Christopher is right that I could make null RHS's part of the syntax of
other rules.. I don't because:
1.) Null alternatives would be easy to create by accident.
2.) Null alternatives are very special in the adverbs they do and do not
allow.
3.) Null rules can make parent symbols nullable, which can have tricky
or unintended results.
4.) Allowing null alternatives does not eliminate the
non-orthogonality. It just moves the issue elsewhere -- null
alternatives in a prioritized rule would almost certanily mean a typo,
or an attempt to do something that won't really work, and probably
should be banned.
All of which means that forcing a user to specify a null RHS as a
special separate rule is in keeping with the Principle of Least
Surprise. Note that one of the people who might be victim of these
surprises is me -- in my own SLIF grammars, I don't regret having to
make my null rules separate.
Finally, I welcome experimentation. It's quite possible to create your
own front-end to the SLIF, if you think you can improve on the SLIF and
handle null rules in a way that combines safety and elegance. As I've
said before, the SLIF exists because of the experiments by Peter
Stuifzand and Andrew Rodland, experiments which took Marpa beyond where
I'd gone and showed me that I could and should improve on Marpa's
original interface (the NAIF).
-- jeffrey
On 06/19/2014 11:42 AM, Christopher Layne wrote:
If you ignore the null rule it'll blow your mind less if you think about it as:
A ::= B | 'hello!'
A ::=
There's no conceptual reason that the null rule couldn't be the last
"alternative" but I believe the DSL grammar won't allow it hence it's a
separate statement.
On Jun 18, 2014, at 9:37, Paul Bennett <[email protected]> wrote:
A ::= B
A ::= 'hello!'
A ::= # empty
Point not taken. Point, in fact, blew my mind. More research is required.
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