Re: [Google Groups] Parsing single expressions of a larger grammar

[Devin Cook]

It would probably be best to have two different grammars.

On Sat, Sep 1, 2012 at 4:23 PM, Aleksander Heintz <alx...@alxandr.me> wrote:
> I'm trying to write a grammar that can be used in 2 different ways, one is
> an interactive console, the other is running a "script" through my
> interpreter. For the later the grammar basically looks like this:
>
> <Program> ::= <Statement List>
> <Statement> ::= <For Stmt> | .... | <Expression Stmt>
> <Expression Stmt> ::= <Expression> ';'
>
> (note a lot has been omitted to make the code short. This means that for
> instance a valid script may look like this:
>
> 2+3;
> 3+5*2;
>
> However, simply writing 2+2 would be invalid, cause that would not be a
> statement. However, when in interactive console, this should be valid, as
> the interactive console should run both valid scripts and valid expressions.
> So I was wondering, do I need to different grammars with different
> start-symbols? Or can I achieve this in some other way?
>
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[Christopher Wells]

You could try this:

1. Define your grammar as follows:

"Start Symbol" = <EitherOr>

<EitherOr> := '1' <Program> | '2' <Console>

<Console> := <Program> | <Expression>

2. At run-time this, prefix either '1' or '2' to the start of what you're parsing, before you invoke the parser.

It may also be also possible to do as follows:

1. Define the grammar as above

2. Add a customization to the run-time parser engine dll, to support changing/setting the engine's parser's "initial LALR state" on the fly