X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=hints%2Fassignment_7_hint_4.mdwn;h=e4f0a7907c95abdd0cb87daa6fe5ae3b1511a269;hp=278ca812a52368fcfced1af38b191d5343a00332;hb=1d5249432378066bbe948bfe0b303704e18468a5;hpb=f5da15db3310f323c87c84ddc650ebcc07e075ed

diff --git a/hints/assignment_7_hint_4.mdwn b/hints/assignment_7_hint_4.mdwn
index 278ca812..e4f0a790 100644
--- a/hints/assignment_7_hint_4.mdwn
+++ b/hints/assignment_7_hint_4.mdwn
@@ -46,13 +46,14 @@
 
 	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 ->
-			fun truth_value ->
+			let eliminate_non_Qxs = fun truth_value ->
 				if truth_value = false
 				then empty_set
-				else unit_set (bind dpm entity_dpm (fun e -> unit_dpm (Q e)))
+				else unit_set (bind_dpm entity_dpm (fun e -> unit_dpm (Q e)))
+			in fun one_dpm -> (bind_dpm one_dpm eliminate_non_Qxs)
 
 	Applied to an `entity_dpm`, that yields a function that we can bind to a `bool dpm set` and that will transform the doubly-wrapped `bool` into a new `bool dpm set`.
 
@@ -64,27 +65,29 @@
 			let obj = List.nth h (r 'x')
 			in (obj, r, h)
 		in let entity_dpm = getx
-		in fun truth_value ->
+		in let eliminate_non_Qxs = fun truth_value ->
 			if truth_value = false
 			then empty_set
 			else unit_set (bind_dpm entity_dpm (fun e -> unit_dpm (Q e)))
+		in fun one_dpm -> (bind_dpm one_dpm eliminate_non_Qxs)
 
 	or, simplifying:
 
 		let getx = fun (r, h) ->
 			let obj = List.nth h (r 'x')
 			in (obj, r, h)
-		in fun truth_value ->
+		in let eliminate_non_Qxs = fun truth_value ->
 			if truth_value
 			then unit_set (bind_dpm getx (fun e -> unit_dpm (Q e)))
 			else empty_set
+		in fun one_dpm -> (bind_dpm one_dpm eliminate_non_Qxs)
 
-	which is:
+	unpacking the definition of `bind_dpm`, that is:
 
 		let getx = fun (r, h) ->
 			let obj = List.nth h (r 'x')
 			in (obj, r, h)
-		in fun truth_value ->
+		in let eliminate_non_Qxs = fun truth_value ->
 			if truth_value
 			then unit_set (
 				fun (r, h) ->
@@ -92,30 +95,35 @@
 					in let u' = (fun e -> unit_dpm (Q e)) a
 					in u' (r', h')
 			) else empty_set
-		
+		in fun one_dpm -> (bind_dpm one_dpm eliminate_non_Qxs)
+
 	which is:
 
-		in fun truth_value ->
+		fun truth_value ->
 			if truth_value
 			then unit_set (
-				fun (r, h) ->
+				let eliminate_non_Qxs = 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 u' (r', h')
 			) else empty_set
-		
+		in fun one_dpm -> (bind_dpm one_dpm eliminate_non_Qxs)
+
 	which is:
 
-		in fun truth_value ->
+		let eliminate_non_Qxs = fun truth_value ->
 			if truth_value
 			then unit_set (
 				fun (r, h) ->
 					let obj = List.nth h (r 'x')
 					in let u' = unit_dpm (Q obj)
-					in u' (r', h')
+					in u' (r, h)
 			) else empty_set
-		
+		in fun one_dpm -> (bind_dpm one_dpm eliminate_non_Qxs)
+
+	This is a function that takes a `bool dpm` as input and returns a `bool dpm set` as output.
+
 	This is a bit different than the \[[Qx]] we had before:
 
 		let eliminate_non_Qxs = (fun truth_value ->
@@ -125,7 +133,7 @@
 					then let obj = List.nth h (r 'x') in Q obj
 					else false
 				in (truth_value', r, h))
-		in (fun one_dpm -> unit_set (bind_dpm one_dpm eliminate_non_Qxs))
+		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`).