2 files changed, 14 insertions, 7 deletions
diff --git a/lib/protocols/Relation.ml b/lib/protocols/Relation.ml
index ece9b8e..54454cf 100644
--- a/lib/protocols/Relation.ml
+++ b/lib/protocols/Relation.ml
@@ -1,9 +1,16 @@
+module Error = struct
+  open Concepts.Condition
+
+  let not_a_relation () =
+    condition "not-a-relation" "A handle was expected to carry the relation protocol and did not"
+      empty
+end
+
 class type implementation = object
   method heading : (Concepts.Hash.hash, Concepts.Condition.condition) result
   method predicate : (Concepts.Hash.hash option, Concepts.Condition.condition) result
   method local_constraints : (Concepts.Hash.hash option, Concepts.Condition.condition) result
-  method tuples : (Concepts.Hash.hash, Concepts.Condition.condition) result
-  method contains : Concepts.Blob.t -> (bool, Concepts.Condition.condition) result
+  method contains : Concepts.Tuple.t -> (bool, Concepts.Condition.condition) result
 end
 
 type Handle.protocol += Relation of implementation
@@ -11,8 +18,8 @@ type t = implementation
 
 let make impl = Relation (impl :> implementation)
 let from handle = Handle.into handle (function Relation impl -> Some impl | _ -> None)
+let require handle = from handle |> Option.to_result ~none:(Error.not_a_relation ())
 let heading i = Handle.invoke i (fun o -> o#heading)
 let predicate i = Handle.invoke i (fun o -> o#predicate)
 let local_constraints i = Handle.invoke i (fun o -> o#local_constraints)
-let tuples i = Handle.invoke i (fun o -> o#tuples)
 let contains i tuple = Handle.invoke i (fun o -> o#contains tuple)
diff --git a/lib/protocols/Relation.mli b/lib/protocols/Relation.mli
index 4aeb562..7e64a3b 100644
--- a/lib/protocols/Relation.mli
+++ b/lib/protocols/Relation.mli
@@ -4,12 +4,12 @@ class type implementation = object
   method heading : (Concepts.Hash.hash, Concepts.Condition.condition) result
   method predicate : (Concepts.Hash.hash option, Concepts.Condition.condition) result
   method local_constraints : (Concepts.Hash.hash option, Concepts.Condition.condition) result
-  method tuples : (Concepts.Hash.hash, Concepts.Condition.condition) result
-  method contains : Concepts.Blob.t -> (bool, Concepts.Condition.condition) result
+  method contains : Concepts.Tuple.t -> (bool, Concepts.Condition.condition) result
 end
 
 val make : #implementation -> Handle.protocol
 val from : Handle.t -> t Handle.interface option
+val require : Handle.t -> (t Handle.interface, Concepts.Condition.condition) result
 val heading : t Handle.interface -> (Concepts.Hash.hash, Concepts.Condition.condition) result
 
 val predicate :
@@ -18,5 +18,5 @@ val predicate :
 val local_constraints :
   t Handle.interface -> (Concepts.Hash.hash option, Concepts.Condition.condition) result
 
-val tuples : t Handle.interface -> (Concepts.Hash.hash, Concepts.Condition.condition) result
-val contains : t Handle.interface -> Concepts.Blob.t -> (bool, Concepts.Condition.condition) result
+val contains :
+  t Handle.interface -> Concepts.Tuple.t -> (bool, Concepts.Condition.condition) result