Expansion Time Binding and Evaluation

Expansion time is the time at which macro and identifier macro definition values are evaluated and macro procedures are invoked. Macros, identifier macros, and primitive syntax are expansion-time values. General expansion-time values can be defined with the define-expansion-time and let-expansion-time syntactic forms. Scoped expansion-time bindings can be obtained with local-expansion-time-value or global-expansion-time-value, and scoping information is available through local-expansion-time-bound?.

• (define-expansion-time id expr) evaluates expr at run time and binds it as an expansion-time value to the global variable id. This form is treated specially by compile-file (see section 15.2.4).
• (let-expansion-time id expr body-expr ) evaluates expr at expansion time and binds an expansion-time value to id within the body-exprs. The run-time result of a let-expansion-time expression is the result of evaluating the last body-expr.

  As with define-macro, an embedded define-expansion-time expression is transformed into a let-expansion-time expression.

• (local-expansion-time-value x) returns the expansion-time binding of x in the scope of the expression currently being expanded. The x argument to the procedure is a symbol; the corresponding identifier must have been bound to an expansion-time value by define-expansion-time or let-expansion-time, otherwise the exn:misc:expansion-time exception is raised. If local-expansion-time-value is invoked at run time, the exn:misc:expansion-time exception is raised.
• (global-expansion-time-value x) is like (local-expansion-time-value x), but x must be bound in the current namespace (and lexical bindings are ignored).
• (local-expansion-time-bound? x) returns #t if the identifier symbol x is locally bound, #f otherwise. If local-expansion-time-bound? is invoked at run time, the exn:misc:expansion-time exception is raised.
• (expansion-time-value? v) returns #t if v is a value created with define-expansion-time, #f otherwise. Note that expansion-time-value? cannot be applied directly to expansion-time identifiers, but expansion-time values can be obtained indirectly with global-defined-value.
• (begin-elaboration-time body-expr ) evaluates each body-expr at expansion time. Once the body-exprs are evaluated, the entire form is replaced with the S-expression result of the last body-expr, and macro expansion continues. This form is treated specially by compile-file (see section 15.2.4).