Re: *which* alternative that matches nothing? (was Re: repetition)

On Thu, 16 Dec 2021 at 15:20, C. M. Sperberg-McQueen
<cmsmcq@blackmesatech.com> wrote:

> If we do want a special symbol for “the alternative that matches nothing”,
> we need to be careful about the two meanings of that phrase.
>
> - An alternative that matches the empty sequence (a sequence
> consisting of nothing, the sequence of length 0) is one thing.  I usually
> write a comment in that alternative to make it easier to see, and also
> usually write it first, so
>
>   X: Y.
>   -Y: {nil}; Z;.
>
> It can also be written in ixml as empty parens or as []?, but since
> the latter re-introduces a question mark, it’s not a good candidate
> for a rewriting system, which will promptly rewrite is as ([]; ).

Empty parens, makes sense. Empty parens  with question mark?
I don't see where that comes from? Either empty or not?


 In
> computer science books, I believe it’s usually written as an epsilon.
>

No thanks. Please keep it to an ASCII keyboard character.


> - An alternative that matches no sequences at all, which no thing
> matches.  This is an expression which denotes the language with
> no sentences, i.e. the empty set.  And this is the meaning most
> naturally associated with the symbol “∅”.

Any reason why [] should not be given this definition?



> When this was discussed earlier, I think the prevailing opinion was that
> if we introduce special symbols like ε and ∅ we have to explain
> what they mean to readers who aren’t familiar with them, some of
> whom at least will see them and start writing off ixml as too
> complicated and mathy.  If we don’t have special symbols for them,
> those who know they want to write expressions with those meanings
> will find expressions like () and [] and ~[] and {empty} and be
> perfectly happy.

I'd be happy with empty parens, but then .....


regards


-- 
Dave Pawson
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Received on Thursday, 16 December 2021 15:31:44 UTC