src/Main.elm

@@ -76,6 +76,7 @@ type Msg
     | Back
     | Enter
     | SetDecimal
+    | SetSign
 
 
 update : Msg -> Model -> Model
@@ -88,6 +89,16 @@ update msg model =
             else
                 { model | currentNum = model.currentNum ++ "." }
 
+        SetSign ->
+            if model.currentNum == "0" then
+                model
+
+            else if String.startsWith "-" model.currentNum then
+                { model | currentNum = String.dropLeft 1 model.currentNum }
+
+            else
+                { model | currentNum = "-" ++ model.currentNum }
+
         ClearAll ->
             initialModel
 
@@ -229,7 +240,8 @@ section =
         , cell (onClick <| InputOperator Add) Single Yellow "+"
         , cell (onClick <| InputNumber 0) Single White "0"
         , cell (onClick SetDecimal) Single White "."
-        , cell (onClick <| Enter) Double Yellow "Enter"
+        , cell (onClick SetSign) Single White "+/-"
+        , cell (onClick <| Enter) Single Yellow "Enter"
         ]