[[!toc]] Jacobson's Variable-Free Semantics as a bare-bones Reader Monad --------------------------------------------------------------- Jacobson's Variable-Free Semantics (e.g., Jacobson 1999, [Towards a Variable-Free Semantics](http://www.springerlink.com/content/j706674r4w217jj5/)) uses combinators to impose binding relationships between argument positions. The system does not make use of assignment functions or variables. We'll see that from the point of view of our discussion of monads, Jacobson's system is essentially a Reader monad in which the assignment function threaded through the computation is limited to at most one variable. It will turn out that Jacobson's geach combinator *g* is exactly our `lift` operator, and her binding combinator *z* is exactly our `bind` with the arguments reversed! Jacobson's system contains two main combinators, *g* and *z*. She calls *g* the Geach rule, and *z* performs binding. Here is a typical computation. This implementation is based closely on email from Simon Charlow, with beta reduction as performed by the on-line evaluator:
```; Analysis of "Everyone_i thinks he_i left"
let g = \f g x. f (g x) in
let z = \f g x. f (g x) x in
let he = \x. x in
let everyone = \P. FORALL x (P x) in

everyone (z thinks (g left he))

~~>  FORALL x (thinks (left x) x)
```
Several things to notice: First, pronouns denote identity functions. As Jeremy Kuhn has pointed out, this is related to the fact that in the mapping from the lambda calculus into combinatory logic that we discussed earlier in the course, bound variables translated to I, the identity combinator (see additional comments below). We'll return to the idea of pronouns as identity functions in later discussions. Second, *g* plays the role of transmitting a binding dependency for an embedded constituent to a containing constituent. Third, one of the peculiar aspects of Jacobson's system is that binding is accomplished not by applying *z* to the element that will (in some pre-theoretic sense) bind the pronoun, here, *everyone*, but rather by applying *z* instead to the predicate that will take *everyone* as an argument, here, *thinks*. The basic recipe in Jacobson's system, then, is that you transmit the dependence of a pronoun upwards through the tree using *g* until just before you are about to combine with the binder, when you finish off with *z*. (There are examples with longer chains of *g*'s below.) Last week we saw a Reader monad for tracking variable assignments:
```type env = (char * int) list;;
type 'a reader = env -> 'a;;
let unit x = fun (e : env) -> x;;
fun (e : env) -> f (u e) e;;
let shift (c : char) (v : int reader) (u : 'a reader) =
fun (e : env) -> u ((c, v e) :: e);;
let lookup (c : char) : int reader = fun (e : env) -> List.assoc c e;;
```
(We've used a simplified term for the bind function here in order to emphasize its similarities with Jacboson's geach combinator.) This monad boxes up a value along with an assignment function, where an assignemnt function was implemented as a list of `char * int`. The idea is that a list like `[('a', 2); ('b',5)]` associates the variable `'a'` with the value 2, and the variable `'b'` with the value 5. Combining this Reader monad with ideas from Jacobson's approach, we can consider the following monad:
```type e = int;;
type 'a link = e -> 'a;;
let unit (a:'a): 'a link = fun x -> a;;
let bind (u: 'a link) (f: 'a -> 'b link) : 'b link = fun (x:e) -> f (u x) x;;
let ap (u: ('a -> 'b) link) (v: 'a link) : 'b link = fun (x:e) -> u x (v x);;
let lift (f: 'a -> 'b) (u: 'a link): ('b link) = ap (unit f) u;;
let g = lift;;
let z (f: 'a -> e -> 'b) (u: 'a link) : e -> 'b = fun (x:e) -> f (u x) x;;
```
```let bind (u: 'a link) (f: 'a -> 'b link) : 'b link = fun (x:e) -> f (u x) x;;
let z (f: 'a -> e -> 'b) (u: 'a link) : e -> 'b = fun (x:e) -> f (u x) x;;
```
`Bind` takes an `'a link`, and a function that maps an `'a` to a `'b link`, and returns a `'b link`, i.e., the result is in the link monad. *z*, on the other hand, takes the same two arguments (in reverse order), but returns something that is not in the monad. Rather, it will be a function from individuals to a computation in which the pronoun in question is bound to that individual. We could emphasize the parallel with the Reader monad even more by writing a `shift` operator that used `unit` to produce a monadic result, if we wanted to. The monad version of *Everyone_i thinks he_i left*, then (remembering that `he = fun x -> x`, and letting `t a f = f a`) is
```everyone (z thinks (g left he))

~~> forall w (thinks (left w) w)

everyone (z thinks (g (t bill) (g said (g left he))))

~~> forall w (thinks (said (left w) bill) w)
```
```    John believes Mary said he thinks she's cute.
|             |         |         |
|             |---------|---------|
|                       |
|-----------------------|
```
It will be convenient to have a counterpart to the lift operation that combines a monadic functor with a non-monadic argument:
```    let g f v = ap (unit f) v;;
let g2 u a = ap u (unit a);;
```
As a first step, we'll bind "she" by "Mary":
```believes (z said (g2 (g thinks (g cute she)) she) mary) john

~~> believes (said (thinks (cute mary) he) mary) john
```
As usual, there is a trail of *g*'s leading from the pronoun up to the *z*. Next, we build a trail from the other pronoun ("he") to its binder ("John").
```believes
said
thinks (cute she) he
Mary
John

believes
z said
(g2 ((g thinks) (g cute she))) he
Mary
John

z believes
(g2 (g (z said)
(g (g2 ((g thinks) (g cute she)))
he))
Mary)
John
```
In the first interation, we build a chain of *g*'s and *g2*'s from the pronoun to be bound ("she") out to the level of the first argument of *said*. In the second iteration, we treat the entire structure as ordinary functions and arguments, without "seeing" the monadic region. Once again, we build a chain of *g*'s and *g2*'s from the currently targeted pronoun ("he") out to the level of the first argument of *believes*. (The new *g*'s and *g2*'s are the three leftmost).
```z believes (g2 (g (z said) (g (g2 ((g thinks) (g cute she))) he)) mary) john

~~> believes (said (thinks (cute mary) john) mary) john
```
Obviously, we can repeat this strategy for any configuration of pronouns and binders. This binding strategy is strongly reminiscent of the translation from the lambda calculus into Combinatory Logic that we studied earlier (see the lecture notes for week 2). Recall that bound pronouns ended up translating to I, the identity combinator, just like Jacobson's identity functions for pronouns; abstracts (\a.M) whose body M did not contain any occurrences of the pronoun to be bound translated as KM, just like our unit, which you will recognize as an instance of K; and abstracts of the form (\a.MN) translated to S[\aM][\aN], just like our ap rule.