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Datum labels

This page is part of the web mail archives of SRFI 110 from before July 7th, 2015. The new archives for SRFI 110 contain all messages, not just those from before July 7th, 2015.

We currently don't say anything about datum labels, a capability in R7RS.  I propose this text for the SRFI:

"A sweet-expression reader MAY implement datum labels with syntax #number=datum. If the first character after the equal sign is not whitespace, such a reader SHOULD read it as a neoteric-expression. If the first character after the equal sign is whitespace, a datum reader MAY reject it. A reader MAY also accept a datum label that is an initial expression of a head production (see the BNF below), with a trailing space or tab, as labelling the rest of the sweet-expression."

I've modified the sample implementation so that it supports datum labels, but only if #NUMBER= is followed by non-whitespace (in which case it calls the neoteric-reader).  Implementing a sweet-expression reader after the "=" is a lot more work, and I don't think we need to try to do that since users can always use neoteric-expressions.  I think datum labels are just too specialized to work that hard at implementing them in other cases; we just need to reserve a syntax so that we COULD add it later.

Well, anyway, that's my position.  Any objections or agreements?

 --- David A. Wheeler