Parsing working.

Modeled on the OCaml code.

implementations/Elm/src/Joy.elm

@@ -1,25 +1,118 @@
-module Joy exposing (joyTermToString)
+module Joy exposing (doit)
+
+import String exposing (replace, words)
 
 
 type JoyType
-    = Symbol String
-    | Integer Int
+    = JoySymbol String
+    | JoyInt Int
     | JoyList (List JoyType)
     | JoyTrue
     | JoyFalse
 
+type alias JoyList = List JoyType
+
+
+
+
+-- Printer
 
 joyTermToString : JoyType -> String
 joyTermToString term =
     case term of
-        Symbol name ->
-            name
-        Integer n ->
-            String.fromInt n
-        JoyTrue ->
-            "true"
-        JoyFalse ->
-            "false"
+        JoySymbol name -> name
+        JoyInt n -> String.fromInt n
+        JoyTrue -> "true"
+        JoyFalse -> "false"
         JoyList list ->
-            "[" ++ (String.join " " (List.map joyTermToString list)) ++ "]"
+            "[" ++ (joyExpressionToString list) ++ "]"
+
+joyExpressionToString expr = String.join " " (List.map joyTermToString expr)
+
+
+
+
+
+-- Use the old S-expression lexing trick.
+
+tokenize : String -> (List String)
+tokenize text = words (replace "[" " [ " (replace "]" " ] " text))
+
+
+tokenator : String -> JoyType
+tokenator tok =
+    case tok of
+        "true" -> JoyTrue
+        "false" -> JoyFalse
+        _ -> case String.toInt tok of
+            Just i -> JoyInt i
+            Nothing -> JoySymbol tok
+
+-- I don't like this because it won't reject "[" and "]"
+-- instead turning them into symbols!
+
+
+
+expect_right_bracket : (List String) -> (List JoyType) -> Result String (List JoyType, List String)
+expect_right_bracket tokens acc =
+    case tokens of
+    [] -> Err "Missing closing bracket."
+    h :: t -> expect_right_bracket_one_token_lookahead h t acc
+
+
+expect_right_bracket_one_token_lookahead : String -> (List String) -> (List JoyType) -> Result String (List JoyType, List String)
+expect_right_bracket_one_token_lookahead token tokens acc =
+    case token of
+    "]" -> Ok (acc, tokens)
+    "[" ->
+        -- (* extract the sub-list *)
+        case expect_right_bracket tokens [] of
+            Err msg -> Err msg
+            Ok (sub_list, rest) ->
+                -- (* continue looking for the expected "]" *)
+                case expect_right_bracket rest acc of
+                    Err msg -> Err msg
+                    Ok (el, rrest) ->
+                        Ok ((JoyList sub_list) :: el, rrest)
+    _ ->
+        case expect_right_bracket tokens acc of
+            Err msg -> Err msg
+            Ok (el, rest) ->
+                Ok ((tokenator token) :: el, rest)
+
+
+---(* token -> token list -> joy_type * token list *)
+one_token_lookahead : String -> (List String) -> Result String (JoyType, List String)
+one_token_lookahead token tokens =
+    case token of
+        "]" -> Err "Extra closing bracket."
+        "[" -> case expect_right_bracket tokens [] of
+            Err msg -> Err msg
+            Ok (list_term, rest_of_tokens) -> Ok (JoyList list_term, rest_of_tokens)
+        _ -> Ok (tokenator token, tokens)
+
+
+parse0 : (List String) -> (List JoyType) -> Result String (List JoyType)
+parse0 tokens acc =
+    case tokens of
+        [] -> Ok acc
+        token :: tokens_tail ->
+            case one_token_lookahead token tokens_tail of
+                Err msg -> Err msg
+                Ok (term, rest_of_tokens) ->
+                    case parse0 rest_of_tokens acc of
+                        Err msg -> Err msg
+                        Ok terms -> Ok (term :: terms)
+
+
+parse tokens = parse0 tokens []
+
+text_to_expression text = parse (tokenize text)
+
+
+
+doit text =
+    case text_to_expression text of
+        Err msg -> Err msg
+        Ok ast -> Ok (joyExpressionToString ast)
 

implementations/Elm/src/Main.elm

@@ -6,7 +6,7 @@ import Html exposing (Html, Attribute, div, input, text)
 import Html.Attributes exposing (..)
 import Html.Events exposing (onInput)
 
-
+import Joy exposing (doit)
 
 -- MAIN
 
@@ -50,7 +50,14 @@ update msg model =
 
 view : Model -> Html Msg
 view model =
+  case doit model.content of
+    Err msg ->
       div []
         [ input [ placeholder "Text to reverse", value model.content, onInput Change ] []
-    , div [] [ text (String.reverse model.content) ]
+        , div [] [ text msg ]
+        ]
+    Ok message ->
+      div []
+        [ input [ placeholder "Text to reverse", value model.content, onInput Change ] []
+        , div [] [ text message ]
         ]