expr.atg

来自「一个Modula-2语言分析器」· ATG 代码 · 共 48 行

COMPILER Expr
(* Simple four-function calculator - FileIO library required *)

  PROCEDURE GetNumber (VAR Int : INTEGER);
  (* Convert latest token to integer value Int *)
    VAR
      I : CARDINAL;
      String : ARRAY [0 .. 20] OF CHAR;
    BEGIN
      ExprS.GetString(ExprS.pos, ExprS.len, String);
      I := 0; Int := 0;
      WHILE String[I] # 0C DO
        Int := 10 * Int + VAL(INTEGER, ORD(String[I]) - ORD('0')); INC(I)
      END;
    END GetNumber;

IGNORE CHR(1) .. CHR(15)

CHARACTERS
  digit =  "0123456789" .

TOKENS
  num = digit {digit} .

PRODUCTIONS
  Expr                              (. VAR r: INTEGER; .)
    = Expression <r> "="            (. IF Successful() THEN
                                         FileIO.WriteInt(FileIO.con, r, 4)
                                       END .) .

  Expression <VAR e: INTEGER>       (. VAR t: INTEGER; .)
    = Term <e>
      {  '+' Term <t>               (. e := e + t .)
      |  '-' Term <t>               (. e := e - t .)
      }.

  Term <VAR t: INTEGER>             (. VAR f: INTEGER; .)
    = Factor <t>
      {  '*' Factor <f>             (. t := t * f .)
      |  '/' Factor <f>             (. t := t DIV f .)
      }.

  Factor <VAR f: INTEGER> =
         num                        (. GetNumber(f) .)
    | '(' Expression <f> ')'.

END Expr.