Re: acceptance of forth

Tinkerer Atlarge wrote: <1j9zm4j.qzwbt6zit6moN%tinkerer@xxxxxxxxxxxxxxx>
The secret might be to forget about constructs and consider the action
of WHILE and REPEAT as entities in their own right which only need
BEGIN as an end-marker for their own purposes. But that is just me
thinking aloud. I don't claim to know yet.

You could do that. It's probably better to start by just using the
basic three constructions, but I do somewhat think of them as individual
pieces.

My understanding goes something like this:

Forth has an unconditional branch and a branch-if-false. It also
differentiates between forward and backward branches. So we have six
basic words to compile these branches, and three convenience words which
combine the basic operations. These words use a stack at compile-time:


Forward Branches
----------------

IF/AHEAD ( -- branch )

Compile a forward (conditional?) branch, leaving a reference so we
can point it to the right place later.

THEN ( branch -- )

Resolve `branch` to jump here.

ELSE ( branch1 -- branch2 ) AHEAD SWAP THEN ; IMMEDIATE

-------------------->
Usually `IF <true-action> ELSE <false-action> THEN`.
---------------------->

It ends the <true-action> section with an unconditional `branch2`
which skips over the <false-action>, and then resolves the
branch-if-false from the IF (so that it jumps here to begin the
<false-action> section).


Backward Branches
-----------------

BEGIN ( -- addr )

Push a marker so that we can branch (loop) back to it.

UNTIL/AGAIN ( addr -- )

Loop (conditially?) back to the most recent BEGIN.

WHILE ( addr -- branch addr ) IF SWAP ; IMMEDIATE

--------------------->
Usually `BEGIN <test> WHILE <action> REPEAT`
<-----------------------

Branch forward out of a loop (putting the branch *under* the BEGIN
addr).

: REPEAT ( branch addr -- ) AGAIN THEN ; IMMEDIATE

loop back to the most recent BEGIN, then resolve the WHILE branch to
jump just outside the loop.


Then you can do crazy things like:

BEGIN <test1> WHILE <test2> WHILE
<action>
REPEAT <failed-test2> ELSE <failed-test1> THEN

:-)

--Josh
.

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