Add a new helper type for SAFE.Client: Optimistic

src/Client/SAFE.fs

@@ -185,4 +185,53 @@ module RemoteData =
     /// `Loaded x -> Loading x`;
     /// `NotStarted -> Loading None`;
     /// `Loading x -> Loading x`;
-    let startLoading (remote: RemoteData<'T>) = remote.StartLoading
+    let startLoading (remote: RemoteData<'T>) = remote.StartLoading
+
+///A type which represents optimistic updates.
+type Optimistic<'T> =
+    | NonExistant
+    | Exists of value:'T * prev:'T option
+    with
+        /// Retrieves the current value
+        member this.Value =
+            match this with
+            | NonExistant -> None
+            | Exists (v, pv) -> Some v
+
+        /// Updates the current value, shifting the existing current value to previous.
+        member this.Update (value: 'T) =
+            match this with
+            | NonExistant -> NonExistant
+            | Exists (v, pv) -> Exists (value, Some v)
+
+        /// Rolls back to the previous value, discarding the current one.
+        member this.Rollback () =
+            match this with
+            | NonExistant -> NonExistant
+            | Exists (_, Some pv) -> Exists (pv , None)
+            | Exists (_, None) -> NonExistant
+
+        /// Maps the underlying optimistic value, when it exists, into another shape.
+        member this.Map (f: 'T -> 'U) =
+            match this with
+            | NonExistant -> NonExistant
+            | Exists (v, pv) -> Exists (f v, pv |> Option.map f)
+
+/// Module containing functions for working with Optimistic type
+module Optimistic =
+    /// Creates a new Optimistic value with no history
+    let create value =
+        Exists (value, None)
+
+    /// Creates an empty Optimistic value
+    let empty =
+        NonExistant
+
+    /// Updates the current value, shifting existing value to previous
+    let update value (optimistic: Optimistic<'T>) = optimistic.Update value
+
+    /// Rolls back to the previous value
+    let rollback (optimistic: Optimistic<'T>) = optimistic.Rollback()
+
+    /// Maps both current and previous values
+    let map f (optimistic: Optimistic<'T>) = optimistic.Map f

test/Client/Program.fs

@@ -1,4 +1,4 @@
-module Client.Tests
+module Client.Tests
 
 open Fable.Mocha
 open SAFE
@@ -124,6 +124,112 @@ let remoteData =
             | RemoteDataCase.LoadingPopulated -> Loading (Some true)
             | RemoteDataCase.Loaded -> Loading (Some true))
     ]
+let optimistic =
+    testList "Optimistic" [
+        testList "create" [
+            testCase "creates new value with no history" <| fun _ ->
+                let opt = Optimistic.create 42
+                match opt with
+                | Exists (value, prev) ->
+                    Expect.equal value 42 "Current value should be set"
+                    Expect.equal prev None "Previous value should be None"
+                | NonExistant ->
+                    failtest "Should not be NonExistant"
+        ]
+
+        testList "empty" [
+            testCase "creates empty optimistic value" <| fun _ ->
+                let opt = Optimistic.empty
+                Expect.equal opt NonExistant "Should be NonExistant"
+        ]
+
+        testList "Value property" [
+            testCase "returns Some for existing value" <| fun _ ->
+                let opt = Optimistic.create 42
+                Expect.equal opt.Value (Some 42) "Should return Some with current value"
+
+            testCase "returns None for NonExistant" <| fun _ ->
+                let opt = Optimistic.empty
+                Expect.equal opt.Value None "Should return None for NonExistant"
+        ]
+
+        testList "update" [
+            testCase "updates value and shifts previous" <| fun _ ->
+                let opt = Optimistic.create 42
+                let updated = opt.Update 84
+                match updated with
+                | Exists (value, prev) ->
+                    Expect.equal value 84 "Current value should be updated"
+                    Expect.equal prev (Some 42) "Previous value should be old current"
+                | NonExistant ->
+                    failtest "Should not be NonExistant"
+
+            testCase "update on NonExistant remains NonExistant" <| fun _ ->
+                let opt = Optimistic.empty
+                let updated = opt.Update 42
+                Expect.equal updated NonExistant "Should remain NonExistant"
+        ]
+
+        testList "rollback" [
+            testCase "rolls back to previous value" <| fun _ ->
+                let opt = Optimistic.create 42 |> fun o -> o.Update 84
+                let rolled = opt.Rollback()
+                match rolled with
+                | Exists (value, prev) ->
+                    Expect.equal value 42 "Current value should be previous"
+                    Expect.equal prev None "Previous value should be None"
+                | NonExistant ->
+                    failtest "Should not be NonExistant"
+
+            testCase "rollback on NonExistant remains NonExistant" <| fun _ ->
+                let opt = Optimistic.empty
+                let rolled = opt.Rollback()
+                Expect.equal rolled NonExistant "Should remain NonExistant"
+        ]
+
+        testList "map" [
+            testCase "maps both current and previous values" <| fun _ ->
+                let opt = Optimistic.create 42 |> fun o -> o.Update 84
+                let mapped = opt.Map string
+                match mapped with
+                | Exists (value, prev) ->
+                    Expect.equal value "84" "Current value should be mapped"
+                    Expect.equal prev (Some "42") "Previous value should be mapped"
+                | NonExistant ->
+                    failtest "Should not be NonExistant"
+
+            testCase "map on NonExistant remains NonExistant" <| fun _ ->
+                let opt = Optimistic.empty
+                let mapped = opt.Map string
+                Expect.equal mapped NonExistant "Should remain NonExistant"
+        ]
+
+        testList "module functions" [
+            testCase "update function matches member" <| fun _ ->
+                let opt = Optimistic.create 42
+                let memberUpdate = opt.Update 84
+                let moduleUpdate = Optimistic.update 84 opt
+                Expect.equal moduleUpdate memberUpdate "Module update should match member update"
+
+            testCase "rollback function matches member" <| fun _ ->
+                let opt = Optimistic.create 42 |> fun o -> o.Update 84
+                let memberRollback = opt.Rollback()
+                let moduleRollback = Optimistic.rollback opt
+                Expect.equal moduleRollback memberRollback "Module rollback should match member rollback"
+
+            testCase "map function matches member" <| fun _ ->
+                let opt = Optimistic.create 42
+                let memberMap = opt.Map string
+                let moduleMap = Optimistic.map string opt
+                Expect.equal moduleMap memberMap "Module map should match member map"
+        ]
+    ]
+
+let allTests =
+    testList "All Tests" [
+        remoteData
+        optimistic
+    ]
 
 [<EntryPoint>]
-let main _ = Mocha.runTests remoteData
+let main _ = Mocha.runTests allTests