On 7/19/12, David A. Wheeler <dwhee...@dwheeler.com> wrote: > Alan Manuel Gloria: >> IIRC some peek-char implementations are just read-char followed by >> unread-char, with only one unread-char lookahead: so the sequence of >> peek-char followed by unread-char might cause strange errors about too >> many unread-char. > > Hmm, I'd think of that as a bug.
Hmm. Here's an X3J13 Common Lisp issue UNREAD-CHAR-AFTER-PEEK-CHAR: http://www.lispworks.com/documentation/lw61/CLHS/Issues/iss356_w.htm Basically the write-up describes peek-char as equivalent to read-char followed by unread-char. Meaning that doing an unread-char after a peek-char is not allowed, as it becomes two unread-char's in sequence. In fact, I think this restriction goes all the way back to C. C has ungetc and getc , but no peekc. To implement Scheme using FILE*, you'd implement peek-char as getc followed by ungetc. Of course modern Scheme implementations will wrap FILE* if they still use it, but most Schemes start out as ridiculously simple implementations (taking advantage of the relative small size of R5RS/R4RS). Less is more and all that jazz. And think about it: R5RS already gives you one character lookahead on ports. Your proposed implementation (wrap R5RS port with cons-buffer) adds another character lookahead buffer, giving a total of 2 characters lookahead. Still, as long as we require that unread-char is only ever used by the "." as indentation whitespace implementation, I think it's OK. I think it's a slippery slope and you'll have to get more characters of lookahead, and thus lose strict R5RS-compliance. -- http://scheme.com/csug8/io.html Chez Scheme mentions that unbuffered ports have a one-character buffer in order to support unread-char and peek-char: "When the unbuffered option is specified, input is unbuffered, but not fully, since one character of buffering is required to support peek-char and unread-char." As the X3J13 UNREAD-CHAR-AFTER-PEEK-CHAR issue points out, this *probably* ends up meaning that peek-char followed by unread-char will fail. ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Readable-discuss mailing list Readable-discuss@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/readable-discuss
