open System let ranStr n = let r = Random() let chars = Array.concat([[|'a' .. 'z'|];[|'A' .. 'Z'|];[|'0' .. '9'|]]) let sz = Array.length chars in String(Array.init n (fun _ -> chars.[r.Next sz])) // > ranStr 100_000_000;; // Real: 00:00:01.555, CPU: 00:00:01.575, GC gen0: 1, gen1: 1