docs: add variables; rework scope (#11062)

Co-authored-by: Valentin Gagarin <valentin@gagarin.work>
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Ryan Hendrickson 2024-07-31 17:39:43 -04:00 committed by GitHub
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- [Data Types](language/types.md) - [Data Types](language/types.md)
- [String context](language/string-context.md) - [String context](language/string-context.md)
- [Syntax and semantics](language/syntax.md) - [Syntax and semantics](language/syntax.md)
- [Variables](language/variables.md)
- [Identifiers](language/identifiers.md) - [Identifiers](language/identifiers.md)
- [Scoping rules](language/scope.md) - [Scoping rules](language/scope.md)
- [String interpolation](language/string-interpolation.md) - [String interpolation](language/string-interpolation.md)

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> >
> *name**identifier* | *string* > *name**identifier* | *string*
Names are used in [attribute sets](./syntax.md#attrs-literal), [`let` bindings](./syntax.md#let-expressions), and [`inherit`](./syntax.md#inheriting attributes). Names are used in [attribute sets](./syntax.md#attrs-literal), [`let` bindings](./syntax.md#let-expressions), and [`inherit`](./syntax.md#inheriting-attributes).
# Keywords # Keywords

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# Scoping rules # Scoping rules
Nix is [statically scoped](https://en.wikipedia.org/wiki/Scope_(computer_science)#Lexical_scope), but with multiple scopes and shadowing rules. A *scope* in the Nix language is a dictionary keyed by [name](./identifiers.md#names), mapping each name to an expression and a *definition type*.
The definition type is either *explicit* or *implicit*.
Each entry in this dictionary is a *definition*.
* primary scope: explicitly-bound variables Explicit definitions are created by the following expressions:
* [`let`](./syntax.md#let-expressions) - [let-expressions](syntax.md#let-expressions)
* [`inherit`](./syntax.md#inheriting-attributes) - [recursive attribute set literals](syntax.md#recursive-sets) (`rec`)
* [function](./syntax.md#functions) arguments - [function literals](syntax.md#functions)
* secondary scope: implicitly-bound variables Implicit definitions are only created by [with-expressions](./syntax.md#with-expressions).
* [`with`](./syntax.md#with-expressions)
Primary scope takes precedence over secondary scope. Every expression is *enclosed* by a scope.
See [`with`](./syntax.md#with-expressions) for a detailed example. The outermost expression is enclosed by the [built-in, global scope](./builtins.md), which contains only explicit definitions.
The respective definition types *extend* their enclosing scope by adding new definitions, or replacing existing ones with the same name.
An explicit definition can replace a definition of any type; an implicit definition can only replace another implicit definition.
Each of the above expressions defines which of its subexpressions are enclosed by the extended scope.
In all other cases, the same scope that encloses an expression is the enclosing scope for its subexpressions.
The Nix language is [statically scoped](https://en.wikipedia.org/wiki/Scope_(computer_science)#Lexical_scope);
the value of a variable is determined only by the variable's enclosing scope, and not by the dynamic context in which the variable is evaluated.
> **Note**
>
> Expressions entered into the [Nix REPL](@docroot@/command-ref/new-cli/nix3-repl.md) are enclosed by a scope that can be extended by command line arguments or previous REPL commands.
> These ways of extending scope are not, strictly speaking, part of the Nix language.

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# Variables
A *variable* is an [identifier](identifiers.md) used as an expression.
> **Syntax**
>
> *expression* → *identifier*
A variable must have the same name as a definition in the [scope](./scope.md) that encloses it.
The value of a variable is the value of the corresponding expression in the enclosing scope.