7 R6RS Conformance

▼ |  | R6RS: Scheme

1 | Using R6RS with Dr Racket
2 | Running Top-Level Programs
3 | Installing Libraries
4 | R6RS Module Language
5 | Libraries and Collections
6 | Language Interoperability
7 | R6RS Conformance
8 | R6RS Libraries
| Index

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### 7`` R6RS Conformance

Racket’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 Racket:

  > (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.

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