R6RS added exceptions for symbols beginning with
"->". Do we accept this, or allow even more
My sense was that most R5RS systems accept anything as an identifier that doesn't parse as a number. Here's what I found using '(+on+ 4bsd -off-):
PLT, Gauche, MIT, Gambit, Chicken, Bigloo, scsh, Scheme 9, Scheme 7, VSCM are okay with it.
Scheme48, Ikarus, Larceny, SSCM, SXM, Chibi report lexical syntax errors.
I don't know of any Scheme that supports extensions to the R5RS/R6RS syntax of numbers that would make these identifiers a problem. I recommend that we allow them.
Chibi accepts +on+ and -off-, it just doesn't allow
symbols beginning with digits. It seems much more
likely that someone started to write a number and
got a suffix wrong. For instance, you propose 4bsd
as a symbol, but 4d0 is valid R5RS numeric syntax.
WG1 accepted Alex Shinn's proposal.
that is, symbols may not begin with a number prefix