Re: VBScript grammar

Anton Belov -- Customer Engineering wrote:
> Does anyone have or know where to get the grammar definition (in any
> form) for VBScript ? I know it has been asked before - but i didn't
> see any answers ...


I've never seen a grammar published. You can't trust the reference
manual either, because it lies. The only way to write a grammar is to
play with an implementation of the language.


I wrote a parser for VB (sorry, not available, I don't own the code),
and since VBScript is based on VB, I'll assume you'll run into similar
problems. Here are a few:


- End requires an extra lookahead, because End is different from
    End If, End While, End Select, etc. Handle it in the scanner
    so that End <something> is returned to the parser as one token,
    not two.
- Line labels are a pain. I think I put some magic in the
    scanner for this.
- There are a zillion reserved words, but some depend on context.
    This can make tokenizing very difficult. For example, "In" is
    a reserved word but can also be a variable name. I think I
    fixed this by having YACC pass state info back to Lex, but
    you have to be careful about lookahead. Another possible fix
    is to allow a syntax error and try to recover in yyerror().
    Not easy...
- Scoping rules are quite complex, and are subtly different for
    variables and types.
- Parenthesis are significant at runtime, so don't throw them
    away during the parse if you ever want to build a runtime.
    In case you're wondering, parenthesis override references,
    so even if a function/subroutine is written to accept a
    parameter by reference, you can pass it by value by
    enclosing the argument in parenthesis.


Writing a runtime is even more exciting...


--
Scott Nicol
snicol@apk.net

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