1 Language Model
2 Syntactic Forms
3 Datatypes
4 Structures
5 Classes and Objects
6 Units
7 Contracts
8 Pattern Matching
9 Control Flow
10 Concurrency
11 Macros
12 Input and Output
13 Reflection and Security
14 Operating System
15 Memory Management
16 Running PLT Scheme
Bibliography
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#%top
Version: 4.0.2

 

2.5 Variable References and #%top

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

(#%top . id)

Refers to a top-level definition that could bind id, even if id has a local binding in its context. Such references are disallowed anywhere within a module form. See also Expansion Steps for information on how the expander introduces #%top identifiers.

Examples:

  > (define x 12)

  > (let ([x 5]) (#%top . x))

  12