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, and arbitrary expressions can be evaluated at expansion time with the  begin-expansion-time syntactic form. Scoped expansion-time bindings can be obtained with  local-expansion-time-value.

• (define-expansion-time id expr) evaluates expr at expansion time and binds it as an expansion-time value to the global variable id.
• (let-expansion-time id expr body) evaluates expr at expansion time and binds an expansion-time value to id within body. The run-time result of a let-expansion-time expression is the result of evaluating body.

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

• (begin-expansion-time body) evaluates body at expansion time. This is similar to   (eval-when (compile eval) body) in other implementations of Scheme.
• (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 must be a symbol; the corresponding identifier must be 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.

The begin-expansion-time form is useful for defining macro packages that can be compiled with  compile-file.

The  expansion-time-value? procedure returns #t if its argument is a value created with define-expansion-time or #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.