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[Ulrich Kühne <ulrichk AT informatik.uni-bremen.de>]

Hi,

I have a (badly designed) imperative language, where statements are not 
separated by semicolons, which gives me plenty of ambiguous parsings. 
One example is return, since there is a plain "return" for void 
functions and "return Exp". Since Exp needs to be included as a valid 
statement, the parser cannot decide whether "return Exp" is one or two 
statements. How can I get rid of this behavior, i.e. I always want to 
have the "return Exp" (the maximum match) alternative? I tried using 
precedences, but I failed. Here is a small example:

module AMBIG-SYNTAX
  syntax Exp ::= Int | Int "+" Int [strict,left]

  syntax Stmt ::=
          "var" Id
        | Id "=" Exp [strict(2)]
        | "return" Exp [strict]
        > "return"
        > Exp
        > Stmt Stmt [right]
endmodule

module AMBIG imports AMBIG-SYNTAX
  syntax Val ::= Int | "nothing"
  syntax KResult ::= Val

  rule <k>return X:Int =>X ...</k>
  rule <k>return => nothing ...</k>
endmodule

resulting in the following warning (using K 3.0):

[Warning] Parser: Parsing ambiguity between: "return" Exp (Stmt1436Syn), 
Stmt Stmt (Stmt1438Syn)    Arbitrarily choosing the first. in the 
following AST:
amb(
    return X:Int,
    return X
)
    File: /home/.../ambig.k
    Location: (17,11,17,23)
    Compilation Phase: Ambiguity filter

My second question: can somebody please explain to me how context rules 
work? I already used some of them, mostly copied from the k-framework 
examples, but still I don't get the real meaning and how to use them 
properly.

Thanks!
Ulrich