The Visual Yacc Distribution

Visual yacc is a tool that takes a lex and a yacc specification pair and generates a parser with the the same behavior as the original parser plus it will draw a representation of the stack maintained by the parser as well as a corresponding parse tree.


Creating the visualizing parser is done with

visual_yacc [-v] file_name

where file_name corresponds to a lex file file_name.l and a yacc file file_name.y. The optional -v (verbose) flag can be used to see what is happening during creation.

The result is a file file_name.out

Typing this command with redirected input will cause a window to appear on your screen. This window has several buttons:

On the left side of the screen, we show the current parse stack. Shifts cause the stack to grow and reductions pop multiple elements from the stack (and push a single element). If you are using the displayable attributes, these values are shown on the stack as well.

On the right side of the screen, we show the parse tree. At the moment no attributes are displayed, but you can see what rules are chosen by YACC during the parse.

Examples to get you started

We have included an Examples directory in the distribution that should help you figure out how to visualize your own grammers. These examples are located in
Each of the ch* directories correspond to examples from the O'Reilly & Associates book "lex & yacc" by John R. Levine, Tony Mason & Doug Brown. Directory gram corresponds to a pascal-oid language. Each directory contains:

Writing your own visualizable grammars

There are several rules you must follow in order to write your own visualizable grammars.
  1. The files must be lex-able and yacc-able on their own, except for the special keyword VYint. The visualization software will check this for you and reject any code that would not be buildable this way.

    Example 1

    This example comes from ch3-02.
    %union {  
       VYint val;
    %type <val> NUMBER expression
    These integers are referenced in the obvious way:
       |    expression '-' NUMBER   { $$ = $1 - $3; }
    In this example, NUMBER is set in the lexor:
          {. . .  yylval.val = . . .  return NUMBER;  }
    Changing VYint to int is also fine except that no value will be shown on the stack.
  2. You can declare other attributes associated with the individual nodes and computations using these attributes can be arbitrarily complex. However, the value of these attributes will not be shown during the visualization. The declarations must occur after the declaration for x.

    Example 3

    In this example, we assume that the lexor sets the dval for NUMBER. (yylval.s.dval = )
    %union {
          double dval;
          VYint val;
          int vblno;
    . . .