,
```nix
graphviz = (import ../tools/graphics/graphviz) {