5 - Forth Interest Group

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the other class's methods. For example, if one had both AUTOMOBILE class and CHEW class, there may be AUTOMOBILE methods that are applicable to the CHEVY class objects. By simply stat- ingAUTOMOBILEINHERIT : >in! ( -- ) \ macro to set input stream pointer. before defining c~~wclass, in-file @ fseek ; it inherits all the methods of AUTOMOBILE. I used this : MESSAGE \ ( -- ) feature to provide STACK objects with the in@ \ remember input stream location. create \ create message >in ! \ put input stream pointer back. bl word count \ grab and compile string in swap 1+ swap 1- ", \ pfa of message. does> ( pfa -- ? ) find if execute \ effect method of action. else the array's elements. Consider the INDEX method the last thing that must be done, after deriving the address of the indexed element within the array, is to switch the class context to that of the abort" method unknownw then I cr . ( oofmssag. fth loaded ) cr elements so subsequent messages will be bound to ating objects for each new the correct instance type (and, grouptype class. Consider thus, the correct method will the following instantiation of be executed). The words a COMPLEX STACK. OBJECTS and :VARS in STACK OBJECT FRED Figure Four assume that the 23 COMPLEX OBJECTS first two instance variables within a grouptype object Thus, FRED is defined as (like ARRAYS, STACKS, the object at the head of the MATRICES, QUEUES, etc) will group of elements, with the be #EL and "EL-CLASS, defining word OBJECT. Then respectively. 23 complex-type objects Figure Eight (page 30) were allotted OBJECTS is gives an example of IN- capable of patching the pre- HERITing a d ~ STACK a viously ~ created word (in this is a specialization (and case, FRED) with the numsuperset) of ARRAY class. ber of elements allotted into Note that the class context FRED'S #EL instance varimust be switched to that of able. the stack's elements just be- The benefit of an objectfore the message is sent to oriented implementation's fetch or store the element security can be seen by comobject in both PUSH and POP. paring the code of Figure One kludge I wanted to Eight with that of Figure Nine avoid in defining grouped (page 31). Both are impleobjects (like ARRAYS and mentations ofstack-type data, STACKS) was the need to but are radically different. predetermine the size of the Note that code using STACK body in the class definition would not break if STACK This would either cause all were changed-due to bethe ARRAYS (or STACKS) to ing forced to use only prohave the same number of cedures within the STACK elements, or else necessitate class's code-definition moda new defining word for cre- ule. Conclusion If an object-oriented programrning language is defined by the characteristics of data abstraction, information hiding, dynamic binding, and inheritance, only a little needs to be added to Forth to make it so. In keeping with an RPN syntax, the process ofbinding a message is shared between the object and the message words. It may appear that this responsibility could have been wholly shifted to the message word, but only because this is a late-binding example. Because late binding has an associated run-time penalty (finding the correct method to execute), early binding is usually used except when late binding is required. It would take about two more words, and about five minutes of coding, to convert the given code to early binding (really!). This will be left as an exercise to the reader. (Oh! I've always wanted to say that!) Maybe I will up date the code in an upcoming article.. . Also, one could optimize the code associated with the fmt instance variable reference. Because it has an offset of zero within the parent object, it is wasteful at run time to add the offset. Object-oriented Forth, as presented, is upwardly compatible with common Forth (Forth-83, specifically); thus, they can be intermixed at will. Programming in OOF promotes grouping all of a data class's routines, just below the declaration of the inner layout of the class. Also, it allows one to focus on objects and actions without constant regard to internal implementation details (one defmition of PUSH works for any kind of STACK). An object orientation is simply the coding disciplines specific to the expression of data, and which are complementary to those for procedures. The intent is to reduce maintenance by minimizing modif~cations caused by a change, and to increase productivity by enhancing reusability of existing code. January 1992 February Figure Three. Defining MESSAGE. I \ oofmssag-fth 910729 rwb : >in@ ( -- ) \ macro to save input stream pointer. in-file @ ftell ; Forth Dimensions
Figure Four. Words to create objects and instance variables. I I I \ oofob jec. fth 910730 rwb I : +objects \ ( 'body -- 'objects ) MACRO dup cell- @ \ ( -- 'body 'el-class ) +/body @ + ; : /body \ ( -- /body ) context @ +/body @ ; : do-object \ ( /body -- allot does> dup @ >context \ select object's context. cell+ \ ( -- 'body ) : object \ ( -- ) Create instance of class. create context @ , /body do-object ; : objects \ ( n -- Create n instances of class. /body over \ ( -- n /body n ) last @ name> >body cell+ \ ( -- n /body n 'body ) context @ over cell+ \ ( -- n /body n '#el 'elclass "elclass ) ! ! \ ( -- n /body * do-object ; : do-var \ ( offset /body -- offset' ) over , + does> ( pfa -- 'body ) dup @ >context cell+ @ + \ ( -- 'var ) , : :var \ ( offset -- offset' ) Create instance variable. create context @ , /body do-var ; : :vars \ ( offset n -- offset' ) Create n instance variables. /body over \ ( -- offset n /body n ) last @ name> >body cell+ \ ( -- offset n /body n 'body ) context @ over cell+ \ ( -- offset n /body n '#el 'elclas "elclass ) ! ! \ ( -- offset n /body ) * do-var ; cr .( oofobjec.fth loaded ) cr Advertisers Index ACM ............................. 22 Ciber ............................... 7 Forth Interest Group ...... .44 Harvard Softworks .......... 41 Miller Microcomputer Services.. ................... .27 Next Generation Systems ...................... 42 Silicon Composers .......... .2 Forth Dimensions 27 January 1992 February
Page 2 and 3: SILICON COMPOSERS INC Announcing th
Page 4 and 5: Objets dyArt 0 bject-oriented progr
Page 6 and 7: Yerk Comes to the PC Rick Grehan Pe
Page 8 and 9: instance variables can re- solve th
Page 10 and 11: :class. The :class word uses a nest
Page 12 and 13: stance variable names in these alte
Page 14 and 15: ules. (Jim Callahan, Harvard Softwo
Page 16 and 17: Dl77 ( magic seed ) swm 3. Page 24,
Page 18 and 19: y an Engish transla- tion or summar
Page 20 and 21: somewhere, should we just say 'It's
Page 22 and 23: ,Partic@a te! ACM SlGForth '92 Fort
Page 24 and 25: methods is called inbetft- ance. De
Page 28 and 29: On-Line Resource Figure Five. The i
Page 30 and 31: of classical Forth.. . And Forth di
Page 32 and 33: (Figure Nine, continued.) : @@ \ (
Page 34 and 35: This partimlar example has been des
Page 36 and 37: (METHODS) DEFINITIONS : ... content
Page 38 and 39: (GEnie/nrm pago 32.) not endorse, a
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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