src/Main.elm

 module Main exposing (main)
 
 import Browser
-import Html exposing (Html, button, div, h1, text)
+import Html exposing (Html, button, div, h1, span, text)
 import Html.Attributes exposing (class)
 import Html.Events exposing (onClick)
 
@@ -12,14 +12,16 @@ import Html.Events exposing (onClick)
 
 type alias Model =
     { stack : List Float
-    , currentNum : Float
+    , currentNum : String
+    , error : Maybe String
     }
 
 
 initialModel : Model
 initialModel =
     { stack = []
-    , currentNum = 0
+    , currentNum = "0"
+    , error = Nothing
     }
 
 
@@ -69,31 +71,55 @@ operatorFunction op =
 type Msg
     = InputOperator Operator
     | InputNumber Float
-    | InputDecimal
     | Clear
     | ClearAll
     | Back
     | Enter
+    | SetDecimal
 
 
 update : Msg -> Model -> Model
 update msg model =
     case msg of
+        SetDecimal ->
+            if String.contains "." model.currentNum then
+                model
+
+            else
+                { model | currentNum = model.currentNum ++ "." }
+
         ClearAll ->
-            { model | currentNum = 0, stack = [] }
+            initialModel
 
         Clear ->
-            { model | currentNum = 0 }
+            { model | currentNum = "0" }
 
         Back ->
-            { model | currentNum = toFloat <| floor <| model.currentNum / 10 }
-
-        Enter ->
+            let
+                newNum =
+                    String.dropRight 1 model.currentNum
+            in
             { model
-                | stack = model.currentNum :: model.stack
-                , currentNum = 0
+                | currentNum =
+                    if String.isEmpty newNum then
+                        "0"
+
+                    else
+                        newNum
             }
 
+        Enter ->
+            let
+                maybeNumber =
+                    String.toFloat model.currentNum
+            in
+            case maybeNumber of
+                Nothing ->
+                    { model | error = Just "PARSE ERR" }
+
+                Just num ->
+                    { model | stack = num :: model.stack, currentNum = "0" }
+
         InputOperator operator ->
             case model.stack of
                 [] ->
@@ -104,19 +130,29 @@ update msg model =
                         op =
                             operatorFunction operator
 
-                        newNum =
-                            op model.currentNum x
+                        maybeNumber =
+                            String.toFloat model.currentNum
                     in
-                    { model
-                        | stack = xs
-                        , currentNum = newNum
-                    }
+                    case maybeNumber of
+                        Nothing ->
+                            { model | error = Just "PARSE ERR" }
+
+                        Just num ->
+                            let
+                                newNum =
+                                    op num x
+                            in
+                            { model
+                                | stack = xs
+                                , currentNum = String.fromFloat newNum
+                            }
 
         InputNumber num ->
-            { model | currentNum = (model.currentNum * 10) + num }
+            if model.currentNum == "0" then
+                { model | currentNum = String.fromFloat num }
 
-        InputDecimal ->
-            model
+            else
+                { model | currentNum = model.currentNum ++ String.fromFloat num }
 
 
 
@@ -192,13 +228,13 @@ section =
         , cell (onClick <| InputNumber 3) Single White "3"
         , cell (onClick <| InputOperator Add) Single Yellow "+"
         , cell (onClick <| InputNumber 0) Single White "0"
-        , cell (onClick InputDecimal) Single White "."
+        , cell (onClick SetDecimal) Single White "."
         , cell (onClick <| Enter) Double Yellow "Enter"
         ]
 
 
-inputBox : Float -> Html Msg
-inputBox num =
+stackBox : Float -> Html Msg
+stackBox num =
     div
         [ class "input-box"
         ]
@@ -206,14 +242,29 @@ inputBox num =
         ]
 
 
+inputBox : Html Msg -> Html Msg
+inputBox num =
+    div
+        [ class "input-box"
+        ]
+        [ num
+        ]
+
+
 view : Model -> Html Msg
 view model =
     div []
         [ h1 [ class "h1" ] [ text "RPN Calculator" ]
         , div
             [ class "calculator" ]
-            (List.map inputBox (List.reverse model.stack)
-                ++ [ inputBox model.currentNum
+            (List.map stackBox (List.reverse model.stack)
+                ++ [ inputBox <|
+                        case model.error of
+                            Nothing ->
+                                text model.currentNum
+
+                            Just err ->
+                                span [ class "error" ] [ text err ]
                    , section
                    ]
             )