clarified signature;

--- a/src/Pure/General/json_api.scala	Tue Dec 21 19:42:20 2021 +0100
+++ b/src/Pure/General/json_api.scala	Tue Dec 21 21:07:26 2021 +0100
@@ -6,6 +6,8 @@
 
 package isabelle
 
+import java.net.URL
+
 
 object JSON_API
 {
@@ -14,11 +16,25 @@
   def api_error(msg: String): Nothing = error("JSON API error: " + msg)
   def api_errors(msgs: List[String]): Nothing = error(("JSON API errors:" :: msgs).mkString("\n  "))
 
+  class Service(val url: URL)
+  {
+    override def toString: String = url.toString
+
+    def get(route: String): HTTP.Content =
+      HTTP.Client.get(if (route.isEmpty) url else new URL(url, route))
+
+    def get_root(route: String = ""): Root =
+      Root(get(if (route.isEmpty) "" else "/" + route).json)
+  }
+
   sealed case class Root(json: JSON.T)
   {
     def get_links: Option[Links] = JSON.value(json, "links").map(Links)
+    def get_next: Option[Service] = get_links.flatMap(_.get_next)
+
     def get_included: List[Resource] = JSON.list(json, "included", Resource.some).getOrElse(Nil)
     def get_data: Option[Data] = JSON.value(json, "data").map(Data(_, get_included))
+
     def get_errors: Option[List[JSON.T]] = JSON.list(json, "errors", Some(_))
 
     def ok: Boolean = get_errors.isEmpty
@@ -29,14 +45,19 @@
         case Some(errs) => api_errors(errs.map(JSON.Format.apply))
       }
     def check_data: Data = check.get_data getOrElse api_error("missing data")
+    def check_resource: Resource = check_data.resource
+    def check_resources: List[Resource] = check_data.resources
 
     override def toString: String = "Root(" + (if (ok) "ok" else "errors") + ")"
   }
 
   sealed case class Links(json: JSON.T)
   {
-    def get_next: Option[Root] =
-      for (next <- JSON.value(json, "next") if next != null) yield Root(next)
+    def get_next: Option[Service] =
+      for {
+        JSON.Value.String(next) <- JSON.value(json, "next")
+        if Url.is_wellformed(next)
+      } yield new Service(Url(next))
 
     override def toString: String =
       if (get_next.isEmpty) "Links()" else "Links(next)"

--- a/src/Pure/Tools/flarum.scala	Tue Dec 21 19:42:20 2021 +0100
+++ b/src/Pure/Tools/flarum.scala	Tue Dec 21 21:07:26 2021 +0100
@@ -17,16 +17,14 @@
 
   def server(url: URL): Server = new Server(url)
 
-  class Server private[Flarum](url: URL)
+  class Server private[Flarum](url: URL) extends JSON_API.Service(url)
   {
-    override def toString: String = url.toString
+    def get_api(route: String): JSON_API.Root =
+      get_root("api" + (if (route.isEmpty) "" else "/" + route))
 
-    def get(route: String): HTTP.Content =
-      HTTP.Client.get(if (route.isEmpty) url else new URL(url, route))
-
-    def get_api(route: String = ""): JSON_API.Root =
-      JSON_API.Root(get("api" + (if (route.isEmpty) "" else "/" + route)).json)
-
-    val root: JSON_API.Root = get_api()
+    val root: JSON_API.Root = get_api("")
+    def users: JSON_API.Root = get_api("users")
+    def groups: JSON_API.Root = get_api("groups")
+    def discussions: JSON_API.Root = get_api("discussions")
   }
 }