X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=hints%2Fassignment_7_hint_3.mdwn;h=842d90ea2f7bcb1e6e031762686b976be7b736ba;hp=ed6bc36afe738833ec552bdf80be18784b548f33;hb=2dd3d25af2cc061385f8544bd7137f4f869d3d98;hpb=de64b1646254627a9e0d10fcdea232c74104e7e5
diff --git a/hints/assignment_7_hint_3.mdwn b/hints/assignment_7_hint_3.mdwn
index ed6bc36a..842d90ea 100644
--- a/hints/assignment_7_hint_3.mdwn
+++ b/hints/assignment_7_hint_3.mdwn
@@ -1,34 +1,34 @@
-* In def 2.5, they say the denotation of an e-type constant α wrt a discourse possibility `(r, g, w)` is whatever object the world `w` associates with α. Since we don't have worlds, this will just be an object.
+* In def 2.5, they say the denotation of an e-type constant α wrt a discourse possibility `(r, h, w)` is whatever object the world `w` associates with α. Since we don't have worlds, this will just be an object.
* They say the denotation of a predicate is whatever extension the world `w` associates with the predicate. Since we don't have worlds, this will just be an extension.
-* They say the denotation of a variable is the object which the store `g` assigns to the index that the assignment function `r` assigns to the variable. In other words, if the variable is `'x'`, its denotation wrt `(r, g, w)` is `g[r['x']]`.
+* They say the denotation of a variable is the object which the store `h` assigns to the index that the assignment function `r` assigns to the variable. In other words, if the variable is `'x'`, its denotation wrt `(r, h, w)` is `h[r['x']]`. In our OCaml implementation, that will be `List.nth h (r 'x')`.
We're going to keep all of that, except dropping the worlds. And instead of talking about
-> \[[expression]] in possibility `(r, g, w)`
+> \[[expression]] in possibility `(r, h, w)`
-we'll just talk about \[[expression]] and let that be a monadic object, implemented in part by a function that takes `(r, g)` as an argument.
+we'll just talk about \[[expression]] and let that be a monadic object, implemented in part by a function that takes `(r, h)` as an argument.
-More specifically, \[[expression]] will be a set of `'a discourse_possibility` monads, where `'a` is the appropriate type for *expression*, and the discourse possibility monads are themselves state monads where `(r, g)` is the state that gets updated. Those are implemented as functions from `(r, g)` to `(a, r', g')`, where `a` is a value of type `'a`, and `r', g'` are possibly altered assignment functions and stores.
+More specifically, \[[expression]] will be a set of `'a discourse_possibility` monads, where `'a` is the appropriate type for *expression*, and the discourse possibility monads are themselves state monads where `(r, h)` is the state that gets updated. Those are implemented as functions from `(r, h)` to `(a, r', h')`, where `a` is a value of type `'a`, and `r', h'` are possibly altered assignment functions and stores.
* In def 2.7, GS&V talk about an operation that takes an existing set of discourse possibilities, and extends each member in the set by allocating a new location in the store, and assigning a variable `'x'` to that location, which holds some object `d` from the domain. It will be useful to have a shorthand way of referring to this operation:
let newpeg_and_bind (bound_variable : char) (d : entity) =
- fun ((r, g) : assignment * store) ->
- let newindex = List.length g
- (* first we store d at index newindex in g, which is at the very end *)
+ fun ((r, h) : assignment * store) ->
+ let newindex = List.length h
+ (* first we store d at index newindex in h, which is at the very end *)
(* the following line achieves that in a simple but very inefficient way *)
- in let g' = List.append g [d]
+ in let h' = List.append h [d]
(* next we assign 'x' to location newindex *)
in let r' = fun v ->
if v = bound_variable then newindex else r v
- in (r',g')
+ in (r',h')
* At the top of p. 13 (this is in between defs 2.8 and 2.9), GS&V give two examples, one for \[[∃xPx]] and the other for \[[Qx]]. In fact it will be easiest for us to break \[[∃xPx]] into two pieces, \[[∃x]] and \[[Px]]. Let's consider expressions like \[[Px]] first.
- They say that the effect of updating an information state `s` with the meaning of "Qx" should be to eliminate possibilities in which the object associated with the peg associated with the variable `x` does not have the property Q. In other words, if we let `Q` be a function from objects to `bool`s, `s` updated with \[[Qx]] should be `s` filtered by the function `fun (r, g) -> let obj = List.nth g (r 'x') in Q obj`. When `...Q obj` evaluates to `true`, that `(r, g)` pair is retained, else it is discarded.
+ They say that the effect of updating an information state `s` with the meaning of "Qx" should be to eliminate possibilities in which the object associated with the peg associated with the variable `x` does not have the property Q. In other words, if we let `Q` be a function from objects to `bool`s, `s` updated with \[[Qx]] should be `s` filtered by the function `fun (r, h) -> let obj = List.nth h (r 'x') in Q obj`. When `...Q obj` evaluates to `true`, that `(r, h)` pair is retained, else it is discarded.
Recall that [we said before](/hints/assignment_7_hint_2) that `List.filter (test : 'a -> bool) (u : 'a set) : 'a set` is the same as:
@@ -36,9 +36,9 @@ More specifically, \[[expression]] will be a set of `'a discourse_possibility` m
Hence, updating `s` with \[[Qx]] should be:
- bind_set s (fun (r, g) -> if (let obj = List.nth g (r 'x') in Q obj) then unit_set (r, g) else empty_set)
+ bind_set s (fun (r, h) -> if (let obj = List.nth h (r 'x') in Q obj) then unit_set (r, h) else empty_set)
- We can call the `(fun (r, g) -> ...)` part \[[Qx]] and then updating `s` with \[[Qx]] will be:
+ We can call the `(fun (r, h) -> ...)` part \[[Qx]] and then updating `s` with \[[Qx]] will be:
bind_set s \[[Qx]]
@@ -62,14 +62,14 @@ More specifically, \[[expression]] will be a set of `'a discourse_possibility` m
Deferring the "property P" part, this says:
s updated with \[[∃x]] ≡
- s >>= (fun (r, g) -> List.map (fun d -> newpeg_and_bind 'x' d) domain)
+ s >>= (fun (r, h) -> List.map (fun d -> newpeg_and_bind 'x' d) domain)
- That is, for each pair `(r, g)` in `s`, we collect the result of extending `(r, g)` by allocating a new peg for object `d`, for each `d` in our whole domain of objects (here designated `domain`), and binding the variable `x` to the index of that peg.
+ That is, for each pair `(r, h)` in `s`, we collect the result of extending `(r, h)` by allocating a new peg for object `d`, for each `d` in our whole domain of objects (here designated `domain`), and binding the variable `x` to the index of that peg.
A later step can then filter out all the possibilities in which the object `d` we did that with doesn't have property P.
- So if we just call the function `(fun (r, g) -> ...)` above \[[∃x]], then `s` updated with \[[∃x]] updated with \[[Px]] is just:
+ So if we just call the function `(fun (r, h) -> ...)` above \[[∃x]], then `s` updated with \[[∃x]] updated with \[[Px]] is just:
s >>= \[[∃x]] >>= \[[Px]]
@@ -87,8 +87,8 @@ More specifically, \[[expression]] will be a set of `'a discourse_possibility` m
Here's how we can represent that:
- bind_set s (fun (r, g) ->
- let u = unit_set (r, g)
+ bind_set s (fun (r, h) ->
+ let u = unit_set (r, h)
in let descendents = u >>= \[[φ]]
in if descendents = empty_set then u else empty_set