Re: How to construct a grammar?

Chris,


thanks for your extensive reply. I only have very few comments below.


Chris F Clark wrote:


> Michael Schuerig <michael@schuerig.de> writes:
>
>> So, I'm wondering, is there a systematic way to get from a suitable
>> list of language samples, i.e, without a formal definition to start
>> with, to a grammar?
>
> There are two answers to that question. A theoretical automated
> answer and a practical hand-done answer. I'm going to start with the
> answer I suspect you care about less, the automated theoretical one
> and dispense quickly with it. Taking a set of examples and generating
> a grammar from itusing an algorithm is a field of research, which I
> think is called roughly "machine learning".


Indeed, that's not what I have in mind, but I see how my wording can
lead to that interpretation. Substitute my "suitable list of language
samples" with "informal understanding of the language to be
recognized".


> This gets us to the next point. Above I said you have a grammar, but
> the grammar may or may not be suitable to your tool. If you've
> written lots of grammars and have developed an intuition for how to
> avoid problems, it may be acceptable as is. However, you need to test
> it by running the grammar through your tool and seeing if the tool
> generates errors. If so, your language isn't acceptible to your tool,
> and may even have flaws where it doesn't even describe what you want.
> Even if your grammar is acceptible to your tool, you need to test it
> to make certain that it is actually what you want, by building a small
> copy of you application that just lexes and parses input and running
> relevant examples through to see if the examples are turned into the
> right kind of parse tree (AST).


That intuition part is what I'm really after. I don't expect to write
that many grammars in my life and therefore my intuition may always be
lacking. That's why I'm interested in a systematic approach that
codifies some of the intuition and experience of others.


If I understand you correctly, your advice (snipped in this reply) would
be to first identify the words of the language (lexer), then work
top-down from sentence level.


> Hope this helps,
> -Chris


Yes, thanks a lot for your effort!


Michael


--
Michael Schuerig
mailto:michael@schuerig.de
http://www.schuerig.de/michael/

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