X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?a=blobdiff_plain;ds=sidebyside;f=week6.mdwn;h=b97a09f3ff51f7655a30f486060afcd833ea3a50;hb=48f733f96a60560cc42360cbfc890f6f9c529d91;hp=221e02077378f650111c0facbaea9cc9d649eb23;hpb=96a8c8c9b81fc914ac7ec368fab0ffa4bcf4177a;p=lambda.git diff --git a/week6.mdwn b/week6.mdwn index 221e0207..b97a09f3 100644 --- a/week6.mdwn +++ b/week6.mdwn @@ -171,3 +171,144 @@ We can use functions that take arguments of type unit to control execution. In Scheme parlance, functions on the unit type are called *thunks* (which I've always assumed was a blend of "think" and "chunk"). +Towards Monads +-------------- + +So the integer division operation presupposes that its second argument +(the divisor) is not zero, upon pain of presupposition failure. +Here's what my OCAML interpreter says: + + # 12/0;; + Exception: Division_by_zero. + +So we want to explicitly allow for the possibility that +division will return something other than a number. +We'll use OCAML's option type, which works like this: + + # type 'a option = None | Some of 'a;; + # None;; + - : 'a option = None + # Some 3;; + - : int option = Some 3 + +So if a division is normal, we return some number, but if the divisor is +zero, we return None: + +
+let div (x:int) (y:int) = + match y with 0 -> None | + _ -> Some (x / y);; + +(* +val div : int -> int -> int option = fun +# div 12 3;; +- : int option = Some 4 +# div 12 0;; +- : int option = None +# 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 +*) ++ +This starts off well: dividing 12 by 3, 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: + +
+let div (x:int option) (y:int option) = + match y with None -> None | + Some 0 -> None | + Some n -> (match x with None -> None | + Some m -> Some (m / n));; + +(* +val div : int option -> int option -> int option =+ +Beautiful, just what we need: now we can try to divide by anything we +want, without fear that we're going to trigger any system errors. + +I prefer to line up the `match` alternatives by using OCAML's +built-in tuple type: + ++# div (Some 12) (Some 4);; +- : int option = Some 3 +# div (Some 12) (Some 0);; +- : int option = None +# div (div (Some 12) (Some 0)) (Some 4);; +- : int option = None +*) +
+let div (x:int option) (y:int option) = + match (x, y) with (None, _) -> None | + (_, None) -> None | + (_, Some 0) -> None | + (Some m, Some n) -> Some (m / n);; ++ +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 +presupposition failure: + +
+let add (x:int option) (y:int option) = + match (x, y) with (None, _) -> None | + (_, None) -> None | + (Some m, Some n) -> Some (m + n);; + +(* +val add : int option -> int option -> int option =+ +This works, but is somewhat disappointing: the `add` operation +doesn't trigger any presupposition of its own, so it is a shame that +it needs to be adjusted because someone else might make trouble. + +But we can automate the adjustment. The standard way in OCAML, +Haskell, etc., is to define a `bind` operator (the name `bind` is not +well chosen to resonate with linguists, but what can you do): + ++# add (Some 12) (Some 4);; +- : int option = Some 16 +# add (div (Some 12) (Some 0)) (Some 4);; +- : int option = None +*) +
+let bind (x: int option) (f: int -> (int option)) = + match x with None -> None | + Some n -> f n;; + +let add (x: int option) (y: int option) = + bind x (fun x -> bind y (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)));; + +(* +# div (div (Some 12) (Some 2)) (Some 4);; +- : int option = Some 1 +# div (div (Some 12) (Some 0)) (Some 4);; +- : int option = None +# add (div (Some 12) (Some 0)) (Some 4);; +- : int option = None +*) ++ +Compare the new definitions of `add` and `div` closely: the definition +for `add` shows what it looks like to equip an ordinary operation to +survive in dangerous presupposition-filled world. Note that the new +definition of `add` does not need to test whether its arguments are +None objects or real numbers---those details are hidden inside of the +`bind` function. + +The definition of `div` shows exactly what extra needs to be said in +order to trigger the no-division-by-zero presupposition. + +For linguists: this is a complete theory of a particularly simply form +of presupposition projection (every predicate is a hole).