[@fmt "%d"] as a shorthand to [@printer fun fmt -> fprintf fmt "%d"]

[XVilka]

Add the new attribute so

[@printer fun fmt -> fprintf fmt "%d"]

would become

[@fmt "%d"]

Well, the reason is quite obvious, just to be type more while the case is quite common, I believe.

[gasche]

Another approach would be for you to declare a local function

let fmt format = fun ppf -> Printf.fprintf ppf format

and use [@printer fmt "%d"].

[XVilka]

@gasche it is what I did exactly, but it is still less concise than my suggestion.