X-Git-Url: http://lambda.jimpryor.net/git/gitweb.cgi?p=lambda.git;a=blobdiff_plain;f=hints%2Fassignment_7_hint_6.mdwn;h=fd8f55d4ff6a160aea1b75693e22634993abe579;hp=0d9f02e8af9f33c454cfadef6dc6ce8ddf37dd95;hb=bf8d964cc93f6b0b44a432bca8c94b1374c05e1f;hpb=c6f572fd09dbe53dff30a91cd07b318b3e5edfd6;ds=sidebyside

diff --git a/hints/assignment_7_hint_6.mdwn b/hints/assignment_7_hint_6.mdwn
index 0d9f02e8..fd8f55d4 100644
--- a/hints/assignment_7_hint_6.mdwn
+++ b/hints/assignment_7_hint_6.mdwn
@@ -19,35 +19,39 @@
                 let new_dpm = fun (r, h) ->
 					(* 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, _, _) = one_dpm (r, h)
+					   set_bind (set_unit 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;;
+                    in (truth_value', r', h')
+				in set_unit new_dpm;;
 
 
+	**Thanks to Simon Charlow** for catching a subtle error in previous versions of this function. Fixed 1 Dec.
+
 *	Representing \[[and φ ψ]] is simple:
 
 		let and_op (phi : clause) (psi : clause) : clause =
-			fun one_dpm -> bind_set (phi one_dpm) psi;;
+			fun one_dpm -> set_bind (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) =
-            fun one_dpm -> unit_set (
+            fun one_dpm -> set_unit (
                 fun (r, h) ->
-					in let truth_value' = (
+					let truth_value' = (
 						truths (phi one_dpm) (r, h) <> [] ||
-						truths (bind_set (negate_op phi one_dpm) psi) (r, h) <> []
+						truths (set_bind (negate_op phi one_dpm) psi) (r, h) <> []
 					) in (truth_value', r, h))
 
 		let if_op (phi : clause) (psi : clause) : clause =
-            fun one_dpm -> unit_set (
+            fun one_dpm -> set_unit (
               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)
@@ -61,21 +65,23 @@
 		type assignment = char -> int;;
 		type store = entity list;;
 		type 'a dpm = assignment * store -> 'a * assignment * store;;
-		let unit_dpm (x : 'a) : 'a dpm = fun (r, h) -> (x, r, h);;
-		let bind_dpm (u: 'a dpm) (f : 'a -> 'b dpm) : 'b dpm =
+		let dpm_unit (x : 'a) : 'a dpm = fun (r, h) -> (x, r, h);;
+		let dpm_bind (u: 'a dpm) (f : 'a -> 'b dpm) : 'b dpm =
 			fun (r, h) ->
 				let (a, r', h') = u (r, h)
 				in let u' = f a
 				in u' (r', h')
 
 		type 'a set = 'a list;;
-		let empty_set : 'a set = [];;
-		let unit_set (x : 'a) : 'a set = [x];;
-		let bind_set (u : 'a set) (f : 'a -> 'b set) : 'b set =
+		let set_empty : 'a set = [];;
+		let set_unit (x : 'a) : 'a set = [x];;
+		let set_bind (u : 'a set) (f : 'a -> 'b set) : 'b set =
 			List.concat (List.map f u);;
 
 		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) ->
@@ -87,20 +93,18 @@
 			fun entity_dpm ->
 				let eliminator = fun truth_value ->
 					if truth_value = false
-					then unit_dpm false
-					else bind_dpm entity_dpm (fun e -> unit_dpm (f e))
-				in fun one_dpm -> unit_set (bind_dpm one_dpm eliminator);;
+					then dpm_unit false
+					else dpm_bind entity_dpm (fun e -> dpm_unit (f e))
+				in fun one_dpm -> set_unit (dpm_bind one_dpm eliminator);;
 
 		(* 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 ->
 					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)))
-				in fun one_dpm -> unit_set (bind_dpm one_dpm eliminator);;
-
-&nbsp;
+					then dpm_unit false
+					else dpm_bind entity1_dpm (fun e1 -> dpm_bind entity2_dpm (fun e2 -> dpm_unit (f e1 e2)))
+				in fun one_dpm -> set_unit (dpm_bind one_dpm eliminator);;
 
 		let new_peg_and_assign (var_to_bind : char) (d : entity) : bool -> bool dpm =
 		  fun truth_value ->
@@ -114,13 +118,15 @@
 		(* from hint 5 *)
 		let exists var : clause =
 			let extend one_dpm (d : entity) =
-				bind_dpm one_dpm (new_peg_and_assign var d)
+				dpm_bind 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 (>>=) = set_bind;;
 		let getx = get 'x';;
 		let gety = get 'y';;
 		let initial_set = [fun (r,h) -> (true,r,h)];;