forth: BEGIN/UNTIL/WHILE/REPEAT/AGAIN (+9)

plans/forth-on-sx.md

@@ -70,7 +70,7 @@ Representation:
 
 ### Phase 3 — control flow + first Hayes tests green
 - [x] `IF`, `ELSE`, `THEN` — compile to SX `if`
-- [ ] `BEGIN`, `UNTIL`, `WHILE`, `REPEAT`, `AGAIN` — compile to loops
+- [x] `BEGIN`, `UNTIL`, `WHILE`, `REPEAT`, `AGAIN` — compile to loops
 - [ ] `DO`, `LOOP`, `+LOOP`, `I`, `J`, `LEAVE` — counted loops (needs a return stack)
 - [ ] Return stack: `>R`, `R>`, `R@`, `2>R`, `2R>`, `2R@`
 - [ ] Vendor John Hayes' test suite to `lib/forth/ans-tests/`
@@ -99,6 +99,20 @@ Representation:
 
 _Newest first._
 
+- **Phase 3 — `BEGIN`/`UNTIL`/`WHILE`/`REPEAT`/`AGAIN` (+9).** Indefinite-loop
+  constructs built on the same PC-driven body runner introduced for `IF`.
+  BEGIN records the current body length on `state.cstack` (a plain numeric
+  back-target). UNTIL/AGAIN pop that back-target and emit a `bif`/`branch`
+  op whose target cell is set to the recorded PC. WHILE emits a forward
+  `bif` with a fresh target cell and pushes it on the cstack *above* the
+  BEGIN marker; REPEAT pops both (while-target first, then back-pc), emits
+  an unconditional branch back to BEGIN, then patches the while-target to
+  the current body length — so WHILE's false flag jumps past the REPEAT.
+  Mixed compile-time layout (numeric back-targets + dict forward targets
+  on the same cstack) is OK because the immediate words pop them in the
+  order they expect. AGAIN works structurally but lacks a test without a
+  usable mid-loop exit; revisit once `EXIT` lands. 161/161 green.
+
 - **Phase 3 start — `IF`/`ELSE`/`THEN` (+18).** `lib/forth/compiler.sx`
   + `tests/test-phase3.sx`. Colon-def body switched from `for-each` to
   a PC-driven runner so branch ops can jump: ops now include dict tags