2.5 Variable References and #%top

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▼ | 2 | Syntactic Forms

2.1 | Modules: module, module*, ...
2.2 | Importing and Exporting: require and provide
2.3 | Literals: quote and #%datum
2.4 | Expression Wrapper: #%expression
2.5 | Variable References and #%top
2.6 | Locations: #%variable-reference
2.7 | Procedure Applications and #%app
2.8 | Procedure Expressions: lambda and case-lambda
2.9 | Local Binding: let, let*, letrec, ...
2.10 | Local Definitions: local
2.11 | Constructing Graphs: shared
2.12 | Conditionals: if, cond, and, and or
2.13 | Dispatch: case
2.14 | Definitions: define, define-syntax, ...
2.15 | Sequencing: begin, begin0, and begin-for-syntax
2.16 | Guarded Evaluation: when and unless
2.17 | Assignment: set! and set!-values
2.18 | Iterations and Comprehensions: for, for/ list, ...
2.19 | Continuation Marks: with-continuation-mark
2.20 | Quasiquoting: quasiquote, unquote, and unquote-splicing
2.21 | Syntax Quoting: quote-syntax
2.22 | Interaction Wrapper: #%top-interaction
2.23 | Blocks: block
2.24 | Internal-Definition Limiting: #%stratified-body
2.25 | Performance Hints: begin-encourage-inline
2.26 | Importing Modules Lazily: lazy-require

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#%top

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#### 2.5`` Variable References and #%top

> syntax id

Refers to a module-level or local binding, when id is not bound as a transformer (see Expansion). At run-time, the reference evaluates to the value in the location associated with the binding.

When the expander encounters an id that is not bound by a module-level or local binding, it converts the expression to (#%top . id) giving #%top the lexical context of the id; typically, that context refers to #%top. See also Expansion Steps.

Examples:
> > (define x 10)
> > x
> 10
> > (let ([x 5]) x)
> 5
> > ((lambda (x) x) 2)
> 2

> syntax (#%top . id)

Refers to a module-level or top-level definition that could bind id, even if id has a local binding in its context.

Within a module form, (#%top . id) expands to just id—with the obligation that id is defined within the module. At phase level 0, (#%top . id) is an immediate syntax error if id is not bound. At phase level 1 and higher, a syntax error is reported if id is not defined at the corresponding phase by the end of module-body partial expansion.

See also Expansion Steps for information on how the expander introduces #%top identifiers.

Examples:
> > (define x 12)
> > (let ([x 5]) (#%top . x))
> 12

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