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Oops - I'm sorry. I uploaded the files to the wrong directory. They should be ok now. My apologies for wasting your time. On 04/17/2013 01:32 PM, John Cowan wrote:
Per Bothner scripsit:http://per.bothner.com/tmp/srfi-108/srfi-108.htmls/using same/using the same/
Thanks - fixed.
Fixed (already in local and new-uploaed version).
Likewise was already fixed.
Period did not get added to tag-subsequent.
Sorry - that was fixed in my local copy/
Rather than referring to R7RS productions, duplicate them in order to help maintain SRFI 109 as a self-contained document.
As a matter of style, I think this questionable - duplicating definitions from existing standards is a mistake. (Where would you stop? Duplicate the entire definition of <expression>?) However, in this case R7RS is not yet ratified. It might be better to refer to R6RS.
Both documents: Is there a need for [ and ] in SRFI 109 in order to escape unbalanced brackets in SRFI-108 constructors?
[ and ] are not required in my copy.
Defining the variables $<<$ and $>>$ as empty strings sounds clever, but unfortunately none of R[4-7]RS require either `eq?` or `eqv?` to be able to distinguish between two instances of the empty string. Scheme implementations are allowed to coalesce them all into a single instance.
(Note you may have read an earlier draft where $<<$ and $>>$ were initialized to "". They are now initialized to (make-string 0).) Not by my reading: make-string returns a "newly allocated string", so while eq? is allowed to #t for the result of the different calls to make-string, it violates most people's expectations of how eq? works. Is there any Scheme implementation where (eq? (make-string 0) (make-string 0)) return #t? I tweaked the draft to discuss this (theoretical) possibility: Note that R6RS and R7RS allows eq? to return #t for distinct calls to (make-string 0). A hypothetical implementation that does so needs to initialize $<<$ and $>>$ some other way. SRFI-109 can't be implemented by a portable library anyway, so it seems fine to make such a requirement. Given that it would be a really weird Scheme implementation where it is an issue.
So if they are to be defined as strings, they need to be non-empty if `$string$` and `$construct:*` are to be definable as functions rather than as macros. I suggest: ;; Calling string-copy guarantees that these are unique objects. (define $<<$ (string-copy "$<<$")) (define $>>$ (string-copy "$>>$")) This means that the sample definition of $string$ has to work a little harder, skipping arguments that are `eqv?` to either of these variables. Literal appearances of "$<<$" or "$>>$" in text, or appearances as the result of evaluating expressions, are guaranteed never to be `eqv?`.
I don't care about making $string$ more complex, but I do care about making $construct$:foo harder to write. True, in many case we can hide the complexity in define-simple-constructor (that might have been missing in the version you reviewed). In many other cases $construct$:foo will be a macro where it doesn't matter what $<<$ and $>>$ evaluate to. In other cases, we can ignore them, and it's ok if they evaluate to "" without it mattering if they're unique. However, I don't think this is an actual problem, and initializing there to (make-string 0) seems fine. -- --Per Bothner per@xxxxxxxxxxx http://per.bothner.com/