* In def 2.5, GS&V say the denotation of an e-type constant <code>α</code> wrt a discourse possibility `(r, h, w)` is whatever entity the world `w` associates with <code>α</code>. Since we don't have worlds, this will just be an entity.
- 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 predicate is whatever extension the world `w` associates with the predicate. Since we don't have worlds, this will just be an extension, or a function from entities to `bool`s.
They say the denotation of a variable is the entity 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'll just talk about \[[expression]] and let that be a monadic value, implemented in part by a function that takes `(r, h)` as an argument.
- More specifically, \[[expression]] will be a set of `'a dpm`s, where `'a` is the appropriate type for *expression*. Each `'a dpm` is implemented by a function that takes `(r, h)` as an argument.
-
* 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 (i) allocating a new location in the store, (ii) putting some entity `d` from the domain in that location, and (iii) assigning variable `x` to that location in the store.
It will be useful to have a shorthand way of referring to this operation:
let new_peg_and_assign (var_to_bind : char) (d : entity) =
fun ((r, h) : assignment * store) ->
(* first we calculate an unused index *)
- let newindex = List.length h
- (* next we store d at h[newindex], which is at the very end of h *)
+ let new_index = List.length h
+ (* next we store d at h[new_index], which is at the very end of h *)
(* the following line achieves that in a simple but inefficient way *)
in let h' = List.append h [d]
- (* next we assign 'x' to location newindex *)
- in let r' = fun v ->
- if v = var_to_bind then newindex else r v
- in (r',h')
+ (* next we assign 'x' to location new_index *)
+ in let r' = fun var ->
+ if var = var_to_bind then new_index else r var
+ (* the reason for returning true as an initial element will emerge later *)
+ in (true, r', h')
-* Is that enough? If not, here are some [more hints](/hints/assignment_7_hint_4).
+* Is that enough? If not, here are some [more hints](/hints/assignment_7_hint_4). But try to get as far as you can on your own.