2 files changed, 38 insertions, 13 deletions
diff --git a/lib/kernel/Source.ml b/lib/kernel/Source.ml
index b3b7389..e5de6e8 100644
--- a/lib/kernel/Source.ml
+++ b/lib/kernel/Source.ml
@@ -34,25 +34,35 @@ let evaluator ~language run =
       method to_string = language ^ "-evaluator"
       method protocols : Protocols.Handle.protocol list = [Protocols.Evaluator.make self]
 
-      method invoke program =
+      method invoke program ~within =
         match text program with
         | None -> Error (Error.not_a_program language)
         | Some (given, _) when given <> language -> Error (Error.foreign language given)
-        | Some (_, source) -> run source
+        | Some (_, source) -> run ~within source
     end
 
-let run root ~language source =
+let evaluators = Path.("system" @/ "evaluator" @/ this)
+
+let bind directory ~within language program =
   let open Utilities.Result in
   let* evaluator =
-    Path.lookup root Path.("evaluator" @/ language @/ this)
+    Path.lookup directory Path.(language @/ this)
     |> Result.map_error (fun _ -> Error.no_evaluator language)
   in
-  let* evaluator = Protocols.Evaluator.require evaluator in
-  Protocols.Evaluator.invoke evaluator (program ~language source)
+  Fun.protect
+    ~finally:(fun () -> Protocols.Handle.release evaluator)
+    (fun () ->
+      let* evaluator = Protocols.Evaluator.require evaluator in
+      Protocols.Evaluator.invoke evaluator program ~within )
+
+let run root ~language ~within source =
+  let open Utilities.Result in
+  let* directory = Path.lookup root evaluators in
+  bind directory ~within language (program ~language source)
 
 let register root ~language evaluator =
   let open Utilities.Result in
-  let* evaluators = Path.lookup root Path.("evaluator" @/ this) in
-  let* registry = Protocols.Handle.require Protocols.Registry.from evaluators in
+  let* directory = Path.lookup root evaluators in
+  let* registry = Protocols.Handle.require Protocols.Registry.from directory in
   let* _ = Protocols.Registry.update registry language None (Some evaluator) in
   Ok ()
diff --git a/lib/kernel/Source.mli b/lib/kernel/Source.mli
index 1a51da2..0961dcc 100644
--- a/lib/kernel/Source.mli
+++ b/lib/kernel/Source.mli
@@ -10,21 +10,36 @@ val program : language:string -> string -> Protocols.Handle.t
 val text : Protocols.Handle.t -> (string * string) option
 
 (** an evaluator for [language]: it runs the text of a program in that
-    language, and refuses anything else. *)
+    language against the directory it is given, and refuses anything
+    else. *)
 val evaluator :
   language:string ->
-  (string -> (Protocols.Handle.t, Concepts.Condition.condition) result) ->
+  (within:Protocols.Handle.t -> string -> (Protocols.Handle.t, Concepts.Condition.condition) result) ->
   Protocols.Handle.t
 
-(** look up the evaluator of [language] under [root]'s \evaluator and run
-    [text] with it. *)
+(** where the evaluators are registered, from the root: \system\evaluator. *)
+val evaluators : Path.t
+
+(** run [program] through the evaluator of [language] found in
+    [directory], against [within]. Usable as an
+    [EphemeralRelation.binding] once [directory] and [within] are given. *)
+val bind :
+  Protocols.Handle.t ->
+  within:Protocols.Handle.t ->
+  string ->
+  Protocols.Handle.t ->
+  (Protocols.Handle.t, Concepts.Condition.condition) result
+
+(** run [text] in [language] against [within], with the evaluator
+    registered under [root]. *)
 val run :
   Protocols.Handle.t ->
   language:string ->
+  within:Protocols.Handle.t ->
   string ->
   (Protocols.Handle.t, Concepts.Condition.condition) result
 
-(** register [evaluator] as \evaluator\[language] under [root]. *)
+(** register [evaluator] for [language] under [root]. *)
 val register :
   Protocols.Handle.t ->
   language:string ->