Add multiplication

However, the simple way in which expressions are evaluated have
to be changed since operator precedence is not taken into
consideration.

src/Calculator.elm

@@ -7,6 +7,15 @@ module Calculator exposing
   )
 
 
+-- TODO:
+--
+-- Respect operator precedence when evaluating expressions.
+--   Yes. 1 + 2 * 3 = 1 + 6 = 7
+--    No. 1 + 2 * 3 = 3 * 3 = 9 <---- This is current behaviour.
+--
+-- The CodePen example uses JavaScript's eval.
+
+
 type Calculator
   = Start
   | Left Int
@@ -25,12 +34,14 @@ type Key
 type Operator
   = Plus
   | Minus
+  | Times
 
 
 type Expr
   = Const Int
   | Add Expr Expr
   | Sub Expr Expr
+  | Mul Expr Expr
 
 
 new : Calculator
@@ -128,6 +139,9 @@ eval expr =
     Sub a b ->
       (eval a) - (eval b)
 
+    Mul a b ->
+      (eval a) * (eval b)
+
 
 operatorToExpr : Operator -> Expr -> Expr -> Expr
 operatorToExpr op =
@@ -138,6 +152,9 @@ operatorToExpr op =
     Minus ->
       Sub
 
+    Times ->
+      Mul
+
 
 type alias Display =
   { expr : String
@@ -192,6 +209,9 @@ exprToString expr =
     Sub a b ->
       (exprToString a) ++ "-" ++ (exprToString b)
 
+    Mul a b ->
+      (exprToString a) ++ "*" ++ (exprToString b)
+
 
 operatorToString : Operator -> String
 operatorToString op =
@@ -201,3 +221,6 @@ operatorToString op =
 
     Minus ->
       "-"
+
+    Times ->
+      "*"

src/Main.elm

@@ -75,7 +75,7 @@ viewCalculator calculator =
               [ text "AC" ]
           , button [ class "r0 c2", disabled True ]
               [ text "÷" ]
-          , button [ class "r0 c3", disabled True ]
+          , button [ class "r0 c3", onClick (Clicked (Operator Times)) ]
               [ text "×" ]
           , button [ class "r1 c0", onClick (Clicked (Digit 7)) ]
               [ text "7" ]

tests/Test/Calculator.elm

@@ -10,7 +10,9 @@ import Calculator exposing (Key(..), Operator(..))
 suite : Test
 suite =
   describe "Calculator"
-    [ processSuite ]
+    [ processSuite
+    , operatorPrecedenceSuite
+    ]
 
 
 processSuite : Test
@@ -185,3 +187,24 @@ processSuite =
                   |> Expect.equal { expr = "12+3-4=11", output = "11" }
           ]
     ]
+
+
+operatorPrecedenceSuite : Test
+operatorPrecedenceSuite =
+  describe "operator precedence" <|
+    [ test "multiplication is done before addition" <|
+        \_ ->
+          let
+            calculator =
+              Calculator.new
+                |> Calculator.process (Digit 1)
+                |> Calculator.process (Operator Plus)
+                |> Calculator.process (Digit 2)
+                |> Calculator.process (Operator Times)
+                |> Calculator.process (Digit 3)
+                |> Calculator.process Equal
+          in
+            calculator
+              |> Calculator.toDisplay
+              |> Expect.equal { expr = "1+2*3=7", output = "7" }
+    ]