Finally, we realize that we're going to have a set of `bool dpm`s to start with, and we need to compose \[[Qx]] with them. We don't want any of the monadic values in the set that wrap `false` to become `true`; instead, we want to apply a filter that checks whether values that formerly wrapped `true` should still continue to do so.
- This is most easily done like this:
+ This could be handled like this:
fun entity_dpm ->
let eliminate_non_Qxs = fun truth_value ->
which is:
- fun truth_value ->
+ let eliminate_non_Qxs = fun truth_value ->
if truth_value
then unit_set (
- let eliminate_non_Qxs = fun (r, h) ->
+ fun (r, h) ->
let obj = List.nth h (r 'x')
let (a, r', h') = (obj, r, h)
in let u' = (fun e -> unit_dpm (Q e)) a
in (truth_value', r, h))
in fun one_dpm -> unit_set (bind_dpm one_dpm eliminate_non_Qxs)
- because that one passed through every `bool dpm` that wrapped a `false`; whereas now we're discarding some of them. But these will work equally well. We can implement either behavior (or, as we said before, the behavior of never passing through a wrapped `false`).
+ because that one passed through every `bool dpm` that wrapped a `false`; whereas now we're discarding some of them. But these will work equally well. We can implement either behavior (or, as we said before, the behavior of never returning any wrapped `false`s).
* Reviewing: now we've determined how to define \[[Q]] and \[[x]] such that \[[Qx]] can be the result of applying the function \[[Q]] to the `entity dpm` \[[x]]. And \[[Qx]] in turn is now a function that takes a `bool dpm` as input and returns a `bool dpm set` as output. We compose this with a `bool dpm set` we already have on hand: