snapshot

It took me _way_ too long to realize that I'm not checking for errors within
the loop, and that will cause it to manifest as an infinite loop as inner
evaluations fail to run.

Debugging notes, for posterity:
  printing one row of a chessboard pattern over fake screen (chessboard screen 4 0 0 15) gets stuck in an infinite loop halfway through
    debug pattern during infinite loop: VWEX. It's still in the loop but it's not executing the body
    raw (fill_rect screen 16 0 20 4 15) works fine
    same number of calls to fill_rect work fine
    replacing calls to fill_rect with pixel inside chessboard2 works fine
    at the point of the infinite loop it's repeatedly going through the hline loop
      -- BUT it never executes the check of the loop (< lo hi) with lo=20, hi=20. Something is returning 1, but it's not inside <
    stream optimization is not implicated

  simple test case with a single loop
    (
      (globals . (
        (foo . (fn () (screen i n)
                 (while (< i n)
                   (pixel screen 4 4 i)
                   (pixel screen 5 4 i)
                   (pixel screen 6 4 i)
                   (pixel screen 7 4 i)
                   (set i (+ i 1)))))
      ))
      (sandbox . (foo screen 0 100))
    )

  simpler (if you reset cursor position before every print):
    (
      (globals . (
        (foo . (fn () (screen i n)
                 (while (< i n)
                   (print screen i)
                   (set i (+ i 1)))))
      ))
      (sandbox . (foo screen 0 210))
    )

  I now believe it has nothing to do with the check. The check always works.
  Sometimes no body is evaluated. And so the set has no effect.

shell/evaluate.mu

@@ -4,8 +4,9 @@
 # 'call-number' is just for showing intermediate progress; this is a _slow_ interpreter
 fn evaluate _in: (addr handle cell), out: (addr handle cell), env-h: (handle cell), globals: (addr global-table), trace: (addr trace), screen-cell: (addr handle cell), keyboard-cell: (addr handle cell), call-number: int {
   # stack overflow?   # disable when enabling Really-debug-print
-  check-stack
-  show-stack-state
+#?   check-stack
+#?   show-stack-state
+  debug-print "^", 7/fg, 0/bg
   # errors? skip
   {
     compare trace, 0
@@ -54,6 +55,7 @@ fn evaluate _in: (addr handle cell), out: (addr handle cell), env-h: (handle cel
     trace trace, "eval", stream
   }
   # }}}
+  debug-print "!", 7/fg, 0/bg
   trace-lower trace
   var in-addr/eax: (addr cell) <- lookup *in
   {
@@ -66,6 +68,7 @@ fn evaluate _in: (addr handle cell), out: (addr handle cell), env-h: (handle cel
     trace-higher trace
     return
   }
+  debug-print "@", 7/fg, 0/bg
   var in-type/ecx: (addr int) <- get in-addr, type
   compare *in-type, 1/number
   {
@@ -211,6 +214,12 @@ fn evaluate _in: (addr handle cell), out: (addr handle cell), env-h: (handle cel
     compare set?, 0/false
     break-if-=
     #
+    {
+      var foo/eax: boolean <- debug-print?
+      compare foo, 0/false
+      break-if-=
+      draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "set|", 7/fg, 0/bg
+    }
     trace-text trace, "eval", "set"
     trace-text trace, "eval", "evaluating second arg"
     var rest/eax: (addr cell) <- lookup *rest-ah
@@ -308,6 +317,8 @@ fn evaluate _in: (addr handle cell), out: (addr handle cell), env-h: (handle cel
     rest-ah <- get rest, right
     var guard-h: (handle cell)
     var guard-ah/esi: (addr handle cell) <- address guard-h
+    # skip first 201 iterations without debug
+    var i/ebx: int <- copy 0
     {
       trace-text trace, "eval", "loop termination check"
       debug-print "V", 4/fg, 0/bg
@@ -319,6 +330,47 @@ fn evaluate _in: (addr handle cell), out: (addr handle cell), env-h: (handle cel
       compare done?, 0/false
       break-if-!=
       evaluate-exprs rest-ah, out, env-h, globals, trace, screen-cell, keyboard-cell, call-number
+      i <- increment
+      compare i, 0xc9/201
+      break-if->
+      loop
+    }
+    turn-on-debug-print
+    i <- copy 0
+    {
+      trace-text trace, "eval", "loop termination check"
+  {
+    compare trace, 0
+    break-if-=
+    var error?/eax: boolean <- has-errors? trace
+    compare error?, 0/false
+    break-if-=
+    return
+  }
+      set-cursor-position 0/screen, 0/x, i
+      i <- increment
+      compare i, 0x20
+      {
+        break-if-<
+        clear-screen 0/screen
+        i <- copy 0
+        set-cursor-position 0/screen, 0/x, i
+      }
+      debug-print "V", 4/fg, 0/bg
+      increment call-number
+      dump-cell-from-cursor-over-full-screen first-arg-ah
+      draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "|", 7/fg, 0/bg
+      evaluate first-arg-ah, guard-ah, env-h, globals, trace, screen-cell, keyboard-cell, call-number
+      debug-print "W", 4/fg, 0/bg
+      dump-cell-from-cursor-over-full-screen guard-ah
+      draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "|", 7/fg, 0/bg
+      var guard-a/eax: (addr cell) <- lookup *guard-ah
+      var done?/eax: boolean <- nil? guard-a
+      compare done?, 0/false
+      break-if-!=
+      dump-cell-from-cursor-over-full-screen rest-ah
+      evaluate-exprs rest-ah, out, env-h, globals, trace, screen-cell, keyboard-cell, call-number
+      draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "]", 7/fg, 0/bg
       loop
     }
     trace-text trace, "eval", "loop terminated"
@@ -453,6 +505,13 @@ fn evaluate-exprs _exprs-ah: (addr handle cell), out: (addr handle cell), env-h:
       debug-print "E", 7/fg, 0/bg
       increment call-number
       evaluate curr-ah, out, env-h, globals, trace, screen-cell, keyboard-cell, call-number
+      {
+        var foo/eax: boolean <- debug-print?
+        compare foo, 0/false
+        break-if-=
+        dump-cell-from-cursor-over-full-screen out
+        draw-text-wrapping-right-then-down-from-cursor-over-full-screen 0/screen, "|", 7/fg, 0/bg
+      }
       debug-print "X", 7/fg, 0/bg
     }
     #

shell/global.mu

@@ -1314,6 +1314,7 @@ fn apply-print _args-ah: (addr handle cell), out: (addr handle cell), trace: (ad
   var stream-storage: (stream byte 0x100)
   var stream/edi: (addr stream byte) <- address stream-storage
   print-cell second-ah, stream, trace
+  set-cursor-position screen, 0, 0
   draw-stream-wrapping-right-then-down-from-cursor-over-full-screen screen, stream, 7/fg, 0/bg
   # return what was printed
   copy-object second-ah, out