-<pre>
-let safe_div (x:int) (y:int) =
- match y with
- | 0 -> None
- | _ -> Some (x / y);;
-
-(*
-val safe_div : int -> int -> int option = fun
-# safe_div 12 2;;
-- : int option = Some 6
-# safe_div 12 0;;
-- : int option = None
-# safe_div (safe_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 `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:
-
-<pre>
-let safe_div2 (u:int option) (v:int option) =
- match u with
- | None -> None
- | Some x ->
- (match v with
- | Some 0 -> None
- | Some y -> Some (x / y));;
-
-(*
-val safe_div2 : int option -> int option -> int option = <fun>
-# safe_div2 (Some 12) (Some 2);;
-- : int option = Some 6
-# safe_div2 (Some 12) (Some 0);;
-- : int option = None
-# safe_div2 (safe_div2 (Some 12) (Some 0)) (Some 3);;
-- : int option = None
-*)
-</pre>
-
-Calling the function now involves some extra verbosity, but it gives us what we need: now we can try to divide by anything we
-want, without fear that we're going to trigger system errors.
-
-I prefer to line up the `match` alternatives by using OCaml's
-built-in tuple type:
-
-<pre>
-let safe_div2 (u:int option) (v:int option) =
- match (u, v) with
- | (None, _) -> None
- | (_, None) -> None
- | (_, Some 0) -> None
- | (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 has already triggered a
-presupposition failure:
-
-<pre>
-let safe_add (u:int option) (v:int option) =
- match (u, v) with
- | (None, _) -> None
- | (_, None) -> None
- | (Some x, Some y) -> Some (x + y);;
-
-(*
-val safe_add : int option -> int option -> int option = <fun>
-# safe_add (Some 12) (Some 4);;
-- : int option = Some 16
-# safe_add (safe_div (Some 12) (Some 0)) (Some 4);;
-- : int option = None
-*)
-</pre>
-
-This works, but is somewhat disappointing: the `safe_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, using the monadic machinery we introduced above.
-As we said, there needs to be different `>>=`, `map2` and so on operations for each
-monad or box type we're working with.
-Haskell finesses this by "overloading" the single symbol `>>=`; you can just input that
-symbol and it will calculate from the context of the surrounding type constraints what
-monad you must have meant. In OCaml, the monadic operators are not pre-defined, but we will
-give you a library that has definitions for all the standard monads, as in Haskell.
-For now, though, we will define our `>>=` and `map2` operations by hand:
-
-<pre>
-let (>>=) (u : 'a option) (j : 'a -> 'b option) : 'b option =
- match u with
- | None -> None
- | Some x -> j x;;
-
-let map2 (f : 'a -> 'b -> 'c) (u : 'a option) (v : 'b option) : 'c option =
- u >>= (fun x -> v >>= (fun y -> Some (f x y)));;
-
-let safe_add3 = map2 (+);; (* that was easy *)
-
-let safe_div3 (u: int option) (v: int option) =
- u >>= (fun x -> v >>= (fun y -> if 0 = y then None else Some (x / y)));;
-</pre>
-
-Haskell has an even more user-friendly notation for defining `safe_div3`, namely:
-
- safe_div3 :: Maybe Int -> Maybe Int -> Maybe Int
- safe_div3 u v = do {x <- u;
- y <- v;
- if 0 == y then Nothing else Just (x `div` y)}
-
-Let's see our new functions in action:
-
-<pre>
-(*
-# safe_div3 (safe_div3 (Some 12) (Some 2)) (Some 3);;
-- : int option = Some 2
-# safe_div3 (safe_div3 (Some 12) (Some 0)) (Some 3);;
-- : int option = None
-# safe_add3 (safe_div3 (Some 12) (Some 0)) (Some 3);;
-- : int option = None
-*)
-</pre>
-
-Compare the new definitions of `safe_add3` and `safe_div3` closely: the definition
-for `safe_add3` shows what it looks like to equip an ordinary operation to
-survive in dangerous presupposition-filled world. Note that the new
-definition of `safe_add3` does not need to test whether its arguments are
-`None` values or real numbers---those details are hidden inside of the
-`bind` function.
-
-Note also that our definition of `safe_div3` recovers some of the simplicity of
-the original `safe_div`, without the complexity introduced by `safe_div2`. We now
-add exactly what extra is needed to track the no-division-by-zero presupposition. Here, too, we don't
-need to keep track of what other presuppositions may have already failed
-for whatever reason on our inputs.
-
-(Linguistics note: Dividing by zero is supposed to feel like a kind of
-presupposition failure. If we wanted to adapt this approach to
-building a simple account of presupposition projection, we would have
-to do several things. First, we would have to make use of the
-polymorphism of the `option` type. In the arithmetic example, we only
-made use of `int option`s, but when we're composing natural language
-expression meanings, we'll need to use types like `N option`, `Det option`,
-`VP option`, and so on. But that works automatically, because we can use
-any type for the `'a` in `'a option`. Ultimately, we'd want to have a
-theory of accommodation, and a theory of the situations in which
-material within the sentence can satisfy presuppositions for other
-material that otherwise would trigger a presupposition violation; but,
-not surprisingly, these refinements will require some more
-sophisticated techniques than the super-simple Option/Maybe monad.)