Simplify manual collection decoders

Author: alfonsogarciacaro

src/Decode.fs

@@ -422,25 +422,7 @@ module Decode =
     // Data structure ///
     ////////////////////
 
-    let list (decoder : Decoder<'value>) : Decoder<'value list> =
-        fun path value ->
-            if Helpers.isArray value then
-                let mutable i = -1
-                let tokens = Helpers.asArray value
-                (Ok [], tokens) ||> Array.fold (fun acc value ->
-                    i <- i + 1
-                    match acc with
-                    | Error _ -> acc
-                    | Ok acc ->
-                        match decoder (path + ".[" + (i.ToString()) + "]") value with
-                        | Error er -> Error er
-                        | Ok value -> Ok (value::acc))
-                |> Result.map List.rev
-            else
-                (path, BadPrimitive ("a list", value))
-                |> Error
-
-    let array (decoder : Decoder<'value>) : Decoder<'value array> =
+    let private arrayWith expectedMsg (mapping: 'value[] -> 'result) (decoder : Decoder<'value>) : Decoder<'result> =
         fun path value ->
             if Helpers.isArray value then
                 let mutable i = -1
@@ -454,10 +436,20 @@ module Decode =
                         match decoder (path + ".[" + (i.ToString()) + "]") value with
                         | Error er -> Error er
                         | Ok value -> acc.[i] <- value; Ok acc)
+                |> Result.map mapping
             else
-                (path, BadPrimitive ("an array", value))
+                (path, BadPrimitive (expectedMsg, value))
                 |> Error
 
+    let list (decoder : Decoder<'value>) : Decoder<'value list> =
+        arrayWith "a list" List.ofArray decoder
+
+    let seq (decoder : Decoder<'value>) : Decoder<'value seq> =
+        arrayWith "a seq" Seq.ofArray decoder
+
+    let array (decoder : Decoder<'value>) : Decoder<'value array> =
+        arrayWith "an array" id decoder
+
     let keyValuePairs (decoder : Decoder<'value>) : Decoder<(string * 'value) list> =
         fun path value ->
             if Helpers.isObject value then