Deferring the "property P" part, this corresponds to:
<pre><code>u updated with \[[∃x]] ≡
- let extend_one = fun one_dpm ->
- fun truth_value ->
- if truth_value = false
- then empty_set
- else List.map (fun d -> new_peg_and_assign 'x' d) domain
+ let extend_one = fun (one_dpm : bool dpm) ->
+ List.map (fun d -> bind_dpm one_dpm (new_peg_and_assign 'x' d)) domain
in bind_set u extend_one
</code></pre>
where `new_peg_and_assign` is the operation we defined in [hint 3](/hints/assignment_7_hint_3):
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 *)
- (* 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
- (* the reason for returning true as an initial element should now be apparent *)
- in (true, r',h')
+ (* we want to return not a function that we can bind to a bool dpm *)
+ fun (truth_value : bool) : bool dpm ->
+ fun ((r, h) : assignment * store) ->
+ (* first we calculate an unused index *)
+ 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 new_index *)
+ in let r' = fun var ->
+ if var = var_to_bind then new_index else r var
+ (* we pass through the same truth_value that we started with *)
+ in (truth_value, r', h')
- What's going on in this representation of `u` updated with \[[∃x]]? For each `bool dpm` in `u` that wraps a `true`, we collect `dpm`s that are the result of extending their input `(r, h)` by allocating a new peg for entity `d`, for each `d` in our whole domain of entities, and binding the variable `x` to the index of that peg. For `bool dpm`s in `u` that wrap `false`, we just discard them. We could if we wanted instead return `unit_set (unit_dpm false)`.
+ What's going on in this representation of `u` updated with \[[∃x]]? For each `bool dpm` in `u`, we collect `dpm`s that are the result of passing through their `bool`, but extending their input `(r, h)` by allocating a new peg for entity `d`, for each `d` in our whole domain of entities, and binding the variable `x` to the index of that peg.
- A later step can then filter out all the `dpm`s according to which the
-entity `d` we did that with doesn't have property P.
+ A later step can then filter out all the `dpm`s according to which the entity `d` we did that with doesn't have property P.
So if we just call the function `extend_one` defined above \[[∃x]], then `u` updated with \[[∃x]] updated with \[[Px]] is just:
<pre><code>bind_set (bind_set u \[[∃x]]) \[[Px]]
</code></pre>
-* Can you figure out how to handle \[[not φ]] on your own? If not, here are some [more hints](/hints/assignment_7_hint_6).
+* Let's compare this to what \[[∃xPx]] would look like on a non-dynamic semantics, for example, where we use a simple reader monad to implement variable binding. Reminding ourselves, we'd be working in a framework like this. (Here we implement environments or assignments as functions from variables to entities, instead of as lists of pairs of variables and entities. An assignment `r` here is what `fun c -> List.assoc c r` would have been in [week6](
+/reader_monad_for_variable_binding).)
+
+ type assignment = char -> entity;;
+ type 'a reader = assignment -> 'a;;
+
+ let unit_reader (x : 'a) = fun r -> x;;
+
+ let bind_reader (u : 'a reader) (f : 'a -> 'b reader) =
+ fun r ->
+ let a = u r
+ in let u' = f a
+ in u' r;;
+
+ let getx = fun r -> r 'x';;
+
+ let lift (predicate : entity -> bool) =
+ fun entity_reader ->
+ fun r ->
+ let obj = entity_reader r
+ in unit_reader (predicate obj)
+
+ `lift predicate` converts a function of type `entity -> bool` into one of type `entity reader -> bool reader`. The meaning of \[[Qx]] would then be:
+
+ <pre><code>\[[Q]] ≡ lift q
+ \[[x]] & equiv; getx
+ \[[Qx]] ≡ \[[Q]] \[[x]] ≡
+ fun r ->
+ let obj = getx r
+ in unit_reader (q obj)
+ </code></pre>
+
+ Recall also how we defined \[[lambda x]], or as [we called it before](/reader_monad_for_variable_binding), \\[[who(x)]]:
+
+ let shift (var_to_bind : char) entity_reader (v : 'a reader) =
+ fun (r : assignment) ->
+ let new_value = entity_reader r
+ (* remember here we're implementing assignments as functions rather than as lists of pairs *)
+ in let r' = fun var -> if var = var_to_bind then new_value else r var
+ in v r'
+
+ Now, how would we implement quantifiers in this setting? I'll assume we have a function `exists` of type `(entity -> bool) -> bool`. That is, it accepts a predicate as argument and returns `true` if any element in the domain satisfies that predicate. We could implement the reader-monad version of that like this:
+
+ fun (lifted_predicate : entity reader -> bool reader) : bool reader ->
+ fun r -> exists (fun (obj : entity) -> lifted_predicate (unit_reader obj) r)
+
+ That would be the meaning of \[[∃]], which we'd use like this:
+
+ <pre><code>\[[∃]] \[[Q]]
+ </code></pre>
+
+ or this:
+
+ <pre><code>\[[∃]] ( \[[lambda x]] \[[Qx]] )
+ </code></pre>
+
+ If we wanted to compose \[[∃]] with \[[lambda x]], we'd get:
+
+ let shift var_to_bind entity_reader v =
+ fun r ->
+ let new_value = entity_reader r
+ in let r' = fun var -> if var = var_to_bind then new_value else r var
+ in v r'
+ in let lifted_exists =
+ fun lifted_predicate ->
+ fun r -> exists (fun obj -> lifted_predicate (unit_reader obj) r)
+ in fun bool_reader -> lifted_exists (shift 'x' getx bool_reader)
+
+ which we can simplify to:
+
+ let shifted v =
+ fun r ->
+ let new_value = r 'x'
+ in let r' = fun var -> if var = 'x' then new_value else r var
+ in v r'
+ in let lifted_exists =
+ fun lifted_predicate ->
+ fun r -> exists (fun obj -> lifted_predicate (unit_reader obj) r)
+ in fun bool_reader -> lifted_exists (shifted bool_reader)
+
+ and simplifying further:
+
+ fun bool_reader ->
+ let shifted v =
+ fun r ->
+ let new_value = r 'x'
+ in let r' = fun var -> if var = 'x' then new_value else r var
+ in v r'
+ let lifted_predicate = shifted bool_reader
+ in fun r -> exists (fun obj -> lifted_predicate (unit_reader obj) r)
+
+ fun bool_reader ->
+ let lifted_predicate = fun r ->
+ let new_value = r 'x'
+ in let r' = fun var -> if var = 'x' then new_value else r var
+ in bool_reader r'
+ in fun r -> exists (fun obj -> lifted_predicate (unit_reader obj) r)
+
+
+* Can you figure out how to handle \[[not φ]] on your own? If not, here are some [more hints](/hints/assignment_7_hint_6). But try to get as far as you can on your own.