appendixa.rst

Appendix A: Overview of Forth (For Newcomers)

The Dictionary

Forth is expressed in words (and numbers) and is separated by spaces:

HAND OPEN  ARM LOWER  HAND CLOSE  ARM RAISE

Such commands may be typed directly from the keyboard, or edited onto mass storage then “LOAD”ed.

All words, whether included with the system or user-defined, exist in the "dictionary," a linked list. A “defining word” is used to add new names to the dictionary. One defining word is : (pronounced “colon”), which is used to define a new word in terms of previously defined words. Here is how one might define a new word called LIFT:

: LIFT   HAND OPEN  ARM LOWER  HAND CLOSE  ARM RAISE ;

The ; terminates the definition. The new word LIFT may now be used instead of the long sequence of words that comprise its definition.

Forth words can be nested like this indefinitely. Writing a Forth application consists of building increasingly powerful definitions, such as this one, in terms of previously defined ones.

Another defining word is CODE, which is used in place of colon to define a command in terms of machine instructions for the native processor. Words defined with CODE are indistinguishable to the user from words defined with colon. CODE definitions are needed only for the most time-critical portions of an applications, if at all.

Data Structures

Still another defining word is CONSTANT, which is used like this:

17 CONSTANT SEVENTEEN

The new word SEVENTEEN can now be used in place of the actual number 17.

The defining word VARIABLE creates a location for temporary data. VARIABLE is used like this:

VARIABLE BANANAS

This reserves a location which is identified by the name BANANAS.

Fetching the contents of this location is the job of the word @ (pronounced “fetch”). For instance,

BANANAS @

fetches the contents of the variable BANANAS. Its counterpart is ! (pronounced “store”), which stores a value into the location, as in:

100 BANANAS !

Forth also provides a word to increment the current value by the given value; for instance, the phrase

2 BANANAS +!

increments the count by two, making it 102.

Forth provides many other data structure operators, but more importantly, it provides the tools necessary for the programmer to create any type of data structure needed for the application.

The Stack

In Forth, variables and arrays are used for saving values that may be required by many other routines and/or at unpredictable times. They are not used for the local passing of data between the definitions. For this, Forth employs a much simpler mechanism: the data stack.

When you type a number, it goes on the stack. When you invoke a word which has numeric input, it will take it from the stack. Thus the phrase

17 SPACES

will display seventeen blanks on the current output device. “17” pushes the binary value 17 onto the stack; the word SPACES consumes it.

A constant also pushes its value onto the stack; thus the phrase:

SEVENTEEN SPACES

has the same effect.

The stack operates on a “last-in, first-out” (LIFO) basis. This means that data can be passed between words in an orderly, modular way, consistent with the nesting of colon definitions.

For instance, a definition called GRID might invoke the phrase 17 SPACES. This temporary activity on the stack will be transparent to any other definition that invokes GRID because the value placed on the stack is removed before the definition of GRID ends. The calling definition might have placed some numbers of its own on the stack prior to calling GRID. These will remain on the stack, unharmed, until GRID has been executed and the calling definition continues.

Control Structures

Forth provides all the control structures needed for structured, GOTO-less programming.

The syntax of the IF THEN construct is as follows:

... ( フラグ ) IF  KNOCK  THEN  OPEN ...

The “flag” is a value on the stack, consumed by IF. A non-zero value indicates true, zero indicates false. A true flag causes the code after IF (in this case, the word KNOCK ) to be executed. The word THEN marks the end of the conditional phrase; execution resumes with the word OPEN. A false flag causes the code between IF and THEN to not be executed. In either case, OPEN will be performed.

The word ELSE allows an alternate phrase to be executed in the false case. In the phrase:

( flag ) IF KNOCK  ELSE  RING  THEN  OPEN ...

the word KNOCK will be performed if the flag is true, otherwise the word RING will be performed. Either way, execution will continue starting with OPEN.

Forth also provides for indexed loops in the form

( limit) ( index) DO ... LOOP

and indefinite loops in the forms:

... BEGIN  ...  ( flag) UNTIL

and

... BEGIN  ...  ( flag) WHILE ... REPEAT ;

For the Whole Story

For a complete introduction to the Forth command set, read Starting Forth, published by Prentice-Hall.