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+;; (read-enable 'curly-infix)
+
+;;; Examples:
+;;; These are do in the conext of the optional monad.
+
+(add-to-load-path (dirname (current-filename)))
+
+(use-modules (control monad)
+             (data monoid)
+             (data optional)
+             (data writer))
+
+(do (just x) <- (just 10)
+    x) ; => 10
+
+(do let y = (just 10)
+    (just x) <- y
+    (+ x 5)) ; => 15
+
+(do (just x) <- (nothing)
+    (+ x 5)) ; => [Nothing]
+
+;;;
+
+(do let either = left 10
+    (left x) <- either
+    x) ; EVALUATION ERROR
+
+;; Int -> Writer Int String
+(define (log-number n)
+  (writer n (format #f "Got nuber: ~a" n)))
+
+(define (mult-with-log)
+  (do (writer a) <- (log-number 3)
+      (writer b) <- (log-number 5)
+      (* a b)))
+
+(do (writer a) <- (log-number 1)
+    a) ; EVALUATION ERROR
+
+(begin { (log-number 1) >>= log-number })
+;; => [Writer 1, "Got nuber: 1, Got nuber: 1"]
+
+(do (writer a) <- (log-number 3)
+    (writer b) <- (log-number 5)
+    (writer (* a b) ""))
+;; EVALUATION ERROR
+;; => [Writer 3, "Got nuber: 3, base"]
+
+(log-number 3) ; => [Writer 3, "Got nuber: 3"]
+(just 1) ; => [Just 1]
+
+(do let y = 5
+    (just x) <- (just 10)
+    (just (* x y)))
+;; => [Just 50]
+
+;;; TODO
+;;; '<- and 'let can't be used after '<-
+
+