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=d69c92e7ed4d65fc9bfef71003559acb4efed46c;hb=0feebbaaa58403d836d7ea6166cf709dd3faf1a8;hpb=04296a25e1123f3e7240727f1e10142a1afadb7e;ds=sidebyside

diff --git a/hints/assignment_7_hint_6.mdwn b/hints/assignment_7_hint_6.mdwn
index d69c92e7..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)