char decoder

src/decoders_deriver.ml

@@ -85,7 +85,11 @@ let rec expr_of_typ (typ : core_type)
   | [%type: float] -> Ast_builder.Default.evar ~loc "D.float"
   | [%type: bool] -> Ast_builder.Default.evar ~loc "D.bool"
   | [%type: char] ->
-      failwith "Cannot directly handle character; please cast to string first"
+      [%expr
+        let open D.Infix in
+        D.string >>= fun s ->
+        if String.length s = 1 then D.succeed @@ String.get s 0
+        else D.fail "Expected a string of length 1"]
   | [%type: string] | [%type: String.t] ->
       Ast_builder.Default.evar ~loc "D.string"
   | [%type: bytes] | [%type: Bytes.t] ->

test/test_decoders.ml

@@ -1,9 +1,6 @@
 module D = Decoders_yojson.Safe.Decode
 
 type my_int = int [@@deriving decoders]
-
-(* type my_int32 = int32 [@@deriving decoders] *)
-(* type my_int64 = int64 [@@deriving decoders] *)
 type my_float = float [@@deriving decoders]
 type my_string = string [@@deriving decoders]
 type my_bool = bool [@@deriving decoders]
@@ -310,3 +307,15 @@ module Ints = struct
     | Ok 5438n -> true
     | _ -> false
 end
+
+type my_char = char [@@deriving decoders]
+
+let%test "char" =
+  (match D.decode_string my_char_decoder {|"c"|} with
+  | Ok 'c' -> true
+  | _ -> false)
+  &&
+  match D.decode_string my_char_decoder {|"abc"|} with
+  (* We expect an error here because the string must have length 1 *)
+  | Error _ -> true
+  | _ -> false