C/C++ Users Group Library 1996 July/C-C++ Users Group Library July 1996.iso/vol_100/122_01/tutorial

Text File | 1984-03-05 | 12.4 KB | 418 lines

TUTORIAL (headings are approximate):
copyright (C) 1983 by E. E. Bergmann
General, the prompt and stack.
Using memory, constants and variables.
Simple i/o, defining new words.
Iteration and conditional.
Numerical i/o.
Redefining and FORGET.
The editor.
::
:::
*********************************************************
*                            *
* PISTOL-Portably Implemented Stack Oriented Language    *
*            Version 2.0            *
* (C) 1983 by    Ernest E. Bergmann            *
*        Physics, Building #16            *
*        Lehigh Univerisity            *
*        Bethlehem, Pa. 18015            *
*                            *
* Permission is hereby granted for all reproduction and *
* distribution of this material provided this notice is *
* included.                        *
*                            *
*********************************************************
::
:::
[A carriage return will continue the scroll;
a "Q" and then a carriage return will abort]
    Before discussing some examples and features, it is
probably best to understand the "prompt".  The prompt is what a
program types at the beginning of the line when it is awaiting
input from the user.  The prompt supplied by PISTOL can be
several characters long and it is to inform or remind the user
about the "state" of the program.  If a number appears at the
beginning of the prompt, it signifies that the parameter stack
is not empty.  Next, a letter is displayed (such as "X") that
indicates the current number base used for i/o ("X", the Roman
numeral for "10", signifies decimal). After this letter there
may be other characters or symbols that are used to indicate if
you are in the middle of some syntactical construction.
Finally, the ">" completes the prompt expression.  In the
examples supplied above and below we have attempted to suggest
the typical prompts that one might expect to see.

    After educating PISTOL by loading PBASE2 or by
restoring CORE2, the system is "smarter" you can try the
following examples:

X> 1 2 23 STACK

The system takes each number as encountered and places them on
the stack.  The word, STACK , prints the current contents of
the stack without changing the stack in any way.

    Now try typing:
3X> + STACK

This should result in the addition of the top two members of
the stack (2 and 23) and placing the result (answer 25) back on
the stack. The word STACK then displays the current contents of
the stack.

    Now try typing:
2X> * stack

The top two items of the stack will be multiplied together and
the result left on the stack.  The word, "stack" is interpreted
as STACK, and we see that the only thing on the stack is the
answer 25.

    To disable the automatic interpretation of lowercase
as uppercase, type:

X> RAISE OFF (or "raise off")

and successive lines will be interpreted without conversion of
lowercase to uppercase.  To revert to conversion, type:

X> RAISE ON

    There are a number of different options that can be
invoked such as ECHO ON and ECHO OFF which control the listing
of files read in by the '<filename> LOAD operation.  One can
use CONSOLE ON and CONSOLE OFF to determine what reaches the
terminal.  An error condion will automatically restore output
to the terminal.  LIST ON and LIST OFF will determine what
output will also be routed to the output list file(of course a
listfile has to be declared first).  This is useful to have a
more permanent record of what happened during a session.  There
is also a SHOWCODE and a NOSHOWCODE which will show the results
of compiling each input line (that is for those who want or
like to know what is going on behind the scenes).

    On the DEC-20 (and also in CP/M) one can exit from
PISTOL by using a control-C, but a more "refined" way is to
type the word, BYE, which will return you to the operating
system.
::
:::
    PISTOL can examine the contents of its own (virtual)
memory. The W@ ("fetch") operator is similar in spirit to
the PEEK function in BASIC.  To place the contents of a
memory location on the stack one simply places the address
on stack and then types W@ .  For example, if we want to
place on the stack the current value of RADIX, the base used
in all I/O number conversion, we need to find the value
stored in the RAM location at USER (i.e. what is RAM[USER]?)
we can type:

X> USER W@

which will place the desired information on stack.  The inverse
operation, which is more dangerous(because a mistake can crash
the program), is to store a new value in the RADIX, hence
change the number base.  The operator, W! ("word store"),
performs this function(for this example we suppose that the
RADIX address is 12345 decimal, usually, it is not):

X> 16 12345 W!

would convert PISTOL to converse in hexadecimal.

    Of course it is awkward to remember constants, such
as the address of RADIX so we can define such constants by:

 > DECIMAL
X> 12345 'RADIX CONSTANT

After such a CONSTANT definition we can redo the examples that
use W@ and W! by (RADIX is in fact already defined in PBASE2):

X> RADIX W@
and
X> 16 RADIX W!

    As an additional convenience the user can define space
for variables with the word, VARIABLE.  For example, If you
wish to define a variable with the initial value 0 and named
ANSWER, you should type:

X> 0 'ANSWER VARIABLE

Later, if you wish to perform the PASCAL statement:

ANSWER := ANSWER + 1;

you would write the PISTOL code:

X> ANSWER W@ 1 + ANSWER W!

Notice that you MUST choose an initial value in VARIABLE and,
of course, CONSTANT definitions.

    Just like the convenience feature of PASCAL,
for which one can use SUCC(ANSWER) to increment ANSWER
by one, there exists an analogous operator (or you can
define your own!):

X> ANSWER 1+W!

::
:::
    The language has many resources beyond those
presented so far.  Perhaps the most significant is the
ability to define new words or definitions to make the
language increasingly smarter. Try examining the contents of
the file, PBASE2 , to see how the definitions may be formed.
As a complex example, the word, = is defined whose function
is to take the top item off of the stack and print its
value.  For example, typing:

X> 8 8 + =

will cause the system to respond with the answer: 16.  Note
that this calculation causes no net change to the stack (it
is the presence of a number before "X>" that indicates the
number of items in the stack at that moment); its contents
before the first "8" was typed is the same as its contents
after the system responds with 16.  Thus the definition of
the word = has increased the convenience of the system for
arithmetic calculations.

    The system can and does handle strings.  Suppose you
would like the system to output "HELLO".  Try typing:

X> 'HELLO MSG

The system will take the 'HELLO as a string to be placed upon
the stack.  The MSG ,"message" takes the top item off of the
stack, assumes it to be a string, and prints it.  The stack
contains the same stuff after MSG is executed as before the
'HELLO was typed.

    Useful I/O words are CR which will output a carriage
return and line feed sequence.  The word SPACE will output a
space. And the word SPACES will pop the top of stack to obtain
the number of spaces to be output.  For example:

X> 'HELLO MSG SPACE 'HELLO MSG CR 5 SPACES 'BYE MSG

should produce:

HELLO HELLO
     BYE


    Standard output would be most tedious if we could not
create definitions to speed up programming.  Here is a humorous
example:

X> 'HELLO : 'HELLO, MSG SPACE 'YOURSELF! CR ;

The use of : and ; provide the means to define a new word
"HELLO" Later, you can type:

X> HELLO

and the system will respond:

HELLO, YOURSELF!

    Thus we see that the pair of symbols, ":" and ";"
delineate a structure used to make definitions.  The material
in between the two symbols becomes the definition of the word
whose name is the string that was lastly placed on the stack
before the ":".

    One can create strings with embedded blanks and tabs up
to 127 characters long by using double quotes to delineate both
ends of the string.  For example, the word, HELLO, defined
above, could have been defined:

'HELLO : "HELLO, YOURSELF!" MSG CR ;

Even with RAISE ON, lowercase characters within double
quotation marks will not be converted.

::
:::
    There are other types of structures.  The pair of
words, DO and LOOP permit an iterative structure.  They use the
top two quantities on the stack as limits of iteration.  So:

X> nn n DO ... LOOP

is equivalent to the PASCAL structure:

    FOR  I := n TO (nn-1) DO
        BEGIN ... END;

To place the current value of the iteration variable on the
stack one uses the word, "I" .  Here is an example that you can
try:

X> 'COUNTING : CR 1 + 1 DO I = SPACE LOOP ;

X> 10 COUNTING
and see PISTOL counting to 10.

    An alternative terminating word to this structure is
+LOOP.  If one uses in PISTOL:

    nn n DO ... m +LOOP
one simulates the BASIC structure:

100 FOR I=n TO (nn-1) STEP m
.
.
.
200 NEXT I

    PISTOL supports a conditional structure of the form:

    IF ... ELSE .... THEN

When the IF is encountered the top of the stack is used as a
boolean variable; it is considered false if equal to zero and
true otherwise (as in LISP).  If true, the actions "..." that
are bracketed between IF and ELSE are carried out; then program
flow skips to what follows THEN.  Whereas, if the top of the
stack was false, the actions "...." between ELSE and THEN are
carried out instead. The "ELSE ...." portion is optional, in
analogy to PASCAL.

    To illustrate, here is an example:

X> 'STATE? :
X:> 'TURNED- MSG W@ IF 'ON ELSE 'OFF THEN
1X:> MSG ;
X>

Trying this new word:

X> CONSOLE STATE?
we get the response:
TURNED-ON

whereas for:

X> LIST STATE?
TURNED-OFF

    PISTOL supports a number of other structures which are
analogous to the PASCAL structures:

        WHILE .. DO ... ;

            and

        REPEAT ... UNTIL NOT .. ;

They are, respectively:

        BEGIN .. IF ... REPEAT

            and

        BEGIN ... .. END

    As in most languages, structures may be nested.  In the
interest of user convenience, the prompt will indicate whether
execution is being deferred, pending completion of unfinished
structures.
::
:::
    PISTOL can communicate in a variety of different number
bases, as was alluded to in the section on W@ and W! . Changing
bases has been formalized by a set of defined words provided in
PBASE2.  These words are: BINARY(B), OCTAL(Q), DECIMAL(X), and
HEX(H); the parenthesized letter is the corresponding symbol
that appears in the prompt.  Thus, in the examples described
above, the number base was decimal.  Here are a few examples of
the use of other number bases:

X> HEX
H> 8 8 + =
10
H> 11 BINARY =
10001
B> 2
2 ?
***PISTOL 2.0***

B>
    Whenever the system responds with ***PISTOL 2.0*** it
has performed an ABORT which resets stacks and prints this
identifying message.
::
:::
    You may "redefine" words, that is to say, the same name
may be used again and again.  Earlier definitions that use the
word will continue to utilize the old meaning; future defini-
tions that reference the redefined word will access the new
meaning.  A warning will be issued when you are redefining.

    It is important to keep in mind that when new words are
defined, their names are added to the "string stack" and the
compiled code is added to the "code stack".  If a recent
definition is not satisfactory, or no longer serves a need, you
may wish to "FORGET" it, so that the "string stack" and the
"code stack" are popped of this useless material.  To discard
this definition AND ALL SUBSEQUENT DEFINITIONS, one should
type:

X> '<name> FORGET

    To obtain the code locations and names of the last ten
definitions, type:

X> TOP10

To obtain information successively on the ten previous
definitions, type:

1X> NEXT10

::
:::
    A crude, line-oriented editor has been implemented
inside PISTOL.  Try it out by typing the following sequence
of commands:

X> LI

X> 3 LI

X> 3 5 LI

X> 4 2 LI

X> 3 INPUT
3: MARY HAD
4: A LITTLE
5: LAMB.
6: <cr>


X> LI

X> 3 DELETE

X> LI

    It is possible to test definitions or other commands
while they are still in the edit buffer.  Make sure that the
edit buffer has the word: ;F at the end ot the section that
you wish to test (it acts as a logical end-of-file).
Determine the line number of the portion where you wish to
start the test, say it is line 14.  All you need to type is:

X> 14 LOAD

This degree of interaction should appeal to those who like
BASIC!?
:
less material.  To discard
this de