5 - Forth Interest Group

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(Figure Nine, continued.) : @@ \ ( 'body -- obj ) \ check not empty \ set element context \ fetch ob j dup empty? not \ ( -- 'body -f ) if dup 'obj swap element-context DUP CELL +>R @ ; We are using here the fact that all of these systems in- crement the instruction pointer and store it on the return stack when a new word executes. We can easily imitate this guide to skip over embedded data of any size, and put any information about it on the stackperhaps just the starting address. (I should mention that an important aspect of Forth in my work is the ability to seamlessly integrate into a Forth system new and unusual data types-some systems have as many as seven new types, each with appropriate mechanisms for storage management, appropriate handlers, operators, etc.) The basic control structures are defined in the same way in all of these systems. For example: : IF COMPILE ?BRANCH HERE 0 , ; IMMEDIATE : THEN HERE SWAP ! ; IMMEDIATE (Compiler security has been ignored I believe all the above systems use the abso- lute address rather than a displacement-but the change is not a major one.) With this information, one can produce any conceiv- able control structure on any of these systems by laying down and resolving the a p propriate branch instructions. Uo be sure, some such structures, like the Eaker case statement, can be synthe- sized using standard control constructs-although with reduced efficiency.) In brief, the user has both knowledge of and control over what is assembled. The standard language provides words (like the control flow words) that introduce variants into the normal succes- sion of addresses constitut- ing the machine language of the abstract machine-but access is there for the user to do something different In effect, the user has as much control over the process of translating a high-level language into "object coden as does the writer of a compiler for a conventional language. The user has the tools to make a high-level language look like anything he wishes-because he has complete control over the process of compilation. And he can do it portably if he uses "traditional Forth." This is a remarkable and somewhat subversive idea: that a user should have power normally reserved to spe- cialists. I wouldn't dismiss it as hogwash if I were you! From: John Wavrik Re: Disenfranchised Mitch Bradley writes, "Where Dr. Wavrik has been specific rather than philosophical (e.g., user- defined control structures), the committee has attempted to deal with the issues. It would have saved me a lot of time if the specific issues had been presented in the form of proposals; then I wouldn't have had to do the work of writing the proposals." In the interest of histori- cal accuracy, Mitch Bradley had a proposal he wanted to submit in this area. He con- sulted me and a few other people. I gave him my im- pression of his proposal, but he submitted it anyway. I do (GEnie continued on page 38.) January 1992 February 32 Forth Dimensions
Simple Ob 'ect- Oriented 6orth C. A. Maynard Wilson, West Australia F orth is a minimalist lan- lation and results in im- tional programming necesguage, by which we proved data security, as only sary to handle a new variamean that the core of those operations which have tion on an object, If the the language provides facili- been designed to work with methods defined for the anties from which the user/ the object's data structure cestor object are valid, the programmer can build his will be executed, and there is objectwill simply obtain them own working environment, no direct access to the data via the inheritance chain. If It has also been described as itself. The user of an object there is the need for a different a syntaxdirected language need have no knowledge of definition of a method for a because, if you can define a the details of data storage or new class, that may be insyntax which will best ex- even details of the appli- cluded in the new class and press your needs, Forth will cable methods. He/she just will only work for the new allow you to create a func- needs to know the valid op class of object and its tional equivalent suitable for erations and any parameter- decendents. This ability is programmer use. passing requirements which called polynwqhisrn. The latest thing in pro- may be necessary to operate Ouerlaading is the ability gramming tools is the use of upon the object. to have more than one defithe objj-oriented approach, The user also needs to nition of the same method, where data and operations know the terminology which and ensuring that the correct upon that data are all part of fits the use of these tech- one is applied to a particular the same 'object." This is niques. There are several object. often refered to as encapsu- variants on the object-ori- Forth has the facilities to create a very simple but effective object-oriented pro- To hide complex methods, we gramming environment. The can set up a new vocabulary. following discussion and development will not produce the fastest objea-oriented theme, but the follow- ented implementation in ing is adequate for our needs. Forth, but will introduce the I A c& definition s~eci- direct application of defin- Clive Maynard is a senior lecturer in fieS the data struciures ing and compiling words to the Department of Computer Engineeded, any initialization, establish an appropriate syntax. The first requirement, then, is to propose an appropriate syntax to represent the object class. (See Figure One.) By analogy with cooking, one should consider that this syntax provides the recipe to create new objeas but that one must use the upon an object. recipe to make an object Inhetftance allows the (i.e., create an instance) beuser to minimize the addi- fore one can use it, neering at Curtin University of ~ ~ ~ h - nology, Western Australia, where this and the operations which article's contents are used as part of may be performed upon the students' Forth instruction. He also -, ~ runs Wawnic AsscciatBS. asvstems- I 1 design consultancy. He teaches real- I An instance is a ~articutime systems using Forth on PCs and lar object of a spec& class, on the Motorola 68HCl I, and is a co author of The Art of Lisp Progamming and has built the data struc- (Springer-Verlag. 1990). Clive has tures defined in the class ' developed a number of embedded &fition. systems for industrial application. . 1 Current interests include the develop A nretbOdis an ment of practical, analytical tods fbr I which can be performed predicting real-time scheduler performance in e m ~ systems; ~ d as well as real-time systems and Al. Forth Dimensions 33 This syntax must provide the compiler with all the information needed to construct the object. The method names must be available for use by any class definition, and so will have to be defined before use. It will also be necessary to provide syntadic delimiters between the method name and the method, and also to separate this from the following method name. These methods may result from short in-line definitions or may need to access predefined and hidden method definitions. Appropriate delimiter pairs which have been selected are: : : and ; ; for in-line definitions, M: andM; for predefined methods. Note for undentanding thefillowing code 1. The operations are made independent of 16-bit or 32-bit Forth implementations by using the constant WSIZE which returns the number of bytes assigned to storage of an integer variable. This technique reduces the efficiency of definitions but ensures portability, which is a reasonable compromise. 2. Hidden methods must be passed the address of the beginning of data within the object itself, and this is shown in the stack comments as "addr" on top of the stack. 3. The stack comments for the methods indicate the parameters needed when applying the method andl or the results produced by the method. 4. The class must inherit from another class or NULL Putting this together for an example class which consists of a point defined by its x,y coordinates and a va- riety of useful (and not-so- useful) methods, [refer to Figure Two]. To create an instance of this class called PT we simply execute: 10 15 POINT PT January 1992 February
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