Re: Trigger Mediator

["James H H Lampert" <jamesl@xxxxxxxxxxx>]

Alan Campin <Alan.Campin@xxxxxxx> wrote:
> . . . but nothing like what I just read about the descent 
> parser, nothing that complex. How do you implement?

A Recursive Descent parser is not, as far as I'm aware, 
something you would normally get out of YACC, or BISON, or 
other parser generators; it is, however, the sort of 
parser that's applicable to any language that can be 
represented as a BNF grammar, as well as fairly easy to 
hand-code, so long as either (1) the language supports 
n-level recursive calls of some sort of subroutine with 
dynamically-scoped local variables (obviously in RPG, it 
would be a "procedure call," rather than an EXSR, just as 
in the QBASICs, it would be a CALL rather than a GOSUB), 
or (2) the language to be parsed is simple enough that 
actual recursion is either unnecessary or limited to 
tail-recursion.

At any rate, to do it in ILE RPG, you're best off writing 
your scanner in ILE C, and doing a cross-module, 
cross-language function call to get your tokens. Then you 
just need to draw up a BNF grammar of your language, and 
then write a recursive subroutine for each production in 
the grammar. Then you just stick your semantics, either 
direct action for an interpreter, or code generation for a 
compiler, into the production routines.

Why do it in ILE RPG? Well, if it has to look up a lot of 
things in a lot of database files, and/or its semantic 
actions affect a lot of database files, RPG's database 
access is somewhat more convenient than C's. Moreover, if 
you're clever, you just might be able to use The Cycle as 
your outermost loop.

--
JHHL