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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
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