[lambda.git] / topics / _week10_gsv.mdwn

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# Doing things with monads

## Extended application: Groenendijk, Stokhof and Veltman's *Coreference and Modality*

GSV are interested in developing and establishing a reasonable theory
of discourse update.  One way of looking at this paper is like this:

  GSV = GS + V

That is, Groenendijk and Stokhof have a well-known theory of dynamic
semantics, and Veltman has a well-known theory of epistemic modality,
and this fragment brings both of those strands together into a single
system.  

We will be interested in this paper both from a theoretical point of
view and from a practical engineering point of view.  On the
theoretical level, these scholars are proposing a strategy for
managing the connection between variables and the objects they
designate in way that is flexible enough to be useful for describing
natural language.  The main way they attempt to do this is by
inserting an extra level in between the variable and the object:
instead of having an assignment function that maps variables directly
onto objects, GSV provide *pegs*: variables map onto pegs, and pegs
map onto objects.  We'll discuss in considerable detail what pegs
allow us to do, since it is highly relevant to one of the main
applications of the course, namely, reference and coreference.

What are pegs?  The term harks back to a paper by Landman called `Pegs
and Alecs'.  There pegs are simply hooks for hanging properties on.
Pegs are supposed to be as anonymous as possible.  Think of hanging
your coat on a physical peg: you don't care which peg it is, only that
there are enough pegs for everyone's coat to hang from.  Likewise, for
the pegs of GSV, all that matters is that there are enough of them.
(Incidentally, there is nothing in Gronendijk and Stokhof's original
DPL paper that corresponds naturally to pegs; but in their Dynamic
Montague Grammar paper, pegs serve a purpose similar to discourse
referents there, though the connection is not simple.)

On an engineering level, the fact that GSV are combining anaphora and
bound quantification with epistemic quantification means that they are
gluing together related but distinct subsystems into a single
fragment.  These subsystems naturally cleave into separate layers in a
way that is obscured in the paper.  We will argue in detail that
re-engineering GSV using monads will lead to a cleaner system that
does all of the same theoretical work.

Empirical targets: on the anaphoric side, GSV want to 

On the epistemic side, GSV aim to account for asymmetries such as

    It might be raining.  It's not raining.
    #It's not raining.  It might be raining.

## Basics

There are a lot of formal details in the paper in advance of the
empirical discussion.  Here are the ones that matter:

    type var = string
    type peg = int
    type refsys = var -> peg
    type ent = Alice | Bob | Carl
    type assignment = peg -> ent

So in order to get from a variable to an object, we have to compose a
refsys `r` with an assignment `g`.  For instance, we might have
r (g ("x")) = Alice.

    type pred = string
    type world = pred -> ent -> bool
    type pegcount = int
    type poss = world * pegcount * refsys * assignment
    type infostate = [poss]

Worlds in general settle all matters of fact in the world.  In
particular, they determine the extensions of predicates and relations.
In this discussion, we'll (crudely) approximate worlds by making them
a function from predicates such as "man" to a function mapping each
entity to a boolean.  

As we'll see, indefinites as a side effect increase the number of pegs
by one.  GSV assume that we can determine what integer the next unused
peg corresponds to by examining the range of the refsys function.
We'll make things easy on ourselves by simply tracking the total
number of used pegs in a counter called `pegcount`.

So information states track both facts about the world (e.g., which
objects count as a man), and facts about the discourse (e.g., how many
pegs have been used).

The formal language the fragment interprets is Predicate Calculus with
equality, existential and universal quantification, and one unary
modality (box and diamond, corresponding to epistemic necessity and
epistemic possibility).

Terms in this language are either individuals such as Alice or Bob, or
else variables.  So in general, the referent of a term can depend on a
possibility:

    ref(i, t) = t if t is an individual, and 
                g(r(t)) if t is a variable, where i = (w,n,r,g)

Here are the main clauses for update (their definition 3.1).  

Following GSV, we'll write `update(s, φ)` (the update of information
state `s` with the information in φ) as `s[φ]`.

    s[P(t)] = {i in s | w(P)(ref(i,t))}

So `man(x)` is the set of live possibilities `i = (w,r,g)` in s such that
the set of men in `w` given by `w(man)` maps the object referred to by
`x`, namely, `r(g("x"))`, to `true`.   That is, update with "man(x)"
discards all possibilities in which "x" fails to refer to a man.

    s[t1 = t2] = {i in s | ref(i,t1) = ref(i,t2)}

    s[φ and ψ] = s[φ][ψ]

When updating with a conjunction, first update with the left conjunct,
then update with the right conjunct.

Existential quantification requires adding a new peg to the set of
discourse referents.  

    s[∃xφ] = {(w, n+1, r[x->n], g[n->a]) | (w,n,r,g) in s and a in ent}[φ]

Here's the recipe: for every possibility (w,n,r,g) in s, and for every
entity a in the domain of discourse, construct a new possibility with
the same world w, an incrementd peg count n+1, and a new r and g
adjusted in such a way that the variable x refers to the object a.

Note that this recipe does not examine φ.  This means that this
analysis treats the formula prefix `∃x` as if it were a meaningful
constituent independent of φ.

Negation is natural enough:

    s[neg φ] =  {i | {i}[φ] = {}}

If updating φ with the information state that contains only the
possibility i returns the empty information state, then not φ is true
with respect to i.

In GSV, disjunction, the conditional, and the universals are defined
in terms of negation and the other connectives.

Exercise: assume that there are two entities in the domain of
discourse, Alice and Bob.  Assume that Alice is a woman, and Bob is a
man.  Show the following computations:

    1. {}[∃x.person(x)]
    2. {}[∃x.man(x)]
    3. {}[∃x∃y.person(x) and person(y)]
    4. {}[∃x∃y.x=x]
    5. {}[∃x∃y.x=y]