X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=hints%2Fassignment_7_hint_6.mdwn;h=e513284bd51d31b5eab302b3ab669e44957bf12f;hp=a371b21c71280a29614f516895747a76fc4f0bba;hb=9efbe94f74c2ea61522fcdb3e3d012fde6034fcd;hpb=60fde0202775a36c5b20c370374649d2a90c6af8

diff --git a/hints/assignment_7_hint_6.mdwn b/hints/assignment_7_hint_6.mdwn
index a371b21c..e513284b 100644
--- a/hints/assignment_7_hint_6.mdwn
+++ b/hints/assignment_7_hint_6.mdwn
@@ -5,41 +5,53 @@
 
 	where `i` *subsists* in <code>s[&phi;]</code> if there are any `i'` that *extend* `i` in <code>s[&phi;]</code>.
 
-	Here's how to do that in our framework. Instead of a possibility subsisting in an updated set of possibilities, we ask what is returned by extensions of a `dpm` when they're given a particular (r, h) as input.
+	Here's how to do that in our framework. Instead of asking whether a possibility subsists in an updated set of possibilities, we ask what is returned by extensions of a `dpm` when they're given a particular (r, h) as input.
 
 		(* filter out which bool dpms in a set are true when receiving (r, h) as input *)
-		let truths set (r, h) = List.filter (fun one_dpm -> let (truth_value, _, _) = one_dpm (r, h) in truth_value) set;;
+		let truths set (r, h) =
+			let test one_dpm =
+				let (truth_value, _, _) = one_dpm (r, h)
+				in truth_value
+			in List.filter test set;;
 
 		let negate_op (phi : clause) : clause =
 			fun one_dpm ->
                 let new_dpm = fun (r, h) ->
-					(* we want to check the behavior of one_dpm when updated with the operation phi *)
-					(* bind_set (unit_set one_dpm) phi === phi one_dpm; do you remember why? *)
-					let truth_value' = truths (phi one_dpm) (r, h) = []
-					(* we return a (bool, r, h) so as to constitute a dpm *)
-                    in (truth_value', r, h)
-				in unit_set new_dpm
+					(* if one_dpm isn't already false at (r, h),
+					   we want to check its behavior when updated with phi
+					   bind_set (unit_set one_dpm) phi === phi one_dpm; do you remember why? *)
+				    let (truth_value, r', h') = one_dpm (r, h)
+					in let truth_value' = truth_value && (truths (phi one_dpm) (r, h) = [])
+					(* new_dpm must return a (bool, r, h) *)
+                    in (truth_value', r', h')
+				in unit_set new_dpm;;
 
 
+	**Thanks to Simon Charlow** for catching a subtle error in previous versions of this function. Fixed 1 Dec.
+
 *	Representing \[[and &phi; &psi;]] is simple:
 
 		let and_op (phi : clause) (psi : clause) : clause =
 			fun one_dpm -> bind_set (phi one_dpm) psi;;
+		(* now u >>= and_op phi psi === u >>= phi >>= psi; do you remember why? *)
+		
 
 *	Here are `or` and `if`:
 
+	(These probably still manifest the bug Simon spotted.)
+
 		let or_op (phi : clause) (psi : clause) =
-			(* NOT: negate_op (and_op (negate_op phi) (negate_op psi)) *)
             fun one_dpm -> unit_set (
                 fun (r, h) ->
-					in let truth_value' = truths (phi one_dpm) (r, h) <> [] || truths (bind_set (negate_op phi one_dpm) psi) (r, h) <> []
-                    in (truth_value', r, h))
+					let truth_value' = (
+						truths (phi one_dpm) (r, h) <> [] ||
+						truths (bind_set (negate_op phi one_dpm) psi) (r, h) <> []
+					) in (truth_value', r, h))
 
 		let if_op (phi : clause) (psi : clause) : clause =
-			(* NOT: negate_op (and_op phi (negate_op psi)) *)
             fun one_dpm -> unit_set (
               fun (r, h) ->
-					in let truth_value' = List.for_all (fun one_dpm ->
+					let truth_value' = List.for_all (fun one_dpm ->
 							let (truth_value, _, _) = one_dpm (r, h)
 							in truth_value = false || truths (psi one_dpm) (r, h) <> []
 						) (phi one_dpm)
@@ -68,6 +80,8 @@
 
 		type clause = bool dpm -> bool dpm set;;
 
+*	More:
+
 		(* this generalizes the getx function from hint 4 *)
 		let get (var : char) : entity dpm =
 			fun (r, h) ->
@@ -77,7 +91,7 @@
 		(* this generalizes the proposal for \[[Q]] from hint 4 *)
 		let lift_predicate (f : entity -> bool) : entity dpm -> clause =
 			fun entity_dpm ->
-				let eliminator = fun (truth_value : bool) ->
+				let eliminator = fun truth_value ->
 					if truth_value = false
 					then unit_dpm false
 					else bind_dpm entity_dpm (fun e -> unit_dpm (f e))
@@ -86,7 +100,7 @@
 		(* doing the same thing for binary predicates *)
 		let lift_predicate2 (f : entity -> entity -> bool) : entity dpm -> entity dpm -> clause =
 			fun entity1_dpm entity2_dpm ->
-				let eliminator = fun (truth_value : bool) ->
+				let eliminator = fun truth_value ->
 					if truth_value = false
 					then unit_dpm false
 					else bind_dpm entity1_dpm (fun e1 -> bind_dpm entity2_dpm (fun e2 -> unit_dpm (f e1 e2)))
@@ -102,11 +116,15 @@
 				in (truth_value, r', h')
 
 		(* from hint 5 *)
-		let exists var : clause = fun one_dpm ->
-			List.map (fun d -> bind_dpm one_dpm (new_peg_and_assign var d)) domain
+		let exists var : clause =
+			let extend one_dpm (d : entity) =
+				bind_dpm one_dpm (new_peg_and_assign var d)
+			in fun one_dpm -> List.map (fun d -> extend one_dpm d) domain
 
 		(* include negate_op, and_op, or_op, and if_op as above *)
 
+*	More:
+
 		(* some handy utilities *)
 		let (>>=) = bind_set;;
 		let getx = get 'x';;