Re: [relax-ng] Compact syntax proposal: allow redundant , and & and |

[James Clark <jjc@jclark.com>]

> I should like to propose that we allow redundant commas (or vertical bars
> or ampersands) just before a right paren, square bracket, or curly brace.
> This would allow:
>
> foo = (
> attribute foo1 {text},
> attribute foo2 {text},
> attribute foo3 {text),
> )
>
> bar = element bar {
> element bar1 {text},
> element bar2 {text},
> element bar3 {text},
> }

So you would make

  foo = (bar, baz,)

legal, but

  foo = bar, baz,

would remain illegal?  That doesn't seem a good idea.  If you are allowed a
trailing comma or vertical bar in some places, then it should be allowed in
all places where you can have patterns or name classes separated by comma or
vertical bar.  Otherwise, it's going to be hard to learn and explain, and
the grammar will get very complicated.

However, if you allow such trailing commas consistently, then you will need
arbitrary lookahead. You don't know till you see the comma after baz whether
baz should be parsed as a pattern or the name of a definition, and baz may
be preceded by annotations of arbitrary length.

Neither of the other possibilities seems desirable to me.  Given that the
benefits of allowing trailing commas are arguable -- the designers of C++
and Java, for example, chose not to allow them -- I would be inclined not to
adopt this proposal.

James