prolog: write/1 + nl/0 via output buffer, 7 tests; built-ins box done

lib/prolog/runtime.sx

@@ -299,6 +299,8 @@
         ((pl-cut? g) (begin (dict-set! cut-box :cut true) (k)))
         ((and (pl-atom? g) (= (pl-atom-name g) "true")) (k))
         ((and (pl-atom? g) (= (pl-atom-name g) "fail")) false)
+        ((and (pl-atom? g) (= (pl-atom-name g) "nl"))
+          (begin (pl-output-write! "\n") (k)))
         ((and (pl-compound? g) (= (pl-fun g) "=") (= (len (pl-args g)) 2))
           (pl-solve-eq! (first (pl-args g)) (nth (pl-args g) 1) trail k))
         ((and (pl-compound? g) (= (pl-fun g) "\\=") (= (len (pl-args g)) 2))
@@ -335,6 +337,10 @@
           (let
             ((call-cb {:cut false}))
             (pl-solve! db (first (pl-args g)) trail call-cb k)))
+        ((and (pl-compound? g) (= (pl-fun g) "write") (= (len (pl-args g)) 1))
+          (begin
+            (pl-output-write! (pl-format-term (first (pl-args g))))
+            (k)))
         (true (pl-solve-user! db g trail cut-box k))))))
 
 (define
@@ -387,6 +393,42 @@
                 (pl-trail-undo-to! trail mark)
                 (pl-solve! db else-goal trail cut-box k)))))))))
 
+(define pl-output-buffer "")
+
+(define pl-output-clear! (fn () (set! pl-output-buffer "")))
+
+(define
+  pl-output-write!
+  (fn (s) (begin (set! pl-output-buffer (str pl-output-buffer s)) nil)))
+
+(define
+  pl-format-args
+  (fn
+    (args)
+    (cond
+      ((empty? args) "")
+      ((= (len args) 1) (pl-format-term (first args)))
+      (true
+        (str
+          (pl-format-term (first args))
+          ", "
+          (pl-format-args (rest args)))))))
+
+(define
+  pl-format-term
+  (fn
+    (t)
+    (let
+      ((w (pl-walk-deep t)))
+      (cond
+        ((pl-var? w) (str "_" (pl-var-id w)))
+        ((pl-atom? w) (pl-atom-name w))
+        ((pl-num? w) (str (pl-num-val w)))
+        ((pl-str? w) (pl-str-val w))
+        ((pl-compound? w)
+          (str (pl-fun w) "(" (pl-format-args (pl-args w)) ")"))
+        (true (str w))))))
+
 (define
   pl-solve-not-eq!
   (fn