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 assignment.
+most one assignment. More specifically, 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* effects binding. (There is a third
-combinator, *l*, which we'll make use of to adjust function/argument
-order to better match English word order; N.B., though, that
-Jacobson's name for this combinator is "lift", but it is different
-from the monadic lift discussed in some detail below.) Here is a
-typical computation (based closely on email from Simon Charlow, with
-beta reduction as performed by the on-line evaluator):
+combinator which following Curry and Steedman, I'll call *T*, which
+we'll make use of to adjust function/argument order to better match
+English word order; N.B., though, that Jacobson's name for this
+combinator is "lift", but it is different from the monadic lift
+discussed in some detail below.) Here is a typical computation (based
+closely on email from Simon Charlow, with beta reduction as performed
+by the on-line evaluator):
<pre>
; 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 everyone = \P. FORALL x (P x) in
let he = \x. x in
-everyone ((z thinks) (g left he))
+let everyone = \P. FORALL x (P x) in
+
+everyone (z thinks (g left he))
~~> FORALL x (thinks (left x) x)
</pre>
embedded constituent to a containing constituent. If the sentence had
been *Everyone_i thinks Bill said he_i left*, there would be an
occurrence of *g* in the most deeply embedded clause (*he left*), and
-another occurrence of (a variant of) *g* in the next most deeply
-embedded clause (*Bill said he left*).
+another occurrence of *g* in the next most deeply
+embedded constituent (*said he left*), and so on (see below).
Third, binding is accomplished by applying *z* not to the element that
will (in some pre-theoretic sense) bind the pronoun, here, *everyone*,
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 that `l a f = f a`) is
+that `he = fun x -> x`, and letting `t a f = f a`) is
<pre>
everyone (z thinks (g left he))
~~> forall w (thinks (left w) w)
-everyone (z thinks (g (l bill) (g said (g left he))))
+everyone (z thinks (g (t bill) (g said (g left he))))
~~> forall w (thinks (said (left w) bill) w)
</pre>
One of Jacobson's main points survives: restricting the reader monad
to a single-value environment eliminates the need for variable names.
+
+It requires some cleverness to use the link monad to bind more than
+one variable at a time. Whereas in the standard reader monad a single
+environment can record any number of variable assignments, because
+Jacobson's monad only tracks a single dependency, binding more than
+one pronoun requires layering the monad, so that intermediate regions
+of the computation will be functors inside of a link monad box inside
+another link monad box, and so on.
+
+[Give details of the readings of *Everyone said someone thinks that he
+likes her*. Jacobson needs to add a variant of g; is it necessary to
+write a link swap that reverses the nesting of the monad boxes?]