From f5b5782cffbfacfdedcd59af0780eb981a564b0c Mon Sep 17 00:00:00 2001 From: Jim Pryor Date: Mon, 1 Nov 2010 02:15:04 -0400 Subject: [PATCH] towards monads tweaks Signed-off-by: Jim Pryor --- towards_monads.mdwn | 24 ++++++++++++------------ 1 file changed, 12 insertions(+), 12 deletions(-) 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 *) -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 = -# 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 *) @@ -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 *) -- 2.11.0