ycalc.cup

java语法解释器生成器

/*
  This example comes from a short article series in the Linux 
  Gazette by Richard A. Sevenich and Christopher Lopes, titled
  "Compiler Construction Tools". The article series starts at

  http://www.linuxgazette.com/issue39/sevenich.html

  Small changes and updates to newest JFlex+Cup versions 
  by Gerwin Klein
*/

/*
  Commented By: Christopher Lopes
  File Name: ycalc.cup
  To Create: > java java_cup.Main < ycalc.cup
*/
   
   
/* ----------------------Preliminary Declarations Section--------------------*/
   
/* Import the class java_cup.runtime.*  */
import java_cup.runtime.*;
   
/* Parser code to change the way the parser reports errors (include
   line and column number of the error). */
parser code {:
    
    /* Change the method report_error so it will display the line and
       column of where the error occurred in the input as well as the
       reason for the error which is passed into the method in the
       String 'message'. */
    public void report_error(String message, Object info) {
   
        /* Create a StringBuffer called 'm' with the string 'Error' in it. */
        StringBuffer m = new StringBuffer("Error");
   
        /* Check if the information passed to the method is the same
           type as the type java_cup.runtime.Symbol. */
        if (info instanceof java_cup.runtime.Symbol) {
            /* Declare a java_cup.runtime.Symbol object 's' with the
               information in the object info that is being typecasted
               as a java_cup.runtime.Symbol object. */
            java_cup.runtime.Symbol s = ((java_cup.runtime.Symbol) info);
   
            /* Check if the line number in the input is greater or
               equal to zero. */
            if (s.left >= 0) {                
                /* Add to the end of the StringBuffer error message
                   the line number of the error in the input. */
                m.append(" in line "+(s.left+1));   
                /* Check if the column number in the input is greater
                   or equal to zero. */
                if (s.right >= 0)                    
                    /* Add to the end of the StringBuffer error message
                       the column number of the error in the input. */
                    m.append(", column "+(s.right+1));
            }
        }
   
        /* Add to the end of the StringBuffer error message created in
           this method the message that was passed into this method. */
        m.append(" : "+message);
   
        /* Print the contents of the StringBuffer 'm', which contains
           an error message, out on a line. */
        System.err.println(m);
    }
   
    /* Change the method report_fatal_error so when it reports a fatal
       error it will display the line and column number of where the
       fatal error occurred in the input as well as the reason for the
       fatal error which is passed into the method in the object
       'message' and then exit.*/
    public void report_fatal_error(String message, Object info) {
        report_error(message, info);
        System.exit(1);
    }
:};
   

   
/* ------------Declaration of Terminals and Non Terminals Section----------- */
   
/* Terminals (tokens returned by the scanner).  

   Terminals that have no value are listed first and then terminals
   that do have an value, in this case an integer value, are listed on
   the next line down. */
terminal           SEMI, PLUS, MINUS, TIMES, DIVIDE, LPAREN, RPAREN;
terminal Integer   NUMBER, ID;
   
/* Non terminals used in the grammar section.  

   Non terminals that have an object value are listed first and then
   non terminals that have an integer value are listed.  An object
   value means that it can be any type, it isn't set to a specific
   type.  So it could be an Integer or a String or whatever. */
non terminal Object     expr_list, expr_part;
non terminal Integer    expr, factor, term;
   

/* -------------Precedence and Associatively of Terminals Section----------- */
   
/*
  Precedence of non terminals could be defined here.  If you do define
  precedence here you won't need to worry about precedence in the
  Grammar Section, i.e. that TIMES should have a higher precedence
  than PLUS.
  
  The precedence defined here would look something like this where the
  lower line always will have higher precedence than the line before it.
   
  precedence left PLUS, MINUS; 
  precedence left TIMES, DIVIDE; 
*/


/* ----------------------------Grammar Section-------------------- */
   
/* The grammar for our parser.
   
   expr_list ::=   expr_list expr_part
                 | expr_part
   expr_part ::=   expr SEMI
   expr      ::=   expr PLUS factor
                 | expr MINUS factor
                 | factor
   factor    ::=   factor TIMES term
                 | factor DIVIDE term
                 | term
   term     ::=    LPAREN expr RPAREN
                 | NUMBER
                 | ID     
*/
   
/* 'expr_list' is the start of our grammar.  It can lead to another
   'expr_list' followed by an 'expr_part' or it can just lead to an
   'expr_part'.  The lhs of the non terminals 'expr_list' and
   'expr_part' that are in the rhs side of the production below need
   to be found.  Then the rhs sides of those non terminals need to be
   followed in a similar manner, i.e. if there are any non terminals
   in the rhs of those productions then the productions with those non
   terminals need to be found and those rhs's followed.  This process
   keeps continuing until only terminals are found in the rhs of a
   production.  Then we can work our way back up the grammar bringing
   any values that might have been assigned from a terminal. */
   
   expr_list ::= expr_list expr_part
                 |
                 expr_part;
   
/* 'expr_part' is an 'expr' followed by the terminal 'SEMI'.  The ':e'
   after the non terminal 'expr' is a label an is used to access the
   value of 'expr' which will be an integer.  The action for the
   production lies between {: and :}.  This action will print out the
   line " = + e" where e is the value of 'expr'.  Before the action
   takes places we need to go deeper into the grammar since 'expr' is
   a non terminal.  Whenever a non terminal is encountered on the rhs
   of a production we need to find the rhs of that non terminal until
   there are no more non terminals in the rhs.  So when we finish
   going through the grammar and don't encounter any more non
   terminals in the rhs productions will return until we get back to
   'expr' and at that point 'expr' will contain an integer value. */
   
   expr_part ::= expr:e
                 {: System.out.println(" = " + e); :}
                 SEMI
                 ;
   
/* 'expr' can lead to 'expr PLUS factor', 'expr MINUS factor', or
   'factor'.  The 'TIMES' and 'DIVIDE' productions are not at this
   level.  They are at a lower level in the grammar which in affect
   makes them have higher precedence.  Actions for the rhs of the non
   terminal 'expr' return a value to 'expr'.  This value that is
   created is an integer and gets stored in 'RESULT' in the action.
   RESULT is the label that is assigned automatically to the rhs, in
   this case 'expr'.  If the rhs is just 'factor' then 'f' refers to
   the non terminal 'factor'.  The value of 'f' is retrieved with the
   function 'intValue()' and will be stored in 'RESULT'.  In the other
   two cases 'f' and 'e' refers to the non terminals 'factor' and
   'expr' respectively with a terminal between them, either 'PLUS' or
   'MINUS'.  The value of each is retrieved with the same function
   'intValue'.  The values will be added or subtracted and then the
   new integer will be stored in 'RESULT'.*/
   
   expr      ::= expr:e PLUS factor:f
                 {: RESULT = new Integer(e.intValue() + f.intValue()); :}
                 |
                 expr:e MINUS factor:f
                 {: RESULT = new Integer(e.intValue() - f.intValue()); :}
                 |
                 factor:f
                 {: RESULT = new Integer(f.intValue()); :}
                 ;
   
/* 'factor' can lead to 'factor TIMES term', 'factor DIVIDE term', or
   'term'.  Since the productions for TIMES and DIVIDE are lower in
   the grammar than 'PLUS' and 'MINUS' they will have higher
   precedence.  The same sort of actions take place in the rhs of
   'factor' as in 'expr'.  The only difference is the operations that
   takes place on the values retrieved with 'intValue()', 'TIMES' and
   'DIVIDE' here instead of 'PLUS' and 'MINUS'.  */
   
   factor    ::= factor:f TIMES term:t
                 {: RESULT = new Integer(f.intValue() * t.intValue()); :}
                 |
                 factor:f DIVIDE term:t
                 {: RESULT = new Integer(f.intValue() / t.intValue()); :}
                 |
                 term:t
                 {: RESULT = new Integer(t.intValue()); :}
                 ;
   
/* 'term' can lead to 'LPAREN expr RPAREN', 'NUMBER', or 'ID'.  The
   first production has the non terminal 'expr' in it so the
   production with its lhs side needs to be found and followed.  The
   next rhs has no non terminals.  So the grammar ends here and can go
   back up.  When it goes back up it will bring the value that was
   retrieved when the scanner encounter the token 'NUMBER'.  'RESULT'
   is assigned 'n', which refers to 'NUMBER', as the action for this
   production.  The same action occurs for 'ID', except the 'i' is
   used to refer to 'ID'.  'ID' is also the only thing on the rhs of
   the production.  And since 'ID' is a terminal the grammar will end
   here and go back up. */
   
   term      ::= LPAREN expr:e RPAREN
                 {: RESULT = e; :}
                 |
                 NUMBER:n
                 {: RESULT = n; :}
                 |
                 ID:i
                 {: RESULT = i; :}
                 ;