diff --git a/elm-todo/Todo.elm b/elm-todo/Todo.elm
index 3a07b24..69c7455 100755
--- a/elm-todo/Todo.elm
+++ b/elm-todo/Todo.elm
@@ -12,7 +12,6 @@ This clean division of concerns is a core part of Elm. You can read more about
 this in <http://guide.elm-lang.org/architecture/index.html>
 -}
 
-import Dom
 import Html exposing (..)
 import Html.Attributes exposing (..)
 import Html.Events exposing (..)
@@ -45,7 +44,6 @@ type alias Model =
 type alias Entry =
     { description : String
     , completed : Bool
-    , editing : Bool
     , id : Int
     }
 
@@ -62,7 +60,6 @@ newEntry : String -> Int -> Entry
 newEntry desc id =
     { description = desc
     , completed = False
-    , editing = False
     , id = id
     }
 
@@ -76,7 +73,6 @@ to them.
 -}
 type Msg
     = UpdateNewEntryField String
-    | EditingEntry Int Bool
     | UpdateEntry Int String
     | Add
     | Delete Int
@@ -102,19 +98,6 @@ update msg model =
         UpdateNewEntryField str ->
             { model | newEntryField = str }
 
-        EditingEntry id isEditing ->
-            let
-                updateEntry t =
-                    if t.id == id then
-                        { t | editing = isEditing }
-                    else
-                        t
-
-                focus =
-                    Dom.focus ("todo-" ++ toString id)
-            in
-                { model | entries = List.map updateEntry model.entries }
-
         UpdateEntry id task ->
             let
                 updateEntry t =
@@ -229,7 +212,7 @@ viewKeyedEntry todo =
 viewEntry : Entry -> Html Msg
 viewEntry todo =
     li
-        [ classList [ ( "completed", todo.completed ), ( "editing", todo.editing ) ] ]
+        [ classList [ ( "completed", todo.completed ) ] ]
         [ div
             [ class "view" ]
             [ input
@@ -240,7 +223,7 @@ viewEntry todo =
                 ]
                 []
             , label
-                [ onDoubleClick (EditingEntry todo.id True) ]
+                []
                 [ text todo.description ]
             , button
                 [ class "destroy"
@@ -248,14 +231,13 @@ viewEntry todo =
                 ]
                 []
             ]
-        , input
+        ,
+        input
             [ class "edit"
             , value todo.description
             , name "title"
             , id ("todo-" ++ toString todo.id)
             , onInput (UpdateEntry todo.id)
-            , onBlur (EditingEntry todo.id False)
-            , onEnter (EditingEntry todo.id False)
             ]
             []
         ]
@@ -313,8 +295,7 @@ viewControlsClear entriesCompleted =
 infoFooter : Html msg
 infoFooter =
     footer [ class "info" ]
-        [ p [] [ text "Double-click to edit a todo" ]
-        , p []
+        [ p []
             [ text "Written by "
             , a [ href "https://github.com/evancz" ] [ text "Evan Czaplicki" ]
             ]
diff --git a/ruby-todo/lib/engine.rb b/ruby-todo/lib/engine.rb
index a257c20..79c9083 100644
--- a/ruby-todo/lib/engine.rb
+++ b/ruby-todo/lib/engine.rb
@@ -4,9 +4,18 @@ def self.run(*args)
   end
 
   def self.run_with_history(model, messages)
+    new_model = model
     messages.map do |msg|
-      msg.apply_to(model)
-      model
+      new_model = msg.apply_to(new_model)
+      new_model
+
+      # is the same as
+      # new_model = msg.apply_to(new_model)
+      # new_new_model = new_model
+      # new_new_model
+
+      # which is different from
+      # msg.apply_to(new_model)
     end
   end
 end
diff --git a/ruby-todo/lib/messages.rb b/ruby-todo/lib/messages.rb
index abe85de..288d2b4 100644
--- a/ruby-todo/lib/messages.rb
+++ b/ruby-todo/lib/messages.rb
@@ -2,13 +2,13 @@ module Msg
 
   class Add
     def apply_to(model)
+      new_entries = model.entries.dup
       unless model.new_entry_field.blank?
-        model.entries << Entry.new(
+        new_entries << Entry.new(
           description: model.new_entry_field,
           id: model.next_id)
       end
-      model.next_id += 1
-      model.new_entry_field = ""
+      Model.new(entries: new_entries, new_entry_field: "", next_id: model.next_id + 1)
     end
   end
 
@@ -17,10 +17,10 @@ def initialize(str)
       @str = str
     end
 
-    attr_reader :str 
+    attr_reader :str
 
     def apply_to(model)
-      model.new_entry_field = str
+      Model.new(entries: model.entries, new_entry_field: str, next_id: model.next_id)
     end
   end
 
@@ -29,14 +29,18 @@ def initialize(id, is_completed)
       @id, @is_completed = id, is_completed
     end
 
-    attr_reader :id, :is_completed 
+    attr_reader :id, :is_completed
 
     def apply_to(model)
+      new_entries = []
       model.entries.each do |entry|
         if entry.id == id
-          entry.completed = is_completed
+          new_entries << Entry.new(id: entry.id, description: entry.description, completed: is_completed)
+        else
+          new_entries << Entry.new(id: entry.id, description: entry.description, completed: entry.completed)
         end
       end
+      Model.new(entries: new_entries, new_entry_field: model.new_entry_field, next_id: model.next_id)
     end
   end
 
@@ -45,16 +49,20 @@ def initialize(id)
       @id = id
     end
 
-    attr_reader :id 
+    attr_reader :id
 
     def apply_to(model)
-      model.entries.reject! { |e| e.id == id }
+      new_entries = model.entries.dup
+      new_entries.reject! { |e| e.id == id }
+      Model.new(entries: new_entries, new_entry_field: model.new_entry_field, next_id: model.next_id)
     end
   end
 
   class DeleteAllCompleted
     def apply_to(model)
-      model.entries.reject!(&:completed)
+      new_entries = model.entries.dup
+      new_entries.reject!(&:completed)
+      Model.new(entries: new_entries, new_entry_field: model.new_entry_field, next_id: model.next_id)
     end
   end
 
diff --git a/ruby-todo/lib/model.rb b/ruby-todo/lib/model.rb
index 75a6085..e64ca6a 100644
--- a/ruby-todo/lib/model.rb
+++ b/ruby-todo/lib/model.rb
@@ -5,7 +5,7 @@ def initialize(entries: [], new_entry_field: "", next_id: nil)
       (entries.lazy.map(&:id).max || -1) + 1
   end
 
-  attr_accessor :entries, :new_entry_field, :next_id
+  attr_reader :entries, :new_entry_field, :next_id
 
   def ==(other)
     !other.nil? &&
@@ -20,7 +20,7 @@ def initialize(id:, description:, completed: false)
     @description, @id, @completed = description, id, completed
   end
 
-  attr_accessor :description, :completed, :id
+  attr_reader :description, :completed, :id
 
   def ==(other)
     !other.nil? &&
diff --git a/ruby-todo/test/todo_test.rb b/ruby-todo/test/todo_test.rb
index f7436c7..a52609c 100644
--- a/ruby-todo/test/todo_test.rb
+++ b/ruby-todo/test/todo_test.rb
@@ -91,7 +91,7 @@
   end
 
   it "supports time travel" do
-    skip "Mutable model does not support time travel"
+    # skip "Mutable model does not support time travel"
 
     actual_history = Engine.run_with_history(Model.new, [
       Msg::UpdateNewEntryField.new("go forward in time"),
@@ -121,7 +121,7 @@
           Entry.new(id: 0, description: "go forward in time", completed: false)
       ]),
       Model.new(next_id: 3, new_entry_field: "", entries: [
-          Entry.new(id: 0, description: "go forward in time", completed: false), 
+          Entry.new(id: 0, description: "go forward in time", completed: false),
           Entry.new(id: 2, description: "delete this item", completed: false)
       ]),
       Model.new(next_id: 3, new_entry_field: "", entries: [
@@ -134,19 +134,19 @@
           Entry.new(id: 0, description: "go forward in time", completed: false)
       ]),
       Model.new(next_id: 4, new_entry_field: "", entries: [
-          Entry.new(id: 0, description: "go forward in time", completed: false), 
+          Entry.new(id: 0, description: "go forward in time", completed: false),
           Entry.new(id: 3, description: "go backward in time", completed: false)
       ]),
       Model.new(next_id: 4, new_entry_field: "", entries: [
-          Entry.new(id: 0, description: "go forward in time", completed: true), 
+          Entry.new(id: 0, description: "go forward in time", completed: true),
           Entry.new(id: 3, description: "go backward in time", completed: false)
       ]),
       Model.new(next_id: 4, new_entry_field: "", entries: [
-          Entry.new(id: 0, description: "go forward in time", completed: true), 
+          Entry.new(id: 0, description: "go forward in time", completed: true),
           Entry.new(id: 3, description: "go backward in time", completed: true)
       ]),
       Model.new(next_id: 4, new_entry_field: "", entries: [
-          Entry.new(id: 0, description: "go forward in time", completed: true), 
+          Entry.new(id: 0, description: "go forward in time", completed: true),
           Entry.new(id: 3, description: "go backward in time", completed: false)
       ]),
       Model.new(next_id: 4, new_entry_field: "", entries: [
@@ -158,6 +158,8 @@
     # but comparing one step at a time gives more readable error messages:
 
     assert_equal expected_history.size, actual_history.size
+    assert_equal expected_history, actual_history
+
     expected_history.zip(actual_history).each.with_index do |(expected, actual), index|
       assert_equal expected, actual, "History mismatch at step #{index}"
     end
diff --git a/swift-todo/Sources/Todo/Engine.swift b/swift-todo/Sources/Todo/Engine.swift
index e3b14a7..c717840 100644
--- a/swift-todo/Sources/Todo/Engine.swift
+++ b/swift-todo/Sources/Todo/Engine.swift
@@ -13,9 +13,10 @@ struct Engine {
     }
 
     static func runWithHistory(on model: Model, applying messages: [Message]) -> [Model] {
+        var newModel = model
         return messages.map { message in
-          message.apply(to: model)
-          return model
+          newModel = message.apply(to: newModel)
+          return newModel
         }
     }
 }
diff --git a/swift-todo/Sources/Todo/Message.swift b/swift-todo/Sources/Todo/Message.swift
index 92dd868..0a31dd4 100644
--- a/swift-todo/Sources/Todo/Message.swift
+++ b/swift-todo/Sources/Todo/Message.swift
@@ -14,30 +14,38 @@ enum Message {
     case delete(Int)
     case deleteAllCompleted
 
-    func apply(to model: Model) {
-        switch(self) {
+    func apply(to model: Model) -> Model {
+        switch (self) {
             case .add:
+                var newEntries = model.entries
                 if !model.newEntryField.isBlank() {
-                    model.entries.append(Entry(id: model.nextID, description: model.newEntryField))
+                    newEntries.append(Entry(id: model.nextID, description: model.newEntryField))
                 }
-                model.nextID += 1
-                model.newEntryField = ""
+                return Model(nextID: model.nextID + 1, entries: newEntries)
 
             case .updateNewEntryField(let str):
-                model.newEntryField = str
+                return Model(nextID: model.nextID, newEntryField: str, entries: model.entries)
 
             case .check(let id, let isCompleted):
+                var newEntries = [Entry]()
                 for entry in model.entries {
-                    if(entry.id == id) {
-                        entry.completed = isCompleted
+                    if entry.id == id {
+                        newEntries.append(Entry(id: entry.id, description: entry.description, completed: isCompleted))
+                    } else {
+                        newEntries.append(Entry(id: entry.id, description: entry.description, completed: entry.completed))
                     }
                 }
+                return Model(nextID: model.nextID, newEntryField: model.newEntryField, entries: newEntries)
 
             case .delete(let id):
-                model.entries.remove { $0.id == id }
+                var newEntries = model.entries
+                newEntries.remove { $0.id == id }
+                return Model(nextID: model.nextID, newEntryField: model.newEntryField, entries: newEntries)
 
             case .deleteAllCompleted:
-                model.entries.remove { $0.completed }
+                var newEntries = model.entries
+                newEntries.remove { $0.completed }
+                return Model(nextID: model.nextID, newEntryField: model.newEntryField, entries: newEntries)
         }
     }
 }
diff --git a/swift-todo/Sources/Todo/Model.swift b/swift-todo/Sources/Todo/Model.swift
index 1cca801..d50d863 100644
--- a/swift-todo/Sources/Todo/Model.swift
+++ b/swift-todo/Sources/Todo/Model.swift
@@ -1,4 +1,4 @@
-class Model {
+struct Model {
     var entries: [Entry]
     var newEntryField: String
     var nextID: Int
@@ -11,7 +11,7 @@ class Model {
     }
 }
 
-class Entry {
+struct Entry {
     var id: Int
     var description: String
     var completed: Bool
diff --git a/swift-todo/Tests/TodoTests/TodoTests.swift b/swift-todo/Tests/TodoTests/TodoTests.swift
index 9220517..e251bc6 100644
--- a/swift-todo/Tests/TodoTests/TodoTests.swift
+++ b/swift-todo/Tests/TodoTests/TodoTests.swift
@@ -26,6 +26,9 @@ class TodoTests: XCTestCase {
              .updateNewEntryField("hop on one foot"),
              .add
          ])
+        
+        print("test test test")
+        print(newModel)
          XCTAssertEqual(3, newModel.entries.count)
          let newEntry = newModel.entries.last!
          XCTAssertEqual(12, newEntry.id)
@@ -77,7 +80,6 @@ class TodoTests: XCTestCase {
          XCTAssertEqual([11], newModel.entries.map { $0.id })
      }
 
-/*
      func testTimeTravel() {
         let actualHistory = Engine.runWithHistory(on: Model(), applying: [
              .updateNewEntryField("go forward in time"),
@@ -145,6 +147,4 @@ class TodoTests: XCTestCase {
              XCTAssertEqual(expected, actual, "History mismatch at step \(index)")
          }
     }
-*/
-
 }