compiler construction - ANTLR4: grammar for arithmetic expression - division by zero check, keywords check -


i need grammar arithmetic expressions.

i notify user error message if tries divide 0 or if use vhdl keywords (target language) variable name.

but new antlr , cant figure out how extend grammar:

grammar arithmetic;  @header {     package generated; }  stat     :   left = variable op = assigment right = expr     # assigment     ;  expr     :   '('   exp = expr ')'                            # parens     |   minus exp = expr                                # unaryminus     |   left = expr op = (times | div)  right = expr    # muldiv     |   left = expr op = (plus  | minus) right = expr   # addsub     |   (variable | constant)                           # element     ;  assigment   :   '=' ; plus        :   '+' ; minus       :   '-' ; times       :   '*' ; div         :   '/' ; lparen      :   '(' ; rparen      :   ')' ;   variable    :   (letter+|digit+|'_')+   ; constant    :   integer                 ;  integer     :   digit+                  ;   letter      :   ('a' .. 'z') | ('a' .. 'z') ; digit       :   ('0' .. '9')    ;  ws          :   [ \r\n\t] + -> skip ;

i found numerous small problems have corrected in grammar below.

  • no eof marker
  • could run 1 statement extended program
  • the @header thing causes java grungui not run
  • _ valid variable name, not want.

  • '5' valid left-hand-side of assignment. 5=6 valid assignment statement, again, not want.

    grammar arithmetic; program : stat+ eof; stat :   left = variable op = assigment right = expr     # assigment ;  expr :   '('   exp = expr ')'                            # parens |   minus exp = expr                                # unaryminus |   left = expr op = (times | div)  right = expr    # muldiv |   left = expr op = (plus  | minus) right = expr   # addsub |   (variable | constant)                           # element ;  assigment   :   '=' ; plus        :   '+' ; minus       :   '-' ; times       :   '*' ; div         :   '/' ; lparen      :   '(' ; rparen      :   ')' ; variable    :   letter+(letter|digit|'_')*   ; constant    :   integer                 ; integer     :   digit+                  ; letter      :   ('a' .. 'z') | ('a' .. 'z') ; digit       :   ('0' .. '9')    ; ws          :   [ \r\n\t] + -> skip ;

now corrects lot of lexing , "good form" issues. next question about, example, division zero.

the grammar not place enforce such rules. example, 3/0 legal mathematical expression. happens evaluate infinity, , therefore guarded against in program. likewise, should handle special cases in code. implementing visitor or listener pattern when right-hand-side of #muldiv context equal zero, should intervene @ point. grammar no place trying implement such sophisticated semantic , context-sensitive rules.

as how program if statement, i'll give peek @ way implement them:

    public override muvalue visitifstmt(lisbasicparser.ifstmtcontext context)     {         lisbasicparser.condition_blockcontext[] conditions = context.condition_block();         bool evaluatedblock = false;         foreach (lisbasicparser.condition_blockcontext condition in conditions)         {             muvalue evaluated = visit(condition.expr());             if (evaluated.asboolean())             {                 evaluatedblock = true;                 visit(condition.stmt_block());                 break;             }         }         if (!evaluatedblock && context.stmt_block() != null)         {             visit(context.stmt_block());         }         return muvalue.void;     }

granted, doesn't make sense out of context, rest assured works. see in full context, please visit bart kiers excellent example of grammar , implementation .


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