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=0d9f02e8af9f33c454cfadef6dc6ce8ddf37dd95;hb=9efbe94f74c2ea61522fcdb3e3d012fde6034fcd;hpb=c6f572fd09dbe53dff30a91cd07b318b3e5edfd6;ds=sidebyside

diff --git a/hints/assignment_7_hint_6.mdwn b/hints/assignment_7_hint_6.mdwn
index 0d9f02e8..e513284b 100644
--- a/hints/assignment_7_hint_6.mdwn
+++ b/hints/assignment_7_hint_6.mdwn
@@ -20,13 +20,15 @@
 					(* 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)
+				    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 (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 φ ψ]] is simple:
 
 		let and_op (phi : clause) (psi : clause) : clause =
@@ -36,10 +38,12 @@
 
 *	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 (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) <> []
 					) in (truth_value', r, h))
@@ -47,7 +51,7 @@
 		let if_op (phi : clause) (psi : clause) : clause =
             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)
@@ -76,6 +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) ->
@@ -100,8 +106,6 @@
 					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;
-
 		let new_peg_and_assign (var_to_bind : char) (d : entity) : bool -> bool dpm =
 		  fun truth_value ->
 			  fun (r, h) ->
@@ -119,6 +123,8 @@
 
 		(* include negate_op, and_op, or_op, and if_op as above *)
 
+*	More:
+
 		(* some handy utilities *)
 		let (>>=) = bind_set;;
 		let getx = get 'x';;