towards monads: tweak error message

[lambda.git] / towards_monads.mdwn

diff --git a/towards_monads.mdwn b/towards_monads.mdwn
index 3671d27..378310f 100644 (file)
--- a/towards_monads.mdwn
+++ b/towards_monads.mdwn
@@ -36,7 +36,7 @@ val div' : int -> int -> int option = fun
 # div' (div' 12 3) 2;;
 Characters 4-14:
   div' (div' 12 3) 2;;
-      ^^^^^^^^^^
+        ^^^^^^^^^^
 Error: This expression has type int option
        but an expression was expected of type int
 *)
@@ -48,13 +48,13 @@ the output of the safe-division function as input for further division
 operations.  So we have to jack up the types of the inputs:
 
 <pre>
-let div' (x:int option) (y:int option) =
-  match y with
+let div' (u:int option) (v:int option) =
+  match v with
          None -> None
     | Some 0 -> None
-       | Some n -> (match x with
+       | Some y -> (match u with
                                          None -> None
-                    | Some m -> Some (m / n));;
+                    | Some x -> Some (x / y));;
 
 (*
 val div' : int option -> int option -> int option = <fun>
@@ -74,25 +74,25 @@ I prefer to line up the `match` alternatives by using OCaml's
 built-in tuple type:
 
 <pre>
-let div' (x:int option) (y:int option) =
-  match (x, y) with
+let div' (u:int option) (v:int option) =
+  match (u, v) with
          (None, _) -> None
     | (_, None) -> None
     | (_, Some 0) -> None
-       | (Some m, Some n) -> Some (m / n);;
+       | (Some x, Some y) -> Some (x / y);;
 </pre>
 
 So far so good.  But what if we want to combine division with
 other arithmetic operations?  We need to make those other operations
-aware of the possibility that one of their arguments will trigger a
+aware of the possibility that one of their arguments has triggered a
 presupposition failure:
 
 <pre>
-let add' (x:int option) (y:int option) =
-  match (x, y) with
+let add' (u:int option) (v:int option) =
+  match (u, v) with
          (None, _) -> None
     | (_, None) -> None
-    | (Some m, Some n) -> Some (m + n);;
+    | (Some x, Some y) -> Some (x + y);;
 
 (*
 val add' : int option -> int option -> int option = <fun>
@@ -112,16 +112,16 @@ Haskell, etc., is to define a `bind` operator (the name `bind` is not
 well chosen to resonate with linguists, but what can you do). To continue our mnemonic association, we'll put a `'` after the name "bind" as well.
 
 <pre>
-let bind' (x: int option) (f: int -> (int option)) =
-  match x with
+let bind' (u: int option) (f: int -> (int option)) =
+  match u with
          None -> None
-    | Some n -> f n;;
+    | Some x -> f x;;
 
-let add' (x: int option) (y: int option)  =
-  bind' x (fun x -> bind' y (fun y -> Some (x + y)));;
+let add' (u: int option) (v: int option)  =
+  bind' u (fun x -> bind' v (fun y -> Some (x + y)));;
 
-let div' (x: int option) (y: int option) =
-  bind' x (fun x -> bind' y (fun y -> if (0 = y) then None else Some (x / y)));;
+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);;