Utilities for mapping custom type constructors to JSON (#28)

* Utilities to make the JSON mapping easier. #25

* Fix compile error. #25

src/Morphir/IR/Type.elm

@@ -290,7 +290,7 @@ matchUnit =
 -}
 field : Name -> Type -> Field
 field fieldName fieldType =
-    Advanced.field fieldName fieldType
+    Advanced.Field fieldName fieldType
 
 
 {-| Matches a field.

src/Morphir/JsonExtra.elm

@@ -0,0 +1,47 @@
+module Morphir.JsonExtra exposing (..)
+
+import Json.Decode as Decode
+import Json.Encode as Encode
+
+
+encodeConstructor : String -> List Encode.Value -> Encode.Value
+encodeConstructor typeTag args =
+    Encode.string typeTag
+        :: args
+        |> Encode.list identity
+
+
+decodeCustomType : (String -> Decode.Decoder a) -> Decode.Decoder a
+decodeCustomType f =
+    Decode.index 0 Decode.string
+        |> Decode.andThen f
+
+
+decodeConstructor : (a -> r) -> Decode.Decoder a -> Decode.Decoder r
+decodeConstructor f a =
+    Decode.map f
+        (Decode.index 1 a)
+
+
+decodeConstructor2 : (a -> b -> r) -> Decode.Decoder a -> Decode.Decoder b -> Decode.Decoder r
+decodeConstructor2 f a b =
+    Decode.map2 f
+        (Decode.index 1 a)
+        (Decode.index 2 b)
+
+
+decodeConstructor3 : (a -> b -> c -> r) -> Decode.Decoder a -> Decode.Decoder b -> Decode.Decoder c -> Decode.Decoder r
+decodeConstructor3 f a b c =
+    Decode.map3 f
+        (Decode.index 1 a)
+        (Decode.index 2 b)
+        (Decode.index 3 c)
+
+
+decodeConstructor4 : (a -> b -> c -> d -> r) -> Decode.Decoder a -> Decode.Decoder b -> Decode.Decoder c -> Decode.Decoder d -> Decode.Decoder r
+decodeConstructor4 f a b c d =
+    Decode.map4 f
+        (Decode.index 1 a)
+        (Decode.index 2 b)
+        (Decode.index 3 c)
+        (Decode.index 4 d)