2 files changed, 57 insertions, 0 deletions
diff --git a/lib/concepts/Mode.ml b/lib/concepts/Mode.ml
new file mode 100644
index 0000000..b7aa814
--- /dev/null
+++ b/lib/concepts/Mode.ml
@@ -0,0 +1,30 @@
+type attribute = string
+type attributes = BatSet.String.t
+type affordance = Decides | Generates of Cardinality.t
+type mode = {bound: attributes; affords: affordance}
+type t = mode list
+
+let attributes_of_list = BatSet.String.of_list
+let attributes_of_map map = BatMap.String.keys map |> BatSet.String.of_enum
+let decides_when bound = {bound= attributes_of_list bound; affords= Decides}
+let enumerable cardinality = {bound= BatSet.String.empty; affords= Generates cardinality}
+let of_list modes = modes
+
+let generation declaration bound =
+  List.fold_left
+    (fun tightest {bound= declared; affords} ->
+      match affords with
+      | Generates cardinality when BatSet.String.subset declared bound ->
+          Some (Option.fold ~none:cardinality ~some:(Cardinality.meet cardinality) tightest)
+      | Generates _ | Decides -> tightest )
+    None declaration
+
+let exhaustible declaration bound =
+  Option.fold ~none:false ~some:Cardinality.exhaustible (generation declaration bound)
+
+let decision declaration bound =
+  exhaustible declaration bound
+  || List.exists
+       (fun {bound= declared; affords} ->
+         match affords with Decides -> BatSet.String.equal declared bound | Generates _ -> false )
+       declaration
diff --git a/lib/concepts/Mode.mli b/lib/concepts/Mode.mli
new file mode 100644
index 0000000..6a906d1
--- /dev/null
+++ b/lib/concepts/Mode.mli
@@ -0,0 +1,27 @@
+type attribute = string
+type attributes = BatSet.String.t
+
+(** what asking a relation under a given set of bound attributes
+    affords. The answer is to whether anything satisfies the binding,
+    or the bindings of the attributes left free together with the
+    class of what is produced. *)
+type affordance = Decides | Generates of Cardinality.t
+
+type mode = {bound: attributes; affords: affordance}
+type t
+
+val attributes_of_list : attribute list -> attributes
+val attributes_of_map : 'a BatMap.String.t -> attributes
+val decides_when : attribute list -> mode
+val enumerable : Cardinality.t -> mode
+val of_list : mode list -> t
+
+(** the tightest class declared for generating under [bound]. A mode
+    declared for fewer bound attributes applies to more, since the
+    attributes left free are then fewer and no more numerous. *)
+val generation : t -> attributes -> Cardinality.t option
+
+val exhaustible : t -> attributes -> bool
+
+(** declared outright, or implied by a generation that can be exhausted. *)
+val decision : t -> attributes -> bool