diff --git a/elm-package.json b/elm-package.json
index 3c3d71d..9ccdb84 100644
--- a/elm-package.json
+++ b/elm-package.json
@@ -10,7 +10,8 @@
     "dependencies": {
         "elm-lang/core": "5.1.1 <= v < 6.0.0",
         "elm-lang/html": "2.0.0 <= v < 3.0.0",
-        "Bogdanp/elm-combine": "3.1.1 <= v < 4.0.0"
+        "Bogdanp/elm-combine": "3.1.1 <= v < 4.0.0",
+        "mhoare/elm-stack": "3.1.1 <= v < 4.0.0"
     },
     "elm-version": "0.18.0 <= v < 0.19.0"
 }
diff --git a/src/Main.elm b/src/Main.elm
index 3aeb66b..976dd13 100644
--- a/src/Main.elm
+++ b/src/Main.elm
@@ -4,22 +4,81 @@ import Html exposing (..)
 import Html.Attributes exposing (..)
 import Html.Events exposing (..)
 
+import Expr exposing (Expr(..), Op(..))
+
+import Stack exposing (Stack(..))
+
 main = Html.beginnerProgram { model = model, update = update, view = view }
 
 type alias Model =
-    { expression : String
-    , result : List Float
+    { result : Result (List String) (List Float)
+    , expression : String
     }
 
 model : Model
-model = Model "" []
+model =
+    { result = Ok []
+    , expression = ""
+    }
 
 type Msg = ExpressionTyped String
 
 update : Msg -> Model -> Model
 update msg model =
-    model
+    case msg of
+        ExpressionTyped str ->
+            Expr.parse ("(" ++ str ++ ")") -- wrap str in brackets so it's recognized as a group
+            |> Result.andThen eval -- Convert stack underflow errors into a list
+            |> \r -> { model | result = r, expression = str }
+
+error : String -> Html a
+error err =
+    div [class "error"] [text err]
+
+stackItem : Float -> Html a
+stackItem n =
+    div [class "item"] [text <| toString n]
 
 view : Model -> Html Msg
 view model =
-    div [] []
\ No newline at end of file
+    let calcOutput = 
+        case model.result of
+            Ok stack ->
+                List.reverse stack -- Puts first items at the top, for nicer looks
+                |> List.map stackItem
+            Err errors ->
+                List.map error errors
+    in div [] (
+        [ input [onInput ExpressionTyped, value model.expression] []
+        ] ++ calcOutput
+    )
+
+-- Run a binary operation on a stack - returns updated stack or an error
+binOp : (Float -> Float -> Float) -> Stack Float -> Result String (Stack Float)
+binOp f s =
+    let (maybeX, s1) = Stack.pop s
+        (maybeY, s2) = Stack.pop s1
+        maybeXY = Maybe.map2 (,) maybeX maybeY
+        result = Maybe.map (\(x, y) -> f y x) maybeXY -- x and y swapped round - this makes "5 1 /" become 5 instead of 0.2.
+        finalStack = Maybe.map (\r -> Stack.push r s2) result
+    in Result.fromMaybe "Stack underflow" finalStack
+
+evalRec : Expr -> Stack Float -> Result String (Stack Float)
+evalRec expr s =
+    case expr of
+        Group es ->
+            List.foldl (\expr s -> Result.andThen (evalRec expr) s) (Ok s) es
+        Num n ->
+            Ok (Stack.push n s)
+        Op o ->
+            case o of
+                Add -> binOp (+) s
+                Subtract -> binOp (-) s
+                Multiply -> binOp (*) s
+                Divide -> binOp (/) s
+
+eval : Expr -> Result (List String) (List Float)
+eval e =
+    evalRec e (Stack.initialise)
+    |> Result.map Stack.toList -- Convert stack to list
+    |> Result.mapError List.singleton -- Wrap possible stackoverflow error in list
\ No newline at end of file