Plov003 : test a recursive descent parser

The next source is 'test3' and it is listed below:

	PROGRAM Number1

	CONSTANTS  pi = 314
		   ee =	272
		   END

	VARIABLES  A B C Def GhYh J123 END

	END Number1

I added VARIABLES. See what it does so far:

an@beryllium:~/modula/Plov$ Plov003 <test3
PROGRAM Number1 CONSTANTS       pi      =       ee      =       END     VARIABLES       A       
Illegal identifier name : A
B       Illegal identifier name : B
C       Illegal identifier name : C
Def     GhYh    J123    END     END     Number1

Number1 Program type.
RETURN  Keyword type.
EXIT    Keyword type.
LOOP    Keyword type.
BEGIN   Keyword type.
END     Keyword type.
IF      Keyword type.
THEN    Keyword type.
ELSIF   Keyword type.
ODD     Keyword type.
PROCEDURE       Keyword type.
PROGRAM Keyword type.
CONSTANTS       Keyword type.
VARIABLES       Keyword type.
pi      Constant =   314 type.
ee      Constant =   272 type.
Def     Variable type.
J123    Variable type.
jan@beryllium:~/modula/Plov$

Something fishy going on here. The Constants are now OK. But the single letter variables are treated as 'illegal'. I have a hunch what's wrong. But from the three multiletter variables (Def GhYh J123) only the first and third are listed in the symbol table....
The initial fix is easy:

      
PROCEDURE isAlpha (chr  : CHAR) : BOOLEAN;

BEGIN
   chr := CAP (chr);
   IF  (chr >= 'A') AND (chr <= 'Z')  THEN
      RETURN TRUE
   END;
   RETURN FALSE
END isAlpha;


PROCEDURE isAlphaNum (chr  : CHAR) : BOOLEAN;

BEGIN
   RETURN  isAlpha (chr) OR isDigit (chr)
END isAlphaNum;


PROCEDURE isNumber (str  : Identifier) : BOOLEAN;

VAR	i	: CARDINAL;

BEGIN
   i := 0;
   REPEAT
      IF  isDigit (str [i]) = FALSE  THEN  RETURN FALSE  END;
      INC (i)
   UNTIL  (str [i] = 0C) OR (i > HIGH (str));
   RETURN TRUE
END isNumber;


PROCEDURE isIdentifier (str  : Identifier) : BOOLEAN;

VAR	i	: CARDINAL;

BEGIN
   IF  NOT isAlpha (str [0])  THEN  RETURN FALSE  END;
   IF  Strings.Length (str) = 1  THEN  RETURN TRUE  END;
   i := 1;
   REPEAT
      IF  NOT isAlphaNum (str [i])  THEN  RETURN FALSE  END;
      INC (i)
   UNTIL  (str [i] = 0C) OR (i > HIGH (str));
   RETURN TRUE
END isIdentifier;

See what it does:

      
jan@beryllium:~/modula/Plov$ Plov003 <test3
PROGRAM Number1 CONSTANTS       pi      =       ee      =       END     VARIABLES       A       
B       C	Def     GhYh	J123     END     Duplicate identifier : END
END     Number1

Number1 Program type.
RETURN  Keyword type.
EXIT    Keyword type.
LOOP    Keyword type.
BEGIN   Keyword type.
END     Keyword type.
IF      Keyword type.
THEN    Keyword type.
ELSIF   Keyword type.
ODD     Keyword type.
PROCEDURE       Keyword type.
PROGRAM 	Keyword type.
CONSTANTS       Keyword type.
VARIABLES       Keyword type.
pi      Constant =   314 type.
ee      Constant =   272 type.
A       Variable type.
C       Variable type.
GhYh    Variable type.

jan@beryllium:~/modula/Plov$

Better now. Single letter variables are now accepted and included in the symbol table, but also here: only two out of three are included. Fixed it: GetSymbol was placed inside the REPEAT loop instead of outside:

PROCEDURE VariableDeclaration;

BEGIN
   GetSymbol;		(* Used to be below the line with 'currentType' in it	*)
   REPEAT
      currentType := Vartype;
      IF  isIdentifier (token)  THEN
         IF  StoreSymbol (token) = TRUE  THEN
	    GetSymbol
	 ELSE
	    ErrorMessage (6)		(* Duplicate identifier *)
	 END
      ELSE
         ErrorMessage (5)		(* Illegal Identifier	*)
      END
   UNTIL Strings.StrEq (token, "END");
   IF  SyntaxError  THEN  Synchronize  END;
   GetSymbol
END VariableDeclaration;

And now fr a new testrun:

jan@beryllium:~/modula/Plov$ Plov003 <test3
PROGRAM Number1 CONSTANTS       pi      =       ee      =       END     VARIABLES       A       B       C
Def     GhYhJ123     END     END     Number1

Number1 Program type.
RETURN  Keyword type.
EXIT    Keyword type.
LOOP    Keyword type.
BEGIN   Keyword type.
END     Keyword type.
IF      Keyword type.
THEN    Keyword type.
ELSIF   Keyword type.
ODD     Keyword type.
PROCEDURE       Keyword type.
PROGRAM Keyword type.
CONSTANTS       Keyword type.
VARIABLES       Keyword type.
pi      Constant =   314 type.
ee      Constant =   272 type.
A       Variable type.
B       Variable type.
C       Variable type.
Def     Variable type.
GhYh    Variable type.
J123    Variable type.
jan@beryllium:~/modula/Plov$

FIXED! Constants and variables are stored in the symbol table! Most problems are so simple to solve, if you feel the force to control the source!

Source to compile:

PROGRAM Number1

CONSTANTS  pi = 314
	   ee =	272
	   END

VARIABLES  A 
	   B 
	   C 
	   Def 
	   GhYh 
	   J123
	   NH4NO3
	   END

BEGIN

END Number1

Compilation result:

jan@beryllium:~/modula/Plov$ Plov003 <test5
PROGRAM Number1 CONSTANTS       pi      =       ee      =       END     VARIABLES       A       B       C
Def     GhYhJ123     NH4NO3  END     BEGIN   END     Number1


Number1 Program type
RETURN  Keyword type
EXIT    Keyword type
LOOP    Keyword type
BEGIN   Keyword type
END     Keyword type
IF      Keyword type
THEN    Keyword type
ELSIF   Keyword type
ODD     Keyword type
PROCEDURE       Keyword type
PROGRAM Keyword type
CONSTANTS       Keyword type
VARIABLES       Keyword type
pi      Constant type =   314
ee      Constant type =   272
A       Variable type
B       Variable type
C       Variable type
Def     Variable type
GhYh    Variable type
J123    Variable type
NH4NO3  Variable type
jan@beryllium:~/modula/Plov$

Nice going!

Source to compile :

PROGRAM Number1

CONSTANTS  pi = 314
	   ee =	272
	   END

VARIABLES  A 
	   B 
	   C 
	   Def 
	   GhYh 
	   J123
	   NH4NO3
	   END

PROCEDURE  AddItUp

BEGIN
END AddItUp

BEGIN
   AddItUp
END Number1

Compilation results :

jan@beryllium:~/modula/Plov$ Plov003 <test4
PROGRAM Number1 CONSTANTS       pi      =       ee      =       END     VARIABLES       A       B       C
Def     GhYhJ123     NH4NO3  END     PROCEDURE       AddItUp BEGIN   END     AddItUp END expected
BEGIN   Procedure names do not match.
AddItUp AddItUp BEGIN expected

jan@beryllium:~/modula/Plov$

Error messages and the program gets stuck in an endless loop. After few seconds the system gets sluggish and the CPU fan goes into third gear. Work to do!

The work to do was a constant juggling with placement of the 'GetSymbol' command. In one function it worked and then when another function was used, the BlockBody function had the GetSymbol call in the wrong place. But now it works...

jan@beryllium:~/modula/Plov$ Plov003 <test4
PROGRAM Number1   
CONSTANTS  pi = ee = END
VARIABLES  A  B  C  Def GhYhJ123  NH4NO3  END
PROCEDURE  AddItUp  BEGIN  END AddItUp   
BEGIN   AddItUp      END Number1

Number1 Program type
RETURN  Keyword type
EXIT    Keyword type
LOOP    Keyword type
BEGIN   Keyword type
END     Keyword type
IF      Keyword type
THEN    Keyword type
ELSIF   Keyword type
ODD     Keyword type
PROCEDURE       Keyword type
PROGRAM Keyword type
CONSTANTS       Keyword type
VARIABLES       Keyword type
pi      Constant type =   314
ee      Constant type =   272
A       Variable type
B       Variable type
C       Variable type
Def     Variable type
GhYh    Variable type
J123    Variable type
NH4NO3  Variable type
AddItUp Procedure type
jan@beryllium:~/modula/Plov$

Source to compile :

PROGRAM Number1

CONSTANTS  pi = 314
	   ee =	272
	   END

VARIABLES  A 
	   B 
	   C 
	   Def 
	   GhYh 
	   J123
	   NH4NO3
	   END

PROCEDURE  Diff

BEGIN
END Diff

PROCEDURE  AddItUp

BEGIN
END AddItUp

BEGIN
   AddItUp
   Diff
END Number1

Compilation results :

jan@beryllium:~/modula/Plov$ Plov003 <test6
PROGRAM Number1 CONSTANTS       pi      =       ee      =       END     VARIABLES       A       B       C
Def     GhYhJ123     NH4NO3  END     PROCEDURE       Diff    BEGIN   END     Diff    PROCEDURE       AddItUp
BEGIN   END     AddItUp BEGINAddItUp Diff    END     Number1


Number1 Program type
RETURN  Keyword type
EXIT    Keyword type
LOOP    Keyword type
BEGIN   Keyword type
END     Keyword type
IF      Keyword type
THEN    Keyword type
ELSIF   Keyword type
ODD     Keyword type
PROCEDURE       Keyword type
PROGRAM Keyword type
CONSTANTS       Keyword type
VARIABLES       Keyword type
pi      Constant type =   314
ee      Constant type =   272
A       Variable type
B       Variable type
C       Variable type
Def     Variable type
GhYh    Variable type
J123    Variable type
NH4NO3  Variable type
Diff    Procedure type
AddItUp Procedure type
jan@beryllium:~/modula/Plov$

More than one variable. More than one cosntant. More than one procedure. All symbols recognized and processed. One should have a fine feeling at such a moment. I am one of those!

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