quick fix; ok json response ~~

src/amount.ml

@@ -1,6 +1,7 @@
 (* TODO
      have safe amount arithmetic
-     make type private *)
+     make type private
+     use config currency option *)
 (* Amounts of currency, serialized as `<Currency>:<IntegerPart>.<FractionalPart>`
      Fixed-precision numbers with 8 decimal places.
      - <Currency> must be at most 11 characters long
@@ -18,22 +19,19 @@ type t = {
 }
 
 let ( let* ) o f = match o with Ok v -> f v | Error _ as e -> e
+let check_not msg = function false -> Ok () | true -> Error msg
+let value_upper_bound = Z.(pow (of_int 2) 52) |> Z.to_int64
+let fraction_upper_bound = Int64.of_int 100_000_000
 
 let make ~sign ~currency ~value ~fraction =
-  let value_upper_bound = Z.(pow (of_int 2) 52) |> Z.to_int64 in
-  let fraction_upper_bound = Int64.of_int 99_999_999 in
-  let* () = if value < Int64.zero then Error "value is negative" else Ok () in
+  let* () = check_not "value is negative" (value < Int64.zero) in
+  let* () = check_not "fraction is negative" (fraction < Int64.zero) in
   let* () =
-    if fraction < Int64.zero then Error "fraction is negative" else Ok ()
+    check_not "value is greater than 2^52" (value > value_upper_bound)
   in
   let* () =
-    if value > value_upper_bound then Error "value is greater than 2^52"
-    else Ok ()
-  in
-  let* () =
-    if fraction > fraction_upper_bound then
-      Error "fraction have more than 8 decimal digits"
-    else Ok ()
+    check_not "fraction has more than 8 decimal digits"
+      (fraction >= fraction_upper_bound)
   in
   Ok { sign; currency; value; fraction }
 
@@ -46,8 +44,8 @@ let to_string =
     in
     let pp_currency ppf = function `Eur -> string ppf "EUR" in
     fun ppf { sign; currency; value; fraction } ->
-      pf ppf "%a%a:%Ld.%Ld" (Fmt.option pp_sign) sign pp_currency currency value
-        fraction
+      pf ppf "%a%a:%Ld.%02Ld" (Fmt.option pp_sign) sign pp_currency currency
+        value fraction
   in
   Fmt.str "%a" pp