Change to use cond

.gitignore

@@ -1,2 +1,2 @@
 # Ignore raket temp files
-*.rkt*
+*.rkt~

Labs/07.rkt

@@ -0,0 +1,49 @@
+#lang typed/racket
+(require (rename-in typed/rackunit [check-equal? test])
+         (only-in typed/rackunit check-exn))
+(define-syntax-rule (test/exn expr msg)
+  (check-exn exn:fail? (lambda () expr) msg))
+
+(define (msg0 [obj : Node] [selector : Symbol]) (obj selector))
+
+(define (node [v : Number] [l : Node] [r : Node]) : Node
+  (lambda (m)
+    (case m
+      [(value) v]
+      [(left)  l]
+      [(right) r]
+      [(sum) (+ v (+ (msg-num l 'sum) (msg-num r 'sum)))]
+      [else (error 'node (symbol->string m))])))
+
+(define (mt)
+  (lambda ([m : Symbol])
+    (case m
+      [(sum) 0]
+      [else (error 'mt (symbol->string m))])))
+
+(define-type Node (Symbol -> (U (U Number Node) Zero)))
+
+(define (msg-num [obj : Node] [selector : Symbol]) : Number
+  (define result (msg0 obj selector))
+  (cond
+    [(number? result) result]
+    [else (error 'msg-num "Expected a number message")]))
+
+(define (msg-node [obj : Node] [selector : Symbol]) : Node
+  (define result (msg0 obj selector))
+  (cond
+    [(not (number? result)) result]
+    [else (error 'msg-node "Expected a node message")]))
+
+(module+ test
+  (define tree1 (node 1 (mt) (mt)))
+  (test (msg0 tree1 'sum) 1)
+  (test (msg0 tree1 'value) 1)
+  (define tree2 (node 1 (node 2 (mt) (mt)) (mt)))
+  (test (msg0 tree2 'value) 1)
+  (test (msg0 (msg-node tree2 'left) 'value) 2)
+  (test (msg0 (msg-node tree2 'right) 'sum) 0)
+  (test/exn (msg0 (mt) 'left) "left")
+  (test/exn (msg0 tree2 'wrong) "wrong")
+  (test/exn (msg-num tree2 'left) "number")
+  (test/exn (msg-node tree2 'value) "node"))