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2.5 Variable References and #%top



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.


> (define x 10)
> x


> (let ([x 5]) x)


> ((lambda (x) x) 2)



(#%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 idwith 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.


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