I'm proposing a minor grammar change, and want to know of any objections.
Currently, in sweet-expressions, if a line has more than one neoteric-expression
and the last expression is ., the . is interpreted as the symbol
whose spelling is the single character .. E.G.:
quote .
= (quote |.|)
I propose making this an error instead, to improve error detection.
It's fairly unusual to want this on *purpose*. Usually . introduces
the cdr of a pair. This would detect accidental use, if you *wanted* to do this,
you'd then have to escape it - say like this:
quote |.|
= (quote |.|)
The grammar change is trivial, and makes the grammar simpler. In ANTLR/Java:
rest returns [Object v]
- : PERIOD /* Improper list */
- (hspace+ pp=post_period {$v = $pp.v;}
- | /*empty*/ {$v = list(.);})
+ : PERIOD hspace+ pp=post_period {$v = $pp.v;} /* Improper list */
A period on a line by *itself* would continue to mean that the next
line (at the same indent level) is the cdr of the given list.
Any objections? I plan to check this into the development branch, so
we can experiment with it, but it'll be easy to pull out if there are
objections.
--- David A. Wheeler
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