5 - Forth Interest Group

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This partimlar example has been designed to initialize the x and y coordinates to 10 and 15, respectively, so the most likely operations to follow are similar to those given in Figure Three. The syntax is, of course, typical RPN where the parameters are put on the stack fmt, then the message or method selector. Finally the operation is performed by the object itself, which is just what object-oriented programming - is all about ~xarn~les new class, and there has to be a valid syntax to access the method In this case, the data structure address is available and only the methods need to be obtained. The use of IN is to bypass the local data struaure address inside PT in favor of that provided on the stack. Syntactically, this has exactly the same effect as INHERIT but may be used to access any object matching the hidden data structure. 73is Figure One. Syntax to represent object class. 1 CLASS DATA ... \ Data structure and \ initialization METHODS .. .. INHERIT or NULL \ Where NULL means \ no ancestor class ENDCLASS - of inheritance, 1 Figure Two. Defining the point class. ~olvmomhism. and overioahing &n easily be developed by extending from a Point class to a Rectangle class, and further to a Square class. To simp* the discussion, we will use a rectangle METHODNAME GETX METHODNAME PUTX METHODNAME GETXY METHODNAME SWAPXY METHODNAME GETY METHODNAME PUTY METHODNAME PUTXY METHODNAMEGRIPE \ Usedas failingmethod oriented parallel to the x and y axes, which can be defined by its two opposite corners: \ The following are useful general method names providing \ for instance initialization and class recognition for the user. upper left (ul) and lower METHODNAME BUILD right 0. (See Figure Four.) METHODNAME ASTEXT Creating the object becomes simply : \ The following are hidden methods. 40 18 6 10 RE~TAN~LE \ These words will not be visible within the Forth dictionary RECT I METHOD: GETXYM ( addr -- x y ) DUP WSIZE + @ SWAP @ METHOD; % rectangle obijct con- I tains two &d it is appropriate that the designer of the new class can operate METHOD: PUTXYM ( x y addr -- ) DUP >R ! R> WSIZE + ! METHOD ; on these hidden or anonymous objects as if they were separate. To obtain the up per left and lower right corner values for the rectangle, METHOD: SWAPXYM ( addr -- ) DUP @ OVER WSIZE + @ 2 PICK ! SWAP WSIZE + ! METHOD ; we have used the structure GETXY IN PT, giving US access to the point methods needed. We could have used, the GETXYM word from the (METHODS ) vocabulary, because it is designed to work with a POINT object. This is only possible because we have not made our Point methods totally hidden to other classes. If we took adanta age of the EXCISE facility included with UWFORTH, we could not cheat by accessing the hidden methods. The programmer may want to use a particular class method which is appropriate for his anonymous. (i.e., unnamed) objects within a CLASS POINT ( x y -- ) DATA 1 , \ Initialize x y to the values on the stack METHODS \ Inline functions PUTX ( x -- ) :: WSIZE + ! ;; GETX ( -- x ) :: WSIZE + @ ;; PUTY ( y -- ) . .. . ! ;; GETY ( -- y ) :: @ ;; ASTEXT ( -- strinaA < . : : " Point" ;; \ Hidden functions defined earlier GETXY ( -- x y ) M: GETXYM M; BUTXY ( x y -- ) M: PUTXYM M; SWAPXY ( -- ) M: SWAPXYM M; BUILD ( x y -- ) M: PUTXYM M; \ An alias for PUTXY here INHERIT NULL ENDCLAS S January 1992 February 34 Forth Dimensions
Figure Three. Using the point class. I GETXY PT . . 15 10 ok \ Note the order printed! SWAPXY PT ok GETXY PT . . 10 15 0k ASTEXT PT COUNT TYPE Point ok 22 7 BUILD PT ok GETXY PT . . 7 22 ok Figure Four. Defining a rectangle class. METHODNAME PUTHEIGHT METHODNAME GETHEIGHT METHODNAME PUTWIDTH METHODNAME GETWIDTH METHODNAME UPPERLEFT METHODNAME LOWERRIGHT METHOD: PUTHEIGHTM ( h addr -- ) DUP @ ROT + \ The new lower right -> ylr SWAP WSIZE 2" + ! METHOD ; METHOD: PUTWIDTHM ( w addr -- ) DUP WSIZE + @ ROT + \ The new xlr SWAP WSIZE 3 * + ! METHOD ; METHOD: GETHEIGHTM ( addr -- h 1 DUP WSIZE 2" + @ SWAP @ - METHOD ; METHOD: GETWIDTHM ( addr -- W DUP WSIZE 3 * + @ SWAP WSIZE + @ - METHOD ; CLASS RECTANGLE ( xlr ylr xu1 yul -- ) \ Rectangle aligned to the x y axes DATA I \ Upper left POINT yul xu1 I I \ Lower right POINT ylr xlr METHODS PUTWIDTH ( w -- ) M: PUTWIDTHM M; GETWIDTH ( -- w ) M: GETWIDTHM M; PUTHEIGHT ( h -- ) M: PUTHEIGHTM M; GETHEIGHT ( -- h ) M: GETHEIGHTM M; UPPERLEFT ( -- x y ) :: GETXY IN PT ;; LOWERRIGHT ( -- x y ) :: WSIZE 2* + GETXY IN PT ;; ASTEXT ( -- stringA ) :: " Rectangle" ;; BUILD ( xlr ylr xu1 yul -- ) :: DUP >R PUTXY IN PT R> WSIZE 2* + PUTXY IN PT ;; INHERIT PT \ NOTE: Instance of POINT needed, not the class. ENDCLASS Forth Dimensions 35 may only be used within a ckass depnitian, as it is only within the class definition that tbm is knowledge of the internal data stncctures to allow theprog~ammerac~4~~. NOW for the SQUARE class, defined in Figure Five. Producing a new square : 5 12 13 SQUARE FRED We now have a single inheritance chain of classes to experiment with! [See Figure Six.] creating the Object Syntax The use of defining and compiling words in Forth provides the programmer with the ability to produce new language constructs, and is the core of the syntax- generation process. To generate the syntax, some preliminary functions will prove useful later. Dur- ing creation of a specific syntax, these will be speci- fied as the need arises; but here we will separate them from the detailed discussion of the syntax itself. To generate distinct method names, it is only a matter of making a func- tional equivalent to a variable without using the storage. In many systems where the dictionary and vocabulary coexist, simple name creation would be enough; but to be completely general, we will define them as follows. : METHODNAME CREATE 0 I ; To hide the more com- plex methods from the normal programming emiron- men4 we can set up a new vocabulary into which all of these definitions can be placed. VOCABULARY (METHODS ) If we wished to interac- tively open this vocabulary for the storage of a new defhtion and then return to the normal FORTH vocabulary, we would enter the sequence: January 1992 February
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Page 36 and 37: (METHODS) DEFINITIONS : ... content
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Page 40 and 41: I still feel that we are debating s
Page 42 and 43: indeed, were not particularly encou
Page 44: L Contributions from the Forth Comm

5 - Forth Interest Group

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