move tupleN to parent module and add empty_list

Author: benbellick

src/decode.ml

@@ -329,6 +329,38 @@ module Make (Decodeable : Decodeable) :
         (fail "Expected a list").dec value
 
 
+  let ( >>=:: ) fst rest = uncons rest fst
+
+  let empty_list =
+    Decoder.of_decode_fun
+    @@ fun value ->
+    match Decodeable.get_list value with
+    | Some [] ->
+        Ok ()
+    | Some _ ->
+        (fail "Expected an empty list").dec value
+    | None ->
+        (fail "Expected a list").dec value
+
+
+  let tuple2 d0 d1 =
+    d0 >>=:: fun arg0 -> d1 >>=:: fun arg1 -> succeed (arg0, arg1)
+
+
+  let tuple3 d0 d1 d2 =
+    d0
+    >>=:: fun arg0 ->
+    d1 >>=:: fun arg1 -> d2 >>=:: fun arg2 -> succeed (arg0, arg1, arg2)
+
+
+  let tuple4 d0 d1 d2 d3 =
+    d0
+    >>=:: fun arg0 ->
+    d1
+    >>=:: fun arg1 ->
+    d2 >>=:: fun arg2 -> d3 >>=:: fun arg3 -> succeed (arg0, arg1, arg2, arg3)
+
+
   let rec at : string list -> 'a decoder -> 'a decoder =
    fun path decoder ->
     match path with
@@ -471,26 +503,5 @@ module Make (Decodeable : Decodeable) :
       custom (optional_decoder (at path value) val_decoder default) next
   end
 
-  module TupleHelper = struct
-    let ( >>=:: ) fst rest = uncons rest fst
-
-    let tuple2 d0 d1 =
-      d0 >>=:: fun arg0 -> d1 >>=:: fun arg1 -> succeed (arg0, arg1)
-
-
-    let tuple3 d0 d1 d2 =
-      d0
-      >>=:: fun arg0 ->
-      d1 >>=:: fun arg1 -> d2 >>=:: fun arg2 -> succeed (arg0, arg1, arg2)
-
-
-    let tuple4 d0 d1 d2 d3 =
-      d0
-      >>=:: fun arg0 ->
-      d1
-      >>=:: fun arg1 ->
-      d2 >>=:: fun arg2 -> d3 >>=:: fun arg3 -> succeed (arg0, arg1, arg2, arg3)
-  end
-
   include Infix
 end

src/sig.ml

@@ -117,7 +117,36 @@ module type S = sig
 
       (If you squint, the uncons operator [>>=::] kind of looks like the cons
       operator [::].)
-  *)
+   *)
+
+  val empty_list : unit decoder
+
+  val tuple2 : 'a decoder -> 'b decoder -> ('a * 'b) decoder
+  (** Decode a collection into an OCaml 2-tuple.
+
+      For example, to decode this json:
+
+      {[
+          [true, "string"]
+      ]}
+
+      we can use this decoder:
+
+      {[
+         tuple2 bool string
+      ]}
+     *)
+
+  val tuple3 : 'a decoder -> 'b decoder -> 'c decoder -> ('a * 'b * 'c) decoder
+  (** Decode a collection into an OCaml 3-tuple. *)
+
+  val tuple4 :
+       'a decoder
+    -> 'b decoder
+    -> 'c decoder
+    -> 'd decoder
+    -> ('a * 'b * 'c * 'd) decoder
+  (** Decode a collection into an OCaml 4-tuple. *)
 
   (** {1 Object primitives} *)
 
@@ -296,37 +325,6 @@ module type S = sig
 
     val custom : 'a decoder -> ('a -> 'b) decoder -> 'b decoder
   end
-
-  (** Helpers for decoding tuples which are encoded as lists *)
-  module TupleHelper : sig
-    val tuple2 : 'a decoder -> 'b decoder -> ('a * 'b) decoder
-    (** Decode a collection into an OCaml 2-tuple.
-
-      For example, to decode this json:
-
-      {[
-          [true, "string"]
-      ]}
-
-      we can use this decoder:
-
-      {[
-         tuple2 bool string
-      ]}
-     *)
-
-    val tuple3 :
-      'a decoder -> 'b decoder -> 'c decoder -> ('a * 'b * 'c) decoder
-    (** Decode a collection into an OCaml 3-tuple. *)
-
-    val tuple4 :
-         'a decoder
-      -> 'b decoder
-      -> 'c decoder
-      -> 'd decoder
-      -> ('a * 'b * 'c * 'd) decoder
-    (** Decode a collection into an OCaml 4-tuple. *)
-  end
 end
 
 (** Signature of things that can be decoded. *)