Here's the mutability homework

elm-todo/Todo.elm

@@ -45,7 +45,6 @@ type alias Model =
 type alias Entry =
     { description : String
     , completed : Bool
-    , editing : Bool
     , id : Int
     }
 
@@ -62,7 +61,6 @@ newEntry : String -> Int -> Entry
 newEntry desc id =
     { description = desc
     , completed = False
-    , editing = False
     , id = id
     }
 
@@ -76,7 +74,6 @@ to them.
 -}
 type Msg
     = UpdateNewEntryField String
-    | EditingEntry Int Bool
     | UpdateEntry Int String
     | Add
     | Delete Int
@@ -102,19 +99,6 @@ 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 +213,7 @@ viewKeyedEntry todo =
 viewEntry : Entry -> Html Msg
 viewEntry todo =
     li
-        [ classList [ ( "completed", todo.completed ), ( "editing", todo.editing ) ] ]
+        [ classList [ ( "completed", todo.completed ) ] ]
         [ div
             [ class "view" ]
             [ input
@@ -240,24 +224,14 @@ viewEntry todo =
                 ]
                 []
             , label
-                [ onDoubleClick (EditingEntry todo.id True) ]
+                [ ]
                 [ text todo.description ]
             , button
                 [ class "destroy"
                 , onClick (Delete todo.id)
                 ]
                 []
             ]
-        , 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 +287,7 @@ 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" ]
             ]

ruby-todo/lib/engine.rb

@@ -5,7 +5,7 @@ def self.run(*args)
 
   def self.run_with_history(model, messages)
     messages.map do |msg|
-      msg.apply_to(model)
+      model = msg.apply_to(model)
       model
     end
   end

ruby-todo/lib/messages.rb

@@ -2,13 +2,16 @@ module Msg
 
   class Add
     def apply_to(model)
+      new_entries = [] + model.entries
+      new_model = Model.new(entries: new_entries,
+                            new_entry_field: "",
+                            next_id: model.next_id+1)
       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 = ""
+      new_model
     end
   end
 
@@ -20,7 +23,9 @@ def initialize(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 +34,22 @@ 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
         end
       end
+      Model.new(entries: new_entries,
+                new_entry_field: model.new_entry_field,
+                next_id: model.next_id)
     end
   end
 
@@ -45,16 +58,22 @@ 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.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.reject(&:completed)
+      Model.new(entries: new_entries,
+                new_entry_field: model.new_entry_field,
+                next_id: model.next_id)
     end
   end
 

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? &&

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"),