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Task/Amb/Scheme/amb.ss

@@ -0,0 +1,34 @@
+(define fail
+  (lambda ()
+    (error "Amb tree exhausted")))
+
+(define-syntax amb
+  (syntax-rules ()
+    ((AMB) (FAIL))                      ; Two shortcuts.
+    ((AMB expression) expression)
+
+    ((AMB expression ...)
+     (LET ((FAIL-SAVE FAIL))
+       ((CALL-WITH-CURRENT-CONTINUATION ; Capture a continuation to
+          (LAMBDA (K-SUCCESS)           ;   which we return possibles.
+            (CALL-WITH-CURRENT-CONTINUATION
+              (LAMBDA (K-FAILURE)       ; K-FAILURE will try the next
+                (SET! FAIL K-FAILURE)   ;   possible expression.
+                (K-SUCCESS              ; Note that the expression is
+                 (LAMBDA ()             ;   evaluated in tail position
+                   expression))))       ;   with respect to AMB.
+            ...
+            (SET! FAIL FAIL-SAVE)      ; Finally, if this is reached,
+            FAIL-SAVE)))))))            ;   we restore the saved FAIL.
+
+
+(let ((w-1 (amb "the" "that" "a"))
+      (w-2 (amb "frog" "elephant" "thing"))
+      (w-3 (amb "walked" "treaded" "grows"))
+      (w-4 (amb "slowly" "quickly")))
+  (define (joins? left right)
+    (equal? (string-ref left (- (string-length left) 1)) (string-ref right 0)))
+  (if (joins? w-1 w-2) '() (amb))
+  (if (joins? w-2 w-3) '() (amb))
+  (if (joins? w-3 w-4) '() (amb))
+  (list w-1 w-2 w-3 w-4))