6 R⁶RS Conformance

PLT Scheme’s R⁶RS support does not conform with the standard in several known ways:

• When guard catches an exception that no clause matches, the exception is re-raiseed without restoring the continuation to the one that raised the exception.

  This difference can be made visible using dynamic-wind. According to R⁶RS, the following program should print “in” and “out” twice, but each prints once using PLT Scheme:

  (guard (exn [(equal? exn 5) 'five])

  (guard (exn [(equal? exn 6) 'six])

  (dynamic-wind

  (lambda () (display "in") (newline))

  (lambda () (raise 5))

  (lambda () (display "out") (newline)))))

  Along similar lines, continuation capture and invocation within an exception handler is restricted. Unless the exception is raised through raise-continuable, a handler can escape only through a continuation that is a tail of the current continuation, and a continuation captured within the handler cannot be invoked after control escapes from the raise.

  The initial exception handler does not return for non-&serious conditions, but raise and raise-continuable both install an uncaught-exception handler (via parameterize and uncaught-exception-handler) to one that returns for non-&serious conditions.

• Inexact numbers are printed without a precision indicator, and precision indicators are ignored on input (e.g., 0.5|7 is read the same as 0.5).

• Word boundaries for string-downcase, string-upcase, and string-titlecase are not determined as specified by Unicode Standard Annex #29.

• When an identifier bound by letrec or letrec* is referenced before it is bound, an exception is not raised; instead, the reference produces #<undefined>.

• A custom textual port must represent positions using integers, and the positions must correspond to bytes in a UTF-8 encoding of the port’s data. For custom ports (byte or character) that support both input and output, beware that buffered input can create a mismatch between the position implemented by the custom procedures and the port’s current position; the result from a custom position procedure is automatically adjusted to account for buffering, and setting the port’s position flushes all buffered bytes, but writing after a read does not automatically reset the port’s position to counteract the effects of buffering.

• The bindings in a namespace produced by null-environment or scheme-report-environment correspond to R⁵RS bindings instead of R⁶RS bindings. In particular, =>, else, _, and ... are not bound.

• Bindings for #%datum, #%app, #%top, and #%top-interaction are imported into every library and program, and at every phase level for which the library or program has imports.