fix amount parser

todo: private type

src/types.ml

@@ -95,20 +95,20 @@ module Amount = struct
     let parse_currency = string "EUR" *> return `Eur in
     let parse_int =
       take_while1 (function '0' .. '9' -> true | _ -> false)
-      >>| Int64.of_string
+      >>| Int64.of_string_opt
+      >>= function
+      | None -> fail "invalid integer"
+      | Some n -> return n
     in
     let parse_t =
       lift4
         (fun sign currency value fraction ->
-          { sign; currency; value; fraction })
+          make ~sign ~currency ~value ~fraction)
         parse_sign parse_currency
         (char ':' *> parse_int)
         (char '.' *> parse_int)
     in
-    fun s ->
-      match parse_string ~consume:Consume.All parse_t s with
-      | Ok v -> Ok v
-      | Error e -> Error e
+    fun s -> parse_string ~consume:Consume.All parse_t s |> Result.join
 end
 
 module Eddsa = struct