From: Jim Pryor <profjim@jimpryor.net>
Date: Mon, 1 Nov 2010 06:15:04 +0000 (-0400)
Subject: towards monads tweaks
X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=commitdiff_plain;h=f5b5782cffbfacfdedcd59af0780eb981a564b0c

towards monads tweaks

Signed-off-by: Jim Pryor <profjim@jimpryor.net>
---

diff --git a/towards_monads.mdwn b/towards_monads.mdwn
index 378310f9..3b217cbd 100644
--- a/towards_monads.mdwn
+++ b/towards_monads.mdwn
@@ -29,20 +29,20 @@ let div' (x:int) (y:int) =
 
 (*
 val div' : int -> int -> int option = fun
-# div' 12 3;;
-- : int option = Some 4
+# div' 12 2;;
+- : int option = Some 6
 # div' 12 0;;
 - : int option = None
-# div' (div' 12 3) 2;;
+# div' (div' 12 2) 3;;
 Characters 4-14:
-  div' (div' 12 3) 2;;
+  div' (div' 12 2) 3;;
         ^^^^^^^^^^
 Error: This expression has type int option
        but an expression was expected of type int
 *)
 </pre>
 
-This starts off well: dividing 12 by 3, no problem; dividing 12 by 0,
+This starts off well: dividing 12 by 2, no problem; dividing 12 by 0,
 just the behavior we were hoping for.  But we want to be able to use
 the output of the safe-division function as input for further division
 operations.  So we have to jack up the types of the inputs:
@@ -58,11 +58,11 @@ let div' (u:int option) (v:int option) =
 
 (*
 val div' : int option -> int option -> int option = <fun>
-# div' (Some 12) (Some 4);;
-- : int option = Some 3
+# div' (Some 12) (Some 2);;
+- : int option = Some 6
 # div' (Some 12) (Some 0);;
 - : int option = None
-# div' (div' (Some 12) (Some 0)) (Some 4);;
+# div' (div' (Some 12) (Some 0)) (Some 3);;
 - : int option = None
 *)
 </pre>
@@ -124,11 +124,11 @@ let div' (u: int option) (v: int option) =
   bind' u (fun x -> bind' v (fun y -> if (0 = y) then None else Some (x / y)));;
 
 (*
-#  div' (div' (Some 12) (Some 2)) (Some 4);;
-- : int option = Some 1
-#  div' (div' (Some 12) (Some 0)) (Some 4);;
+#  div' (div' (Some 12) (Some 2)) (Some 3);;
+- : int option = Some 2
+#  div' (div' (Some 12) (Some 0)) (Some 3);;
 - : int option = None
-# add' (div' (Some 12) (Some 0)) (Some 4);;
+# add' (div' (Some 12) (Some 0)) (Some 3);;
 - : int option = None
 *)
 </pre>