forth: Replace "SKIP?" with "(?br)"

There is an alternate git history where I continued the Forth-ification of
words, including "SKIP?", but that was a bad idea: because that word was
written by flow control immediates, I stepped into quicksands where stability
became necessary in z80c.fs and I couldn't gracefully get out of it.

I'm stepping back and take this opportunity to replace the shoddy SKIP? algo
with a more straightforward (?br) implementation.

(br) and (?br) will always stay in boot code where it's easier manage a stable
ABI.

forth/core.fs

@@ -2,17 +2,17 @@
 : -^ SWAP - ;
 : [ INTERPRET 1 FLAGS ! ; IMMEDIATE
 : ] R> DROP ;
-: LIT [ JTBL 26 + LITN ] , ;
+: LIT JTBL 26 + , ;
 : LITS LIT SCPY ;
 : LIT< WORD LITS ; IMMEDIATE
 : _err LIT< word-not-found (print) ABORT ;
-: ' WORD (find) SKIP? _err ;
-: ['] WORD (find) SKIP? _err LITN ; IMMEDIATE
+: ' WORD (find) NOT (?br) [ 4 , ] _err ;
+: ['] ' LITN ; IMMEDIATE
 : COMPILE ' LITN ['] , , ; IMMEDIATE
 : [COMPILE] ' , ; IMMEDIATE
 : BEGIN H@ ; IMMEDIATE
 : AGAIN COMPILE (br) H@ - , ; IMMEDIATE
-: UNTIL COMPILE SKIP? COMPILE (br) H@ - , ; IMMEDIATE
+: UNTIL COMPILE (?br) H@ - , ; IMMEDIATE
 : ( BEGIN LIT< ) WORD SCMP NOT UNTIL ; IMMEDIATE
 ( Hello, hello, krkrkrkr... do you hear me?
   Ah, voice at last! Some lines above need comments
@@ -29,8 +29,7 @@
 : ALLOT HERE +! ;
 
 : IF                ( -- a | a: br cell addr )
-    COMPILE SKIP?   ( if true, don't branch )
-    COMPILE (br)
+    COMPILE (?br)
     H@              ( push a )
     2 ALLOT         ( br cell allot )
 ; IMMEDIATE
@@ -73,7 +72,7 @@
   the RS )
 : LOOP
     COMPILE R> 1 LITN COMPILE + COMPILE DUP COMPILE >R
-    COMPILE I' COMPILE = COMPILE SKIP? COMPILE (br)
+    COMPILE I' COMPILE = COMPILE (?br)
     H@ - ,
     COMPILE R> COMPILE DROP COMPILE R> COMPILE DROP
 ; IMMEDIATE

forth/dictionary.txt

@@ -73,10 +73,9 @@ Note about flow words: flow words can only be used in definitions. In the
 INTERPRET loop, they don't have the desired effect because each word from the
 input stream is executed immediately. In this context, branching doesn't work.
 
-(fbr)       --              Branches forward by the number specified in its
-                            atom's cell.
-(bbr)       --              Branches backward by the number specified in its
-                            atom's cell.
+(br)        --              Branches by the number specified in the 2 following
+                            bytes. Can be negative.
+(?br)       f --            Branch if f is false.
 [           --              Begin interetative mode. In a definition, words
                             between here and "]" will be executed instead of
                             compiled.
@@ -92,9 +91,6 @@ INTERPRET   --              Get a line from stdin, compile it in tmp memory,
                             then execute the compiled contents.
 QUIT        R:drop --       Return to interpreter prompt immediately
 RECURSE     R:I -- R:I-2    Run the current word again.
-SKIP?       f --            If f is true, skip the execution of the next atom.
-                            Use this right before ";" and you're gonna have a
-                            bad time.
 THEN        I:a --          *I* Set branching cell at a.
 UNTIL       f --            *I* Jump backwards to BEGIN if f is *false*.
 

forth/forth.asm

@@ -670,61 +670,28 @@ BR:
 	ld	(IP), hl
 	jp	next
 
-; Skip the compword where HL is currently pointing. If it's a regular word,
-; it's easy: we inc by 2. If it's a NUMBER, we inc by 4. If it's a LIT, we skip
-; to after null-termination.
-	.db	"SKIP?"
+.fill 72
+
+	.db	"(?br)"
 	.dw	$-BR
 	.db	5
-CSKIP:
+CBR:
 	.dw	nativeWord
 	pop	hl
 	call	chkPS
 	ld	a, h
 	or	l
-	jp	z, next		; False, do nothing.
-	ld	hl, (IP)
-	ld	de, NUMBER
-	call	.HLPointsDE
-	jr	z, .isNum
-	ld	de, BR
-	call	.HLPointsDE
-	jr	z, .isNum
-	ld	de, LIT
-	call	.HLPointsDE
-	jr	nz, .isWord
-	; We have a literal
-	inc	hl \ inc hl
-	call	strskip
-	inc	hl		; byte after word termination
-	jr	.end
-.isNum:
-	; skip by 4
-	inc	hl
-	inc	hl
-	; continue to isWord
-.isWord:
-	; skip by 2
-	inc	hl \ inc hl
-.end:
-	ld	(IP), hl
+	jp	z, BR+2		; False, branch
+	; True, skip next 2 bytes and don't branch
+	ld	hl, IP
+	inc	(hl)
+	inc	(hl)
 	jp	next
 
-; Sets Z if (HL) == E and (HL+1) == D
-.HLPointsDE:
-	ld	a, (hl)
-	cp	e
-	ret	nz		; no
-	inc	hl
-	ld	a, (hl)
-	dec	hl
-	cp	d		; Z has our answer
-	ret
-
-.fill 45
+.fill 18
 
 	.db	","
-	.dw	$-CSKIP
+	.dw	$-CBR
 	.db	1
 WR:
 	.dw	nativeWord
@@ -1046,3 +1013,5 @@ CMP:
 	.db	"_bend"
 	.dw	$-CMP
 	.db	5
+; Offset: 06ee
+.out $

forth/icore.fs

@@ -54,7 +54,7 @@
 ( This is only the "early parser" in earlier stages. No need
   for an abort message )
 : (parse)
-    (parsed) SKIP? _c ABORT
+    (parsed) NOT IF _c ABORT THEN
 ;
 
 ( a -- )

forth/parse.fs

@@ -75,10 +75,10 @@
 ;
 
 : (parse)           ( a -- n )
-    (parsec) NOT SKIP? EXIT
-    (parseh) NOT SKIP? EXIT
-    (parseb) NOT SKIP? EXIT
-    (parsed) NOT SKIP? EXIT
+    (parsec) IF EXIT THEN
+    (parseh) IF EXIT THEN
+    (parseb) IF EXIT THEN
+    (parsed) IF EXIT THEN
     ( nothing works )
     ABORT" unknown word! "
 ;

tests/forth/harness.fs

@@ -4,6 +4,6 @@
 
 : fail SPC ." failed" LF 1 1 PC! BYE ;
 
-: # SKIP? fail SPC ." pass" LF ;
+: # IF SPC ." pass" LF ELSE fail THEN ;
 
 : #eq 2DUP SWAP . SPC '=' EMIT SPC . '?' EMIT = # ;