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It would be straightforward to extend this to higher level imports. First, the import syntax might be modified to something similar to:
(for "library" (phase 0)
(phase 1)
(phase 3)) ; imports into phases 0, 1 and 3
where (phase 0) = RUN
(phase 1) = SYNTAX
Example:
(library "m" "r6rs"
(export x)
(define x 1))
(library "n" "r6rs"
(import (for "m" (phase 2)))
(let-syntax ((a (lambda (exp)
(let-syntax ((b (lambda (exp) x))) ; use of x
(b)))))
(a))
The change in the semantics would then be:
To visit a library at phase N:
* Visit at phase N any library that is imported by this library
for ... (phase 0), and that is not yet visited at phase N.
[MODIFIED]
* For each k >= 1, invoke at phase N+k any library that is imported
by this library for .... (phase k), and that is not yet invoked at
phase N+k.
[ADDED]
* For each k >= 1, visit at phase N+1 any library that is imported by
this library for .... (phase k), and that is not yet visited at phase
N+k.
* Evaluate all syntax definitions within the library.
(Top-level expressions are not evaluated, and the right-hand sides
of variable definitions are not evaluated.)
Sometimes it is useful to import a library for all phases. For example, this
may be what is implicitly done for the r6rs language, so one could also have
an option: (for "library" all) == (for "library" (phase 0) (phase 1) (phase 2) .....) Regards Andre