diff --git a/hkmc2/jvm/src/test/scala/hkmc2/JSBackendDiffMaker.scala b/hkmc2/jvm/src/test/scala/hkmc2/JSBackendDiffMaker.scala
index 67e4570ad..4cb790056 100644
--- a/hkmc2/jvm/src/test/scala/hkmc2/JSBackendDiffMaker.scala
+++ b/hkmc2/jvm/src/test/scala/hkmc2/JSBackendDiffMaker.scala
@@ -13,6 +13,8 @@ abstract class JSBackendDiffMaker extends MLsDiffMaker:
   val sjs = NullaryCommand("sjs")
   val showRepl = NullaryCommand("showRepl")
   val silent = NullaryCommand("silent")
+  val expect = Command("expect"): ln =>
+    ln.trim
   
   private val baseScp: codegen.js.Scope =
     codegen.js.Scope.empty
@@ -61,6 +63,7 @@ abstract class JSBackendDiffMaker extends MLsDiffMaker:
         output(s"JS:")
         output(jsStr)
       def mkQuery(prefix: Str, jsStr: Str) =
+        import hkmc2.Message.MessageContext
         val queryStr = jsStr.replaceAll("\n", " ")
         val (reply, stderr) = host.query(queryStr, expectRuntimeErrors.isUnset && fixme.isUnset && todo.isUnset)
         reply match
@@ -73,11 +76,15 @@ abstract class JSBackendDiffMaker extends MLsDiffMaker:
                     output(s"> ${line}")
               content match
               case "undefined" =>
-              case _ => output(s"$prefix= ${content}")
+              case _ =>
+                expect.get match
+                  case S(expected) if content != expected => raise:
+                    ErrorReport(msg"Expected: ${expected}, got: ${content}" -> N :: Nil,
+                      source = Diagnostic.Source.Runtime)
+                  case _ => output(s"$prefix= ${content}")
           case ReplHost.Empty =>
           case ReplHost.Unexecuted(message) => ???
           case ReplHost.Error(isSyntaxError, message) =>
-            import hkmc2.Message.MessageContext
             if (isSyntaxError) then
               // If there is a syntax error in the generated code,
               // it should be a code generation error.
diff --git a/hkmc2/shared/src/test/mlscript/ucs/general/DualOptions.mls b/hkmc2/shared/src/test/mlscript/ucs/general/DualOptions.mls
index a8f6f0e4b..fbf736498 100644
--- a/hkmc2/shared/src/test/mlscript/ucs/general/DualOptions.mls
+++ b/hkmc2/shared/src/test/mlscript/ucs/general/DualOptions.mls
@@ -10,6 +10,7 @@ class Some[T](value: T) extends Option[T]
 module None extends Option[nothing]
 class Pair[A, B](x: A, y: B)
 
+// All `add_n` functions should be inferred to have the same type.
 
 fun add_1(x, y) =
   if
@@ -95,25 +96,25 @@ fun add_4(x, y) =
 :fixme
 add_4(None, None)
 //│ ╔══[ERROR] Name not found: add_4
-//│ ║  l.96: 	add_4(None, None)
+//│ ║  l.97: 	add_4(None, None)
 //│ ╙──      	^^^^^
 
 :fixme
 add_4(Some(5), None)
 //│ ╔══[ERROR] Name not found: add_4
-//│ ║  l.102: 	add_4(Some(5), None)
+//│ ║  l.103: 	add_4(Some(5), None)
 //│ ╙──       	^^^^^
 
 :fixme
 add_4(None, Some(9))
 //│ ╔══[ERROR] Name not found: add_4
-//│ ║  l.108: 	add_4(None, Some(9))
+//│ ║  l.109: 	add_4(None, Some(9))
 //│ ╙──       	^^^^^
 
 :fixme
 add_4(Some(5), Some(9))
 //│ ╔══[ERROR] Name not found: add_4
-//│ ║  l.114: 	add_4(Some(5), Some(9))
+//│ ║  l.115: 	add_4(Some(5), Some(9))
 //│ ╙──       	^^^^^
 
 
@@ -158,6 +159,7 @@ add_6(Some(5), Some(9))
 //│ = 14
 
 
+// Functions from now on have a predicate `p` that can be used to add some preconditions.
 
 
 fun add_6(p, x, y) =
@@ -169,15 +171,23 @@ fun add_6(p, x, y) =
     y is None     and x is None     then 0
 
 
+:expect 0
 add_6((x) => true, None, None)
 //│ = 0
 
+:expect 42
 add_6((x) => true, Some(5), None)
 //│ = 42
 
+:expect 5
+add_6((x) => false, Some(5), None)
+//│ = 5
+
+:expect 9
 add_6((x) => true, None, Some(9))
 //│ = 9
 
+:expect 14
 add_6((x) => true, Some(5), Some(9))
 //│ = 14
 
@@ -192,18 +202,23 @@ fun add_7(p, x, y) =
     y is None     and x is None     then 0
 
 
+:expect 0
 add_7((x) => x > 0, None, None)
 //│ = 0
 
+:expect 5
 add_7((x) => x > 0, Some(5), None)
 //│ = 5
 
+:expect 36
 add_7((x) => x > 0, None, Some(9))
 //│ = 36
 
+:expect -9
 add_7((x) => x > 0, None, Some(-9))
 //│ = -9
 
+:expect 14
 add_7((x) => x > 0, Some(5), Some(9))
 //│ = 14
 
@@ -218,21 +233,27 @@ fun add_8(p, x, y) =
     y is None     and x is None     then 0
 
 
+:expect 0
 add_8((x) => x > 0, None, None)
 //│ = 0
 
+:expect 42
 add_8((x) => true, Some(9), None)
 //│ = 42
 
+:expect 5
 add_8((x) => x > 0, Some(5), None)
 //│ = 5
 
+:expect 36
 add_8((x) => x > 0, None, Some(9))
 //│ = 36
 
+:expect -9
 add_8((x) => x > 0, None, Some(-9))
 //│ = -9
 
+:expect 14
 add_8((x) => x > 0, Some(5), Some(9))
 //│ = 14