Initial data commit

Task/Monads-Writer-monad/EchoLisp/monads-writer-monad.l

@@ -0,0 +1,42 @@
+(define (Writer.unit x (log #f))
+	(if log (cons log x)
+		(cons (format "init → %d" x) x)))
+
+;; f is a lisp function
+;; (Writer.lift f) returns a  Writer function which returns a Writer element
+
+(define (Writer.lift f name)
+	(lambda(elem)
+             (Writer.unit
+             	(f (rest elem))
+                (format "%a \n %a  → %a" (first elem) name (f (rest elem))))))
+
+;; lifts and applies
+(define (Writer.bind f elem) ((Writer.lift f (string f)) elem))
+
+(define (Writer.print elem) (writeln 'result (rest elem)) (writeln (first elem)))
+		
+;; Writer monad versions
+(define w-root  (Writer.lift sqrt "root"))
+(define w-half  (Writer.lift (lambda(x) (// x 2)) "half"))
+(define w-inc  ( Writer.lift add1 "add-one"))
+
+
+;; no binding required, as we use Writer lifted functions
+(->  5 Writer.unit w-root w-inc w-half Writer.print)
+
+result 1.618033988749895
+init → 5
+root → 2.23606797749979
+add-one → 3.23606797749979
+half → 1.618033988749895
+
+;; binding
+(->>  0 Writer.unit (Writer.bind sin) (Writer.bind cos)  w-inc w-half Writer.print)
+
+result 1
+init → 0
+sin → 0
+cos → 1
+add-one → 2
+half → 1