str () t is formatted output function that matches "%a" conversion format specifier in functions, that prints to string, e.g., sprintf, failwithf, errorf and, suprisingly all Lwt printing function, including Lwt_io.printf and logging (or any other function with type ('a,unit,string,...) formatN`. Example:
Or_error.errorf "type %a is not valid for %a"
Type.str ty Exp.str exp