Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!willett!ForthNet
From: ForthNet@willett.UUCP (ForthNet articles from GEnie)
Newsgroups: comp.lang.forth
Subject: Forth Implementation
Message-ID: <616.UUL1.3#5129@willett.UUCP>
Date: 6 Mar 90 01:04:20 GMT
Organization: Latest link in the ForthNet chain.  (Pgh, PA)
Lines: 105

Category 3,  Topic 24
Message 52        Mon Mar 05, 1990
R.BERKEY [Robert]            at 05:48 PST
 
 
 Re:  ?FOR , a version of FOR that executes zero times.

Frank Sergeant writes:

 > Rob Chapman (via Usenet) suggests that  u  FOR  ...  NEXT   should
 > execute u times.
 >
 > Rob, thanks for your suggestion...
    ...
 > I have about decided that I agree with Rob.
    ...
 > cmFORTH has the word -ZERO that accomplishes this change to u
 > instead  of u+1.  It is used like this
 >       : STARS  ( u -)   FOR  -ZERO  STAR   THEN  NEXT  ;
    ...
 > Rather than use -ZERO, I think I will build it in as the default.
    ...
 > Anyone have any thoughts on why we shouldn't make this change?


I like Rob Chapman's structure, also.

However, I think that using the name FOR impedes readability, portability of
code, and potential for standardization.  I don't see any basic technical
problem with FOR (the name NEXT and using I equivalent to R@ with FOR are
different issues).  We went through this after Forth-83--there were those who
chose to code or recode DO as ?DO .

As to a zero executing version of FOR , here's an implementation I've added
after seeing Rob Chapman's posting.  The loop code is in the target system. 
Here are some of the target compiler implementations.


 thread, BRANCH,         \ target compile BRANCH
 thread, ?FOR,           \ target compile (?FOR)
 thread, ?LOOP,          \ the (loop) that get's compiled can vary
 0 value -1LOOP          \ returns the compilation token of (-1LOOP)

 T: ?FOR   ( -- >mark )    \ T: adds the word to a compiler vocabulary
    ?FOR,                  \ compile (?FOR)
    BRANCH, >MARK          \ (inefficient: BRANCH, should be eliminated)
    IP-R>   SAVE> ?LOOP,   \ save what ?LOOP compiles on return stack
    IP->R                  \    (should be left on parameter stack?)
    -1LOOP =: ?LOOP,       \ set ?LOOP,
    ;

 T: ?LOOP   ( >mark -- )
    DUP >RESOLVE             \ resolve the branch after ?FOR
    ?LOOP,   <RESOLVE        \ compile loop and resolve backward branch
    IP-R>   RESTORE> ?LOOP,   IP->R   \ put back previous copy in ?LOOP,
    ;

This design is so that the syntax ?LOOP can be used to terminate a variety of
loops.  Also, ?FOR puts branch addresses on the stack as would BEGIN so that
using WHILE inside a ?FOR loop is like using it inside a BEGIN  ...  UNTIL
loop.  I'm not at all sure about this--the ANS ?DO requires two addresses. 
Any comments?

The following code is for the target system.  On the 80x8x, using the overflow
flag for (LOOP) and (NEXT) is a bit faster than using the carry flag (here
it's 15 bytes memory access vs. 16).  This is the same basic loop body I've
always implemented for (LOOP) , except that this version has no provision for
LEAVE .  Maybe I'll add that later for compatibility, but for now I plan on
using WHILE and UNLOOP for new code.

 CODE (-1LOOP)   ( -- )
    0 [+BP] WORD DEC
    OV<> IF
       ES: LODSB   CBW   AX, SI ADD   NEXT JMP
    THEN
    SI INC   4 #, BP ADD
    NEXT JMP   END-CODE

' (-1LOOP) =: -1LOOP

 CODE (?FOR)   ( u -- )
    AX POP   $80 #, AH XOR
    SWITCH,   AX PUSH   AX PUSH   SWITCH,
    NEXT JMP   END-CODE

 ' (?FOR) =: ?FOR,

I'm not using I with these loops right now, but it could be coded as follows. 
This coding for I makes

                    ?FOR  ...  I  ...
 the same as       0 ?DO  ...  I  ...

 CODE I   ( -- w )
    \  get the current index of the innermost loop
    2 [+BP], AX MOV   0 [+BP], AX SUB
    AX PUSH   NEXT JMP   END-CODE


Robert


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