22 March 2007

A simple Parsing-based parser example

As requested by several people, I have uploaded a simple example parser that uses the Parsing module. It is pretty self explanatory, so I encourage you to take a look at it, run it, and experiment with changes.

That done, I should say a bit more about how I actually use the Parsing module. Well, the first thing I did with it was to write a parser for a parser generator input language similar to what Elkhound supports. The parser translates the input to two output files, Token.py and Ast.py, which contain code that the Parsing module can generate a parser from. Here are a few example productions from Lyken's grammar specification:
token comment=xclass:"comment" {
inline Init ${
self.val = raw
}$
}

fail pStmtList <{pExpr};
nonterm StmtList=[e] {
-> Stmt(Stmt, semicolon);
-> DelimitedExpr(DelimitedExpr) [pStmtList];

-> ExtendStmt(StmtList, Stmt, semicolon);
-> ExtendDelimitedExpr(StmtList, DelimitedExpr) [pStmtList];
}

nonterm Stmts {
-> Empty;
-> Stmt(Stmt);
-> StmtList(StmtList);
}

start Module=[S] {
-> Module(boi, DocStr, ModuleDecl, Version, InitialCBlock, Stmts);
}
There are numerous features not represented above, but the general idea should be apparent. Note that embedded code can be associated with productions. This allows me to do some pretty highly stylized code generation, yet still embed custom code where necessary.

One of the non-obvious clauses above is the "=[S]" that follows "start Module". This is extra annotation that says the Module production provides an outer lexical scope. By supporting such custom annotations, I am able to automatically generate code that deals with many aspects of semantic analysis. This is also one of the main reasons I haven't seen fit to release the grammar specification parser -- it is not obvious to me how to generalize such features in a way that everyone can benefit. At the moment I am of the opinion that the low level docstring-based interface to the Parsing module is good for small- to medium-size parsers, and that for large parsers, you need to write a custom translator that I can't hope to guess the needs of.

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