diff --git a/README.md b/README.md
index 8198dc4..87d6075 100644
--- a/README.md
+++ b/README.md
@@ -90,9 +90,7 @@ let config =
     ;b2b = [Crc32c]}
   ;index_location = Start};;
 let codec_chain =
-  {a2a = []
-  ;a2b = ShardingIndexed config
-  ;b2b = [Crc32c]};;
+  {a2a = []; a2b = ShardingIndexed config; b2b = []};;
 
 let shard_node = Result.get_ok @@ ArrayNode.(group_node / "another");;
 
@@ -124,7 +122,6 @@ FilesystemStore.array_exists store shard_node;;
 FilesystemStore.group_exists store group_node;;
 
 let a, g =
-  Result.get_ok @@
   FilesystemStore.find_child_nodes store group_node;;
 List.map ArrayNode.to_path a;;
 (*- : string list = ["/some/group/name"; "/some/group/another"] *)
diff --git a/dune-project b/dune-project
index 3f69e7e..3cee859 100644
--- a/dune-project
+++ b/dune-project
@@ -23,15 +23,15 @@
    dune
    (ocaml (>= 4.14.2))
    yojson
-   ppx_deriving_yojson
+   ppx_deriving
    ezgzip
-   checkseum
+   owl
    stdint
+   checkseum
    (ounit2 :with-test)
    (odoc :with-doc)
    (bisect_ppx
-     (and :dev (>= 2.5.0) :with-test))
-   owl)
+     (and :dev (>= 2.5.0) :with-test)))
  (tags
   (topics "to describe" your project)))
 
diff --git a/lib/codecs/array_to_array.ml b/lib/codecs/array_to_array.ml
index e929f46..8de6b96 100644
--- a/lib/codecs/array_to_array.ml
+++ b/lib/codecs/array_to_array.ml
@@ -7,7 +7,7 @@ type array_to_array =
   [@@deriving show]
 
 type error =
-  [ `Invalid_transpose_order of dimension_order * string ]
+  [ `Transpose_order of dimension_order * string ]
 
 (* https://zarr-specs.readthedocs.io/en/latest/v3/codecs/transpose/v1.0.html *)
 module TransposeCodec = struct
@@ -26,7 +26,7 @@ module TransposeCodec = struct
           let msg =
             "transpose order leads to a change in encoded
             representation size, which is prohibited." in
-          Result.error @@ `Invalid_transpose_order (t, msg)
+          Result.error @@ `Transpose_order (t, msg)
         else
           Ok {decoded with shape}
       with
@@ -34,13 +34,13 @@ module TransposeCodec = struct
         let msg =
           "transpose order max element is larger than
           the decoded representation dimensionality." in
-        Result.error @@ `Invalid_transpose_order (t, msg)
+        Result.error @@ `Transpose_order (t, msg)
 
 
   let parse_order o =
     if Array.length o = 0 then
       let msg = "transpose order cannot be empty." in
-      Result.error @@ `Invalid_transpose_order (o, msg)
+      Result.error @@ `Transpose_order (o, msg)
     else
       let o' = Array.copy o in
       Array.fast_sort Int.compare o';
@@ -48,7 +48,7 @@ module TransposeCodec = struct
         let msg =
           "order must not have any repeated dimensions
           or negative values." in
-        Result.error @@ `Invalid_transpose_order (o, msg)
+        Result.error @@ `Transpose_order (o, msg)
       else
         Result.ok @@ Transpose o
 
@@ -64,18 +64,18 @@ module TransposeCodec = struct
       let msg =
         "Transpose order must have the same length
         as the decoded representation's number of dims." in
-      Result.error @@ `Invalid_transpose_order (o, msg)
+      Result.error @@ `Transpose_order (o, msg)
     else if not @@ Array.for_all (fun x -> x <= max) o then
       let msg =
         "Largest value of transpose order must not be larger than
         then dimensionality of the decoded representation." in
-      Result.error @@ `Invalid_transpose_order (o, msg)
+      Result.error @@ `Transpose_order (o, msg)
     else
       Ok ()
 
   let encode o x =
     try Ok (Ndarray.transpose ~axis:o x) with
-    | Failure s -> Error (`Invalid_transpose_order (o, s))
+    | Failure s -> Error (`Transpose_order (o, s))
 
   let decode o x =
     let inv_order = Array.(make (length o) 0) in
diff --git a/lib/codecs/array_to_array.mli b/lib/codecs/array_to_array.mli
index 599909a..fcdb587 100644
--- a/lib/codecs/array_to_array.mli
+++ b/lib/codecs/array_to_array.mli
@@ -6,7 +6,7 @@ type array_to_array =
   | Transpose of dimension_order
 
 type error =
-  [ `Invalid_transpose_order of dimension_order * string ]
+  [ `Transpose_order of dimension_order * string ]
 
 val pp_array_to_array : Format.formatter -> array_to_array -> unit
 val show_array_to_array : array_to_array -> string
diff --git a/lib/codecs/array_to_bytes.ml b/lib/codecs/array_to_bytes.ml
index 0782e6c..a71261f 100644
--- a/lib/codecs/array_to_bytes.ml
+++ b/lib/codecs/array_to_bytes.ml
@@ -32,12 +32,10 @@ and chain =
   [@@deriving show]
 
 type error =
-  [ `Bytes_encode_error of string
-  | `Bytes_decode_error of string
-  | `Sharding_shape_mismatch of int array * int array * string
-  | Extensions.error
+  [ Extensions.error
   | Array_to_array.error
-  | Bytes_to_bytes.error ]
+  | Bytes_to_bytes.error
+  | `Sharding of int array * int array * string ]
 
 (* https://zarr-specs.readthedocs.io/en/latest/v3/codecs/bytes/v1.0.html *)
 module BytesCodec = struct
@@ -205,6 +203,15 @@ end = struct
 
   type t = shard_config  
 
+  let parse_chain repr chain =
+    List.fold_left
+      (fun acc c ->
+         acc >>= fun r ->
+         ArrayToArray.parse r c >>= fun () ->
+         ArrayToArray.compute_encoded_representation c r) (Ok repr) chain.a2a
+    >>= fun repr' ->
+    ArrayToBytes.parse repr' chain.a2b
+
   let parse 
     : type a b.
       (a, b) Util.array_repr ->
@@ -217,10 +224,9 @@ end = struct
       let msg =
         "sharding chunk_shape length must equal the dimensionality of
         the decoded representaton of a shard." in
-      Result.error @@
-      `Sharding_shape_mismatch (t.chunk_shape, repr.shape, msg))
+      Result.error @@ `Sharding (t.chunk_shape, repr.shape, msg))
     >>= fun () ->
-    match
+    (match
       Array.for_all2 (fun x y -> (x mod y) = 0) repr.shape t.chunk_shape
     with
     | true -> Ok ()
@@ -228,8 +234,10 @@ end = struct
       let msg =
         "sharding chunk_shape must evenly divide the size of the shard shape."
       in
-      Result.error @@
-      `Sharding_shape_mismatch (t.chunk_shape, repr.shape, msg)
+      Result.error @@ `Sharding (t.chunk_shape, repr.shape, msg))
+    >>= fun () ->
+    parse_chain repr t.codecs >>= fun () ->
+    parse_chain repr t.index_codecs
 
   let compute_encoded_size input_size t =
     List.fold_left BytesToBytes.compute_encoded_size
diff --git a/lib/codecs/array_to_bytes.mli b/lib/codecs/array_to_bytes.mli
index 1596483..8a85458 100644
--- a/lib/codecs/array_to_bytes.mli
+++ b/lib/codecs/array_to_bytes.mli
@@ -27,9 +27,7 @@ type error =
   [ Extensions.error
   | Array_to_array.error
   | Bytes_to_bytes.error
-  | `Bytes_encode_error of string
-  | `Bytes_decode_error of string
-  | `Sharding_shape_mismatch of int array * int array * string ]
+  | `Sharding of int array * int array * string ]
 
 module ArrayToBytes : sig
   val parse
diff --git a/lib/extensions.ml b/lib/extensions.ml
index 3b0cb25..1606e44 100644
--- a/lib/extensions.ml
+++ b/lib/extensions.ml
@@ -1,10 +1,5 @@
-type grid_info =
-  {msg : string
-  ;chunk_shape : int array
-  ;array_shape : int array}
-
 type error =
-  [ `Grid of grid_info ]
+  [ `Extension of string ]
 
 module RegularGrid = struct
   type t = int array
@@ -15,10 +10,10 @@ module RegularGrid = struct
     match chunk_shape, array_shape with
     | c, a when Array.(length c <> length a) ->
       let msg = "grid chunk and array shape must have the same the length." in
-      Result.error @@ `Grid {msg; array_shape; chunk_shape}
+      Result.error @@ `Extension msg
     | c, a when Util.(max c > max a) -> 
       let msg = "grid chunk dimension size must not be larger than array's." in
-      Result.error @@ `Grid {msg; array_shape; chunk_shape}
+      Result.error @@ `Extension msg
     | c, _ -> Ok c
 
   let ceildiv x y =
diff --git a/lib/extensions.mli b/lib/extensions.mli
index 2ac37d1..78a62ef 100644
--- a/lib/extensions.mli
+++ b/lib/extensions.mli
@@ -1,10 +1,5 @@
-type grid_info =
-  {msg : string
-  ;chunk_shape : int array
-  ;array_shape : int array}
-
 type error =
-  [ `Grid of grid_info ]
+  [ `Extension of string ]
 
 module RegularGrid : sig
   type t
diff --git a/lib/metadata.ml b/lib/metadata.ml
index 27fbb71..c3638bf 100644
--- a/lib/metadata.ml
+++ b/lib/metadata.ml
@@ -2,8 +2,7 @@ open Extensions
 open Util.Result_syntax
 
 type error =
-  [ Extensions.error
-  | `Json_decode of string ]
+  [ `Metadata of string ]
 
 module FillValue = struct
   type t =
@@ -126,6 +125,7 @@ module ArrayMetadata = struct
     chunks
     =
     RegularGrid.create ~array_shape:shape chunks
+    >>? (fun (`Extension msg) -> `Metadata msg)
     >>| fun chunk_grid ->
     {shape
     ;codecs
@@ -223,7 +223,7 @@ module ArrayMetadata = struct
     | xs ->
       RegularGrid.of_yojson xs >>= fun grid -> 
       RegularGrid.(create ~array_shape:shape @@ chunk_shape grid)
-      >>? fun (`Grid {msg; _}) -> msg)
+      >>? fun (`Extension msg) -> msg)
     >>= fun chunk_grid ->
 
     (match member "chunk_key_encoding" x with 
@@ -305,8 +305,7 @@ module ArrayMetadata = struct
     Yojson.Safe.to_string @@ to_yojson t
 
   let decode b = 
-    of_yojson @@ Yojson.Safe.from_string b >>? fun s ->
-    `Json_decode s
+    of_yojson @@ Yojson.Safe.from_string b
 
   let update_attributes t attrs =
     {t with attributes = attrs}
@@ -404,8 +403,7 @@ module GroupMetadata = struct
     Ok {zarr_format; node_type; attributes}
 
   let decode s = 
-    of_yojson @@ Yojson.Safe.from_string s >>? fun b ->
-    `Json_decode b
+    of_yojson @@ Yojson.Safe.from_string s
 
   let encode t =
     Yojson.Safe.to_string @@ to_yojson t
diff --git a/lib/metadata.mli b/lib/metadata.mli
index 926b73f..203854d 100644
--- a/lib/metadata.mli
+++ b/lib/metadata.mli
@@ -6,9 +6,8 @@
     [zarr.json] within the prefix of a group or array.*)
 
 type error =
-  [ Extensions.error
-  | `Json_decode of string ]
-(** A type for JSON decoding errors. *)
+  [ `Metadata of string ]
+(** A type for Metadata operation errors. *)
 
 module FillValue : sig
   type t =
@@ -51,7 +50,7 @@ module ArrayMetadata : sig
   val encode : t -> string
   (** [encode t] returns a byte string representing a JSON Zarr array metadata. *)
 
-  val decode : string -> (t, [> error ]) result
+  val decode : string -> (t, string) result
   (** [decode s] decodes a bytes string [s] into a {!ArrayMetadata.t}
       type, and returns an error if the decoding process fails. *)
 
@@ -135,7 +134,7 @@ module GroupMetadata : sig
   val encode : t -> string
   (** [encode t] returns a byte string representing a JSON Zarr group metadata. *)
 
-  val decode : string -> (t, [> error ]) result
+  val decode : string -> (t, string) result
   (** [decode s] decodes a bytes string [s] into a {!t} type, and returns
       an error if the decoding process fails. *)
 
diff --git a/lib/node.ml b/lib/node.ml
index a0cd8b1..0069010 100644
--- a/lib/node.ml
+++ b/lib/node.ml
@@ -12,7 +12,6 @@ module GroupNode = struct
   type t =
     | Root
     | Cons of t * string
-    [@@deriving show]
 
   let create parent name =
     if rep_ok name then
@@ -87,14 +86,16 @@ module GroupNode = struct
     | _, Root -> false
     | v, Cons (parent, _) -> parent = v
 
-  let show n = to_path n
+  let show = to_path
+
+  let pp fmt t =
+    Format.fprintf fmt "%s" @@ show t
 end
 
 module ArrayNode = struct
   type t =
     {parent : GroupNode.t
     ;name :  string}
-    [@@deriving show]
 
   let create parent name =
     if rep_ok name then
@@ -145,4 +146,7 @@ module ArrayNode = struct
   let to_metakey p = to_key p ^ "/zarr.json"
   
   let show = to_path
+
+  let pp fmt t =
+    Format.fprintf fmt "%s" @@ show t
 end
diff --git a/lib/storage/memory.ml b/lib/storage/memory.ml
index d68ad93..c0afff1 100644
--- a/lib/storage/memory.ml
+++ b/lib/storage/memory.ml
@@ -1,10 +1,4 @@
-module HashableString = struct
-  type t = string
-  let hash = Hashtbl.hash
-  let equal = String.equal
-end
-
-module StrMap = Hashtbl.Make (HashableString)
+module StrMap = Util.StrMap
 
 let create () = StrMap.create 16
 
@@ -13,7 +7,8 @@ module Impl = struct
 
   let get t key =
     Option.to_result
-      ~none:(`Store_read key) @@ StrMap.find_opt t key
+      ~none:(`Store_read (key ^ " not found.")) @@
+      StrMap.find_opt t key
 
   let set t key value =
     StrMap.replace t key value
diff --git a/lib/storage/storage.ml b/lib/storage/storage.ml
index 4dd8cdd..e6274e0 100644
--- a/lib/storage/storage.ml
+++ b/lib/storage/storage.ml
@@ -65,20 +65,12 @@ module Make (M : STORE) : S with type t = M.t = struct
     Some (ArrayNode.parent node)
 
   let group_metadata node t =
-    if not @@ group_exists t node then
-      Result.error @@
-      `Store_read (GroupNode.show node ^ " is not a member of this store.")
-    else
-      get t @@ GroupNode.to_metakey node >>= fun bytes ->
-      GM.decode bytes
+    get t @@ GroupNode.to_metakey node >>= fun bytes ->
+    GM.decode bytes >>? fun msg -> `Store_read msg
 
   let array_metadata node t =
-    if not @@ array_exists t node then
-      Result.error @@
-      `Store_read (ArrayNode.show node ^ " is not a member of this store.")
-    else
-      get t @@ ArrayNode.to_metakey node >>= fun bytes ->
-      AM.decode bytes
+    get t @@ ArrayNode.to_metakey node >>= fun bytes ->
+    AM.decode bytes >>? fun msg -> `Store_read msg
 
   (* Assumes without checking that [metakey] is a valid node metadata key.*)
   let unsafe_node_type t metakey =
@@ -87,22 +79,16 @@ module Make (M : STORE) : S with type t = M.t = struct
     |> Util.member "node_type" |> Util.to_string
 
   let find_child_nodes t node =
-    if group_exists t node then
-      Result.ok @@
-      List.fold_left
-        (fun (lacc, racc) pre ->
-          let p = "/" ^ String.(length pre - 1 |> sub pre 0) in
-          if unsafe_node_type t (pre ^ "zarr.json") = "array" then
-            let x = Result.get_ok @@ ArrayNode.of_path p in
-            x :: lacc, racc
-          else
-            let x = Result.get_ok @@ GroupNode.of_path p in
-            lacc, x :: racc)
-        ([], []) (snd @@ list_dir t @@ GroupNode.to_prefix node)
-    else
-      let msg =
-        GroupNode.show node ^ " is not a node in this heirarchy." in
-      Result.error @@ `Store_read msg
+    List.fold_left
+      (fun (lacc, racc) pre ->
+        let p = "/" ^ String.(length pre - 1 |> sub pre 0) in
+        if unsafe_node_type t (pre ^ "zarr.json") = "array" then
+          let x = Result.get_ok @@ ArrayNode.of_path p in
+          x :: lacc, racc
+        else
+          let x = Result.get_ok @@ GroupNode.of_path p in
+          lacc, x :: racc)
+      ([], []) (snd @@ list_dir t @@ GroupNode.to_prefix node)
 
   let find_all_nodes t =
     let keys =
@@ -138,11 +124,11 @@ module Make (M : STORE) : S with type t = M.t = struct
     Owl_types.index array ->
     (a, b) Ndarray.t ->
     t ->
-    (unit, [> error]) result
+    (unit, [> error ]) result
   = fun node slice x t ->
     let open Util in
     get t @@ ArrayNode.to_metakey node >>= fun bytes ->
-    AM.decode bytes >>= fun meta ->
+    AM.decode bytes >>? (fun msg -> `Store_write msg) >>= fun meta ->
     (if Ndarray.shape x = Indexing.slice_shape slice @@ AM.shape meta then 
         Ok ()
       else
@@ -199,7 +185,7 @@ module Make (M : STORE) : S with type t = M.t = struct
   = fun node slice kind t ->
     let open Util in
     get t @@ ArrayNode.to_metakey node >>= fun bytes ->
-    AM.decode bytes >>= fun meta ->
+    AM.decode bytes >>? (fun msg -> `Store_read msg) >>= fun meta ->
     (if AM.is_valid_kind meta kind then
         Ok ()
       else
@@ -245,7 +231,8 @@ module Make (M : STORE) : S with type t = M.t = struct
   let reshape t node shape =
     let mkey = ArrayNode.to_metakey node in
     get t mkey  >>= fun bytes ->
-    AM.decode bytes >>= fun meta ->
+    AM.decode bytes >>? (fun msg -> `Store_write msg)
+    >>= fun meta ->
     (if Array.length shape = Array.length @@ AM.shape meta then
       Ok ()
     else
diff --git a/lib/storage/storage_intf.ml b/lib/storage/storage_intf.ml
index f880fec..0017f1d 100644
--- a/lib/storage/storage_intf.ml
+++ b/lib/storage/storage_intf.ml
@@ -7,10 +7,7 @@ type range = ByteRange of int * int option
 
 type error =
   [ `Store_read of string
-  | `Store_write of string
-  | Node.error
-  | Metadata.error
-  | Codecs.error ]
+  | `Store_write of string ]
 
 module type STORE = sig
   (** The abstract STORE interface that stores should implement.
@@ -31,7 +28,7 @@ module type STORE = sig
 
   type t
   val get : t -> key -> (string, [> error]) result
-  val get_partial_values : t -> (key * range) list -> string option list
+  val get_partial_values : t -> (key * range) list -> (string list, [> error ]) result
   val set : t -> key -> string -> unit
   val set_partial_values : t -> (key * int * string) list -> (unit, [> error]) result
   val erase : t -> key -> unit
@@ -66,7 +63,7 @@ module type S = sig
       'a ->
       ArrayNode.t ->
       t ->
-      (unit, [> error ]) result
+      (unit, [> Codecs.error | Extensions.error | Metadata.error ]) result
   (** [create_array ~sep ~dimension_names ~attributes ~codecs ~shape ~chunks kind fill node t]
       creates an array node in store [t] where:
       - Separator [sep] is used in the array's chunk key encoding.
@@ -79,19 +76,20 @@ module type S = sig
       This operation can fail if the codec chain is not well defined. *)
 
   val array_metadata
-    : ArrayNode.t -> t -> (ArrayMetadata.t, [> error]) result
+    : ArrayNode.t -> t -> (ArrayMetadata.t, [> error ]) result
   (** [array_metadata node t] returns the metadata of array node [node].
       This operation returns an error if node is not a member of store [t]. *)
 
   val group_metadata
-    : GroupNode.t -> t -> (GroupMetadata.t, [> error]) result
+    : GroupNode.t -> t -> (GroupMetadata.t, [> error ]) result
   (** [group_metadata node t] returns the metadata of group node [node].
       This operation returns an error if node is not a member of store [t].*)
 
   val find_child_nodes
-    : t -> GroupNode.t -> (ArrayNode.t list * GroupNode.t list, [> error]) result
+    : t -> GroupNode.t -> ArrayNode.t list * GroupNode.t list
   (** [find_child_nodes t n] returns a tuple of child nodes of group node [n].
-      This operation can fail if [n] is not a member of store [t]. *)
+      This operation returns a pair of empty lists if node [n] has no
+      children or is not a member of store [t]. *)
 
   val find_all_nodes : t -> ArrayNode.t list * GroupNode.t list
   (** [find_all_nodes t] returns [Some p] where [p] is a pair of lists
@@ -126,7 +124,7 @@ module type S = sig
       Owl_types.index array ->
       ('a, 'b) Ndarray.t ->
       t ->
-      (unit, [> error]) result
+      (unit, [> error | Node.error | Codecs.error ]) result
   (** [set_array n s x t] writes n-dimensional array [x] to the slice [s]
       of array node [n] in store [t]. This operation fails if:
       - the ndarray [x] size does not equal slice [s].
@@ -139,7 +137,7 @@ module type S = sig
       Owl_types.index array ->
       ('a, 'b) Bigarray.kind ->
       t ->
-      (('a, 'b) Ndarray.t, [> error]) result
+      (('a, 'b) Ndarray.t, [> error | Node.error | Codecs.error ]) result
   (** [get_array n s k t] reads an n-dimensional array of size determined
       by slice [s] from array node [n]. This operation fails if:
       - If there is a problem decoding/encoding node [n] chunks.
@@ -217,39 +215,29 @@ module Base = struct
     in
     StrSet.(elements keys, elements prefixes)
 
-  let rec get_partial_values ~get_fn t kr_pairs =
-    match kr_pairs with
-    | [] -> [None]
-    | (k, r) :: xs ->
-      match get_fn t k with
-      | Error _ ->
-        None :: (get_partial_values ~get_fn t xs)
-      | Ok v ->
-        try
-          let sub = match r with
-            | ByteRange (rs, None) ->
-              String.sub v rs @@ String.length v 
-            | ByteRange (rs, Some rl) ->
-              String.sub v rs rl in
-          Some sub :: (get_partial_values ~get_fn t xs)
-        with
-        | Invalid_argument _ ->
-          None :: (get_partial_values ~get_fn t xs)
-    
-  let rec set_partial_values ~set_fn ~get_fn t = function
-    | [] -> Ok ()
-    | (k, rs, v) :: xs ->
-      match get_fn t k with
-      | Error _ ->
-        set_fn t k v;
-        set_partial_values ~set_fn ~get_fn t xs
-      | Ok ov ->
-        try
-          let ov' = Bytes.of_string ov in
-          String.(length v |> blit v 0 ov' rs);
-          set_fn t k @@ Bytes.to_string ov';
-          set_partial_values ~set_fn ~get_fn t xs
-        with
-        | Invalid_argument s ->
-          Error (`Store_read s)
+  let get_partial_values ~get_fn t kr_pairs =
+    let open Util.Result_syntax in
+    List.fold_right
+      (fun (k, ByteRange (rs, len)) acc ->
+        acc >>= fun xs ->
+        get_fn t k >>| fun v ->
+        (match len with
+        | None -> String.sub v rs @@ String.length v - rs
+        | Some l -> String.sub v rs l) :: xs) kr_pairs (Ok [])
+
+  let set_partial_values ~set_fn ~get_fn t krv =
+    let open Util.Result_syntax in
+    let module StrMap = Util.StrMap in
+    let tbl = StrMap.create @@ List.length krv in
+    List.fold_right
+      (fun (k, rs, v) acc ->
+        acc >>= fun () ->
+        (match StrMap.find_opt tbl k with
+        | None ->
+          get_fn t k
+        | Some ov -> Ok ov)
+        >>| fun ov ->
+        let ov' = Bytes.of_string ov in
+        String.(length v |> blit v 0 ov' rs);
+        set_fn t k @@ Bytes.to_string ov') krv (Ok ())
 end
diff --git a/lib/util.ml b/lib/util.ml
index 78656cb..616cd07 100644
--- a/lib/util.ml
+++ b/lib/util.ml
@@ -24,10 +24,18 @@ module ComparableArray = struct
   let compare = Stdlib.compare
 end
 
+module HashableString = struct
+  type t = string
+  let hash = Hashtbl.hash
+  let equal = String.equal
+end
+
 module ArraySet = Set.Make (ComparableArray)
 
 module Arraytbl = Hashtbl.Make (HashableArray)
 
+module StrMap = Hashtbl.Make (HashableString)
+
 module Result_syntax = struct
   let ( >>= ) = Result.bind
 
diff --git a/lib/util.mli b/lib/util.mli
index 1960a15..3deea05 100644
--- a/lib/util.mli
+++ b/lib/util.mli
@@ -12,6 +12,9 @@ module ExtPoint : sig
   val ( = ) : ('a -> 'a -> bool) -> 'a t -> 'a t -> bool
 end
 
+module StrMap : sig include Hashtbl.S with type key = string end
+(** A hashtable with string keys. *)
+
 module Arraytbl : sig include Hashtbl.S with type key = int array end
 (** A hashtable with integer array keys. *)
 
diff --git a/test/test_node.ml b/test/test_node.ml
index c39d38f..9b620dc 100644
--- a/test/test_node.ml
+++ b/test/test_node.ml
@@ -129,7 +129,7 @@ let array_node = [
   let s = "/some/dir/moredirs/path/pname" in
   let n = ArrayNode.of_path s |> Result.get_ok in
   assert_equal "pname" @@ ArrayNode.name n;
-  assert_equal ~printer:Fun.id s @@ ArrayNode.to_path n;
+  assert_equal ~printer:Fun.id s @@ ArrayNode.show n;
 
   (* parent tests *)
   assert_equal
@@ -138,8 +138,10 @@ let array_node = [
     ArrayNode.parent (ArrayNode.of_path "/nodename" |> Result.get_ok);
 
   (* equality tests *)
+  let n' = (ArrayNode.of_path s |> Result.get_ok) in
   assert_equal
-    true @@ ArrayNode.(n = (ArrayNode.of_path s |> Result.get_ok));
+    ~printer:ArrayNode.show n n';
+  assert_equal true ArrayNode.(n = n');
   assert_equal
     false @@
     ArrayNode.(n = Result.get_ok @@ ArrayNode.of_path (s ^ "/more"));
diff --git a/test/test_storage.ml b/test/test_storage.ml
index 30f1794..2ae9697 100644
--- a/test/test_storage.ml
+++ b/test/test_storage.ml
@@ -133,7 +133,7 @@ let test_store
   let child = GroupNode.of_path "/some/child" |> Result.get_ok in
   M.create_group store child;
   (match M.find_child_nodes store gnode with
-  | Ok (arrays, groups) ->
+  | arrays, groups ->
     assert_equal
       ~printer:string_of_list
       ["/arrnode"] @@
@@ -141,19 +141,11 @@ let test_store
     assert_equal
       ~printer:string_of_list
       ["/some"] @@
-      List.map GroupNode.to_path groups
-  | Error _ ->
-    assert_failure
-      "a store with more than one node
-      should return children for a root node.");
+      List.map GroupNode.to_path groups);
 
   (* test getting child nodes of a group not a member of this store. *)
-  let r =
-    M.find_child_nodes store @@
-    (Result.get_ok @@ GroupNode.(root / "fakegroup")) in
-  assert_bool
-    "finding child nodes of a non-store group node should fail." @@
-    Result.is_error r;
+  let g = (Result.get_ok @@ GroupNode.(root / "fakegroup")) in
+  assert_equal ([], []) @@ M.find_child_nodes store g;
 
   let ac, gc = M.find_all_nodes store in
   let got =
diff --git a/zarr.opam b/zarr.opam
index a536498..4735c34 100644
--- a/zarr.opam
+++ b/zarr.opam
@@ -13,14 +13,14 @@ depends: [
   "dune" {>= "3.15"}
   "ocaml" {>= "4.14.2"}
   "yojson"
-  "ppx_deriving_yojson"
+  "ppx_deriving"
   "ezgzip"
-  "checkseum"
+  "owl"
   "stdint"
+  "checkseum"
   "ounit2" {with-test}
   "odoc" {with-doc}
   "bisect_ppx" {dev & >= "2.5.0" & with-test}
-  "owl"
 ]
 build: [
   ["dune" "subst"] {dev}