[istd] delete unused [is_singleton]

Summary:
It started with wondering which function the "O(1)" comment was about
but I ended up deleting `is_singleton` because it's unused and it can be
easily re-implemented.

Reviewed By: ngorogiannis

Differential Revision: D21425225

fbshipit-source-id: 5ee3e189c

infer/src/istd/IContainer.ml

@@ -4,7 +4,6 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *)
-(* Extension of Base.Container, i.e. generic definitions of container operations in terms of fold. *)
 
 open! IStd
 module F = Format
@@ -17,8 +16,6 @@ let singleton_or_more ~fold t =
           match acc with Empty -> Singleton item | _ -> return More ) )
 
 
-let is_singleton ~fold t = match singleton_or_more ~fold t with Singleton _ -> true | _ -> false
-
 let mem_nth ~fold t index =
   With_return.with_return (fun {return} ->
       let (_ : int) =

infer/src/istd/IContainer.mli

@@ -8,16 +8,14 @@
 open! IStd
 module F = Format
 
+(** Extension of {!Base.Container}, i.e. generic definitions of container operations in terms of a
+    [fold] function. *)
+
 type 'a singleton_or_more = Empty | Singleton of 'a | More
 
 val singleton_or_more :
   fold:('t, 'a, 'a singleton_or_more) Container.fold -> 't -> 'a singleton_or_more
 
-(* O(1) *)
-
-val is_singleton : fold:('t, 'a, 'a singleton_or_more) Container.fold -> 't -> bool
-  [@@warning "-32"]
-
 val mem_nth : fold:('t, _, int) Container.fold -> 't -> int -> bool
 
 val forto : (int, int, 'accum) Container.fold