Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!sun-barr!decwrl!ucbvax!ucsd!sdcc6!ir230
From: ir230@sdcc6.ucsd.EDU (john wavrik)
Newsgroups: comp.lang.forth
Subject: Learning Forth
Message-ID: <4600@sdcc6.ucsd.EDU>
Date: 3 Aug 89 05:48:28 GMT
Organization: University of California, San Diego
Lines: 75


 
   SCFVM.BITNET!ZMLEB (Lee Brotzman) writes:
 
> I don't think that Forth is easy to learn.  But, I think that time spent
> learning Forth is time well spent... I have used the techniques I gained 
> from an intimate knowledge of Forth in other, "academically accepted", 
> languages.
 
> This may be a harsh statement, but I still think that the only way that
> Forth can be truly appreciated is to write a Forth interpreter on you own.
 
> At its heart, Forth is extremely simple.  In its soul, Forth is something 
> that approaches enlightenment.  With a small amount of knowledge about how 
> computers work, and some assembly language programming, Forth is yours. 
> Just remember that Forth is not hard, it is only different.
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                              LEARNING FORTH
 
 
    FORTRAN, BASIC, Pascal, C, ADA, etc. belong to a family of similar 
languages. Learning one after having learned one of the others is a 
relatively minor matter of coping with a new syntax. Compound statements may 
be delimited by BEGIN .. END in one and curly brackets in another; statements 
may be terminated or separated; the terminator/separator may be a semi-colon 
in one language and a comma in another; keywords may vary: "case" may be 
called "switch"; "print" may be called "write"; etc.  One of my students, a 
faculty member steeped in the lore of these languages, expressed his 
frustration at being unable to translate his pre-conceived ideas to Forth by 
saying "What kind of crazy language has 800 keywords?". 
 
    Any learning depends on "organizing principles": techniques or devices 
used to make sense of a body of information. Forth can appear hard to those 
who have learned other languages because they have the wrong organizing 
principles. They have been trained to avoid involving themselves with how the 
how a language is implemented. Compilers for conventional languages are very 
complex, and conventional wisdom is that the user should not have to know 
anything about them. ("How a compiler works is no more of interest to a 
programmer than the details of an internal combustion engine is to the driver 
of a car." --anon) 
 
    Forth requires a different set of organizing principles. It is learned 
most easily by understanding how it works. Indeed, Forth's main claim to fame 
is that it trades built-in features for users' knowledge. Forth is an assembly 
language for a virtual machine which can be implemented on any real computer.
The virtual machine has two stacks, two stack pointers, and an instruction 
pointer. Code is compiled as a sequence of addresses of component words. Etc.
Words can be understood in terms of their actions (typically in terms of what 
they do to the stack). A mental image of the underlying virtual machine 
provides a powerful organizing principle. It is not necessary to write a Forth 
system to learn Forth -- but one should learn on a system for which the source 
code is accessible on-line. 
 
   Forth is more a toolkit for building languages than a language itself. By 
providing a low level base, Forth allows the user to make extensions and 
modifications which, for conventional languages, would require the services of 
a professional compiler writer. By providing a mechanism for extensibility, 
Forth permits the user to build a tailor-made high level language. Unlike most 
assembly languages, the commands of Forth can be used interactively. This has 
been found to speed development (and makes it easy to produce an interactive 
application language). Since Forth includes the elements of an operating 
system, highly portable complete systems can be created which include I/O from 
keyboard, console, and mass storage. Forth, as conceived of by Charles Moore, 
has both power and portability. 
 
   A user's access to the implementation is only half of the virtue of Forth.
The other half is that the implementation is simple enough for the access to 
be useful. The simplicity of Forth's implementation is the key both to 
understanding the language and to its power. 
 
                                                --J. Wavrik