Fix exercism#286 - Linked List: change pop and shift to return option

exercises/linked-list/Example.fs

@@ -12,19 +12,19 @@ let addToEmpty newValue linkedList =
 
 let pop linkedList =
     match linkedList.last with
-    | None -> failwith "Cannot pop from empty list"
+    | None -> None
     | Some oldLast ->
         linkedList.last <- oldLast.prev
         linkedList.last |> Option.iter (fun el -> el.next <- None)
-        oldLast.value
+        Some oldLast.value
 
 let shift linkedList =
     match linkedList.first with
-    | None -> failwith "Cannot shift from empty list"
+    | None -> None
     | Some oldFirst ->
         linkedList.first <- oldFirst.next
         linkedList.first |> Option.iter (fun el -> el.prev <- None)
-        oldFirst.value
+        Some oldFirst.value
 
 let push newValue linkedList =
     match linkedList.last with

exercises/linked-list/LinkedListTest.fs

@@ -4,6 +4,14 @@ open NUnit.Framework
 open LinkedList
 
 [<Test>]
+let ``Pop returns None if the list is empty`` () =
+    let linkedList = mkLinkedList ()
+    let value = pop linkedList
+
+    Assert.That(value, Is.EqualTo(None))
+
+[<Test>]
+[<Ignore("Remove to run test")>]
 let ``Push and pop are first in last out order`` () =
     let linkedList = mkLinkedList ()
     linkedList |> push 10
@@ -12,8 +20,15 @@ let ``Push and pop are first in last out order`` () =
     let val1 = pop linkedList
     let val2 = pop linkedList
 
-    Assert.That(val1, Is.EqualTo(20))
-    Assert.That(val2, Is.EqualTo(10))
+    Assert.That(val1, Is.EqualTo(Some 20))
+    Assert.That(val2, Is.EqualTo(Some 10))
+
+[<Test>]
+let ``Shift returns None if the list is empty`` () =
+    let linkedList = mkLinkedList ()
+    let value = shift linkedList
+
+    Assert.That(value, Is.EqualTo(None))
 
 [<Test>]
 [<Ignore("Remove to run test")>]
@@ -25,8 +40,8 @@ let ``Push and shift are first in first out order`` () =
     let val1 = shift linkedList
     let val2 = shift linkedList
 
-    Assert.That(val1, Is.EqualTo(10))
-    Assert.That(val2, Is.EqualTo(20))
+    Assert.That(val1, Is.EqualTo(Some 10))
+    Assert.That(val2, Is.EqualTo(Some 20))
 
 [<Test>]
 [<Ignore("Remove to run test")>]
@@ -38,8 +53,8 @@ let ``Unshift and shift are last in first out order`` () =
     let val1 = shift linkedList
     let val2 = shift linkedList
 
-    Assert.That(val1, Is.EqualTo(20))
-    Assert.That(val2, Is.EqualTo(10))
+    Assert.That(val1, Is.EqualTo(Some 20))
+    Assert.That(val2, Is.EqualTo(Some 10))
 
 [<Test>]
 [<Ignore("Remove to run test")>]
@@ -51,8 +66,8 @@ let ``Unshift and pop are last in last out order`` () =
     let val1 = pop linkedList
     let val2 = pop linkedList
 
-    Assert.That(val1, Is.EqualTo(10))
-    Assert.That(val2, Is.EqualTo(20))
+    Assert.That(val1, Is.EqualTo(Some 10))
+    Assert.That(val2, Is.EqualTo(Some 20))
 
 [<Test>]
 [<Ignore("Remove to run test")>]
@@ -66,9 +81,9 @@ let ``Push and pop can handle multiple values`` () =
     let val2 = pop linkedList
     let val3 = pop linkedList
 
-    Assert.That(val1, Is.EqualTo(30))
-    Assert.That(val2, Is.EqualTo(20))
-    Assert.That(val3, Is.EqualTo(10))
+    Assert.That(val1, Is.EqualTo(Some 30))
+    Assert.That(val2, Is.EqualTo(Some 20))
+    Assert.That(val3, Is.EqualTo(Some 10))
 
 [<Test>]
 [<Ignore("Remove to run test")>]
@@ -82,9 +97,9 @@ let ``Unshift and shift can handle multiple values`` () =
     let val2 = shift linkedList
     let val3 = shift linkedList
 
-    Assert.That(val1, Is.EqualTo(30))
-    Assert.That(val2, Is.EqualTo(20))
-    Assert.That(val3, Is.EqualTo(10))
+    Assert.That(val1, Is.EqualTo(Some 30))
+    Assert.That(val2, Is.EqualTo(Some 20))
+    Assert.That(val3, Is.EqualTo(Some 10))
 
 [<Test>]
 [<Ignore("Remove to run test")>]
@@ -95,12 +110,12 @@ let ``All methods of manipulating the linkedList can be used together`` () =
 
     let val1 = pop linkedList
 
-    Assert.That(val1, Is.EqualTo(20))
+    Assert.That(val1, Is.EqualTo(Some 20))
 
     linkedList |> push 30
     let val2 = shift linkedList
 
-    Assert.That(val2, Is.EqualTo(10))
+    Assert.That(val2, Is.EqualTo(Some 10))
 
     linkedList |> unshift 40
     linkedList |> push 50
@@ -109,6 +124,6 @@ let ``All methods of manipulating the linkedList can be used together`` () =
     let val4 = pop linkedList
     let val5 = shift linkedList
 
-    Assert.That(val3, Is.EqualTo(40))
-    Assert.That(val4, Is.EqualTo(50))
-    Assert.That(val5, Is.EqualTo(30))
+    Assert.That(val3, Is.EqualTo(Some 40))
+    Assert.That(val4, Is.EqualTo(Some 50))
+    Assert.That(val5, Is.EqualTo(Some 30))