Add part4

advanced/part4/src/Email.elm

@@ -0,0 +1,45 @@
+module Email exposing (Email, decoder, encode, toString)
+
+import Json.Decode as Decode exposing (Decoder)
+import Json.Encode as Encode exposing (Value)
+
+
+{-| An email address.
+
+Having this as a custom type that's separate from String makes certain
+mistakes impossible. Consider this function:
+
+updateEmailAddress : Email -> String -> Http.Request
+updateEmailAddress email password = ...
+
+(The server needs your password to confirm that you should be allowed
+to update the email address.)
+
+Because Email is not a type alias for String, but is instead a separate
+custom type, it is now impossible to mix up the argument order of the
+email and the password. If we do, it won't compile!
+
+If Email were instead defined as `type alias Email = String`, we could
+call updateEmailAddress password email and it would compile (and never
+work properly).
+
+This way, we make it impossible for a bug like that to compile!
+
+-}
+type Email
+    = Email String
+
+
+toString : Email -> String
+toString (Email str) =
+    str
+
+
+encode : Email -> Value
+encode (Email str) =
+    Encode.string str
+
+
+decoder : Decoder Email
+decoder =
+    Decode.map Email Decode.string