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docs: add variables; rework scope (#11062)
Co-authored-by: Valentin Gagarin <[email protected]>
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# Scoping rules | ||
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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*. | ||
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* primary scope: explicitly-bound variables | ||
* [`let`](./syntax.md#let-expressions) | ||
* [`inherit`](./syntax.md#inheriting-attributes) | ||
* [function](./syntax.md#functions) arguments | ||
Explicit definitions are created by the following expressions: | ||
- [let-expressions](syntax.md#let-expressions) | ||
- [recursive attribute set literals](syntax.md#recursive-sets) (`rec`) | ||
- [function literals](syntax.md#functions) | ||
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* secondary scope: implicitly-bound variables | ||
* [`with`](./syntax.md#with-expressions) | ||
Implicit definitions are only created by [with-expressions](./syntax.md#with-expressions). | ||
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Primary scope takes precedence over secondary scope. | ||
See [`with`](./syntax.md#with-expressions) for a detailed example. | ||
Every expression is *enclosed* by a scope. | ||
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. | ||
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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. | ||
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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. | ||
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> **Note** | ||
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> 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 | ||
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A *variable* is an [identifier](identifiers.md) used as an expression. | ||
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> **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. |