Expression parser added

elm-package.json

@@ -9,7 +9,8 @@
     "exposed-modules": [],
     "dependencies": {
         "elm-lang/core": "5.1.1 <= v < 6.0.0",
-        "elm-lang/html": "2.0.0 <= v < 3.0.0"
+        "elm-lang/html": "2.0.0 <= v < 3.0.0",
+        "Bogdanp/elm-combine": "3.1.1 <= v < 4.0.0"
     },
     "elm-version": "0.18.0 <= v < 0.19.0"
 }

src/Expr.elm

@@ -0,0 +1,44 @@
+module Expr exposing (Expr(..), Op(..), parse)
+
+import Combine exposing (..)
+import Combine.Num as Num
+
+type Op
+    = Add
+    | Subtract
+    | Multiply
+    | Divide
+
+type Expr 
+    = Num Float
+    | Op Op
+    | Group (List Expr)
+
+num : Parser () Expr
+num =
+    Num <$> Num.float <|> Num << toFloat <$> Num.int
+
+stringIs : String -> a -> Parser s a
+stringIs str val =
+    string str *> succeed val
+
+op : Parser () Expr
+op =
+    stringIs "+" Add
+    <|> stringIs "-" Subtract
+    <|> stringIs "*" Multiply
+    <|> stringIs "/" Divide
+    |> map Op
+
+group : Parser () Expr
+group =
+    between (string "(") (string ")") (sepBy1 whitespace (lazy <| \_ -> parser)) -- Avoid bad recursion issues using lazy parser evaluation
+    |> map Group
+
+parser : Parser () Expr
+parser =
+    (lazy <| \_ -> group) <|> op <|> num
+
+parse : String -> Result (ParseErr ()) (ParseOk () Expr)
+parse =
+    Combine.parse parser