Correctly implement For for TaskOption

Author: TheAngryByrd

src/FsToolkit.ErrorHandling.TaskResult/TaskOptionCE.fs

@@ -418,12 +418,20 @@ type TaskOptionBuilderBase() =
                     true)
         )
 
-    member inline _.For
+    member inline this.For
         (
             sequence: seq<'T>,
             body: 'T -> TaskOptionCode<'TOverall, unit>
         ) : TaskOptionCode<'TOverall, unit> =
-        ResumableCode.For(sequence, body)
+        ResumableCode.Using(
+            sequence.GetEnumerator(),
+            // ... and its body is a while loop that advances the enumerator and runs the body on each element.
+            (fun e ->
+                this.While(
+                    (fun () -> e.MoveNext()),
+                    TaskOptionCode<'TOverall, unit>(fun sm -> (body e.Current).Invoke(&sm))
+                ))
+        )
 
     member inline internal this.TryFinallyAsync
         (

tests/FsToolkit.ErrorHandling.TaskResult.Tests/TaskOptionCE.fs

@@ -391,28 +391,35 @@ let ceTests =
               task {
 
                   let mutable loopCount = 0
-                  let expected = Error "error"
+                  let mutable wasCalled = false
+
+                  let sideEffect () =
+                      wasCalled <- true
+                      "ok"
+
+                  let expected = None
 
                   let data =
-                      [ Ok "42"
-                        Ok "1024"
+                      [ Some "42"
+                        Some "1024"
                         expected
-                        Ok "1M"
-                        Ok "1M"
-                        Ok "1M" ]
+                        Some "1M"
+                        Some "1M"
+                        Some "1M" ]
 
                   let! actual =
-                      taskResult {
+                      taskOption {
                           for i in data do
                               let! x = i
                               loopCount <- loopCount + 1
                               ()
 
-                          return "ok"
+                          return sideEffect ()
                       }
 
                   Expect.equal loopCount 2 "Should only loop twice"
                   Expect.equal actual expected "Should be an error"
+                  Expect.isFalse wasCalled "No additional side effects should occur"
               } ]
 
 let specialCaseTask returnValue =