<code>mid (/εmaidεnt@tI/): P -> <u>P</u></code>
-> In Haskell, this is called `Control.Monad.return` and `Control.Applicative.pure`. In other theoretical contexts it is sometimes called `unit` or `η`. In the class presentation Jim called it `𝟭`; but now we've decided that `mid` is better. (Think of it as "m" plus "identity", not as the start of "midway".) This notion is exemplified by `Just` for the box type `Maybe α` and by the singleton function for the box type `List α`.
+> This notion is exemplified by `Just` for the box type `Maybe α` and by the singleton function for the box type `List α`. It will be a way of boxing values with your box type that plays a distinguished role in the various Laws and interdefinitions we present below.
+
+> In Haskell, this is called `Control.Monad.return` and `Control.Applicative.pure`. In other theoretical contexts it is sometimes called `unit` or `η`. All of these names are somewhat unfortunate. First, it has little to do with `η`-reduction in the Lambda Calculus. Second, it has little to do with the `() : unit` value we discussed in earlier classes. Third, it has little to do with the `return` keyword in C and other languages; that's more closely related to continuations, which we'll discuss in later weeks. Finally, this doesn't perfectly align with other uses of "pure" in the literature. `mid`'d values _will_ generally be "pure" in the other senses, but other boxed values can be too.
+
+> For all these reasons, we're thinking it will be clearer in our discussion to use a different name. In the class presentation Jim called it `𝟭`; but now we've decided that `mid` is better. (Think of it as "m" plus "identity", not as the start of "midway".)
<code>m$ or mapply (/εm@plai/): <u>P -> Q</u> -> <u>P</u> -> <u>Q</u></code>
<code>>=> or flip mcomp : (P -> <u>Q</u>) -> (Q -> <u>R</u>) -> (P -> <u>R</u>)</code>
-> In Haskell, this is `Control.Monad.>=>`. In the class handout, we gave the types for `>=>` twice, and once was correct but the other was a typo. The above is the correct typing.
+> In Haskell, this is `Control.Monad.>=>`. We will move freely back and forth between using `<=<` (aka `mcomp`) and using `>=>`, which
+is just `<=<` with its arguments flipped. `<=<` has the virtue that it corresponds more
+closely to the ordinary mathematical symbol `○`. But `>=>` has the virtue
+that its types flow more naturally from left to right.
+
+> In the class handout, we gave the types for `>=>` twice, and once was correct but the other was a typo. The above is the correct typing.
<code>>>= or mbind : (<u>Q</u>) -> (Q -> <u>R</u>) -> (<u>R</u>)</code>
+> Haskell uses the symbol `>>=` but calls it "bind". This is not well chosen from the perspective of formal semantics, since it's only loosely connected with what we mean by "binding." But the name is too deeply entrenched to change. We've at least preprended an "m" to the front of "bind". In some presentations this operation is called `★`.
+
<code>=<< or flip mbind : (Q -> <u>R</u>) -> (<u>Q</u>) -> (<u>R</u>)</code>
<code>join: <span class="box2">P</span> -> <u>P</u></code>
> In Haskell, this is `Control.Monad.join`. In other theoretical contexts it is sometimes called `μ`.
-Haskell uses the symbol `>>=` but calls it "bind". This is not well chosen from the perspective of formal semantics, but it's too deeply entrenched to change. We've at least preprended an "m" to the front of "bind".
-
-Haskell's names "return" and "pure" for `mid` are even less well chosen, and we think it will be clearer in our discussion to use a different name. (Also, in other theoretical contexts this notion goes by other names, anyway, like `unit` or `η` --- having nothing to do with `η`-reduction in the Lambda Calculus.)
-
The menagerie isn't quite as bewildering as you might suppose. Many of these will be interdefinable. For example, here is how `mcomp` and `mbind` are related: <code>k <=< j ≡ \a. (j a >>= k)</code>. We'll state some other interdefinitions below.
-We will move freely back and forth between using `>=>` and using `<=<` (aka `mcomp`), which
-is just `>=>` with its arguments flipped. `<=<` has the virtue that it corresponds more
-closely to the ordinary mathematical symbol `○`. But `>=>` has the virtue
-that its types flow more naturally from left to right.
-
These functions come together in several systems, and have to be defined in a way that coheres with the other functions in the system:
* ***Mappable*** (in Haskelese, "Functors") At the most general level, box types are *Mappable*
For the third failure, that is examples of MapNables that aren't Monads, we'll just state that lists where the `map2` operation is taken to be zipping rather than taking the Cartesian product (what in Haskell are called `ZipList`s), these are claimed to exemplify that failure. But we aren't now in a position to demonstrate that to you.
-
-## Safe division ##
-
-As we discussed in class, there are clear patterns shared between lists and option types and trees, so perhaps you can see why people want to figure out the general structures. But it probably isn't obvious yet why it would be useful to do so. To a large extent, this will only emerge over the next few classes. But we'll begin to demonstrate the usefulness of these patterns by talking through a simple example, that uses the monadic functions of the Option/Maybe box type.
-
-Integer division 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.
-
-Say we want to explicitly allow for the possibility that
-division will return something other than a number.
-To do that, 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`. As a mnemonic aid, we'll prepend a `safe_` to the start of our new divide function.
-
-<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.)
-