Looks pretty ML:ish to me, even in a segment like this:

   let parse_int64 (local_ buf) (sp : span) : int64# =
     let mutable acc : int64# = #0L in
     let mutable i = 0 in
     let mutable valid = true in
     while valid && i < I16.to_int sp.#len do
       let c = Bytes.get buf (I16.to_int sp.#off + i) in
       match c with
       | '0' .. '9' ->
         acc <- I64.add (I64.mul acc #10L) (I64.of_int (Char.code c - 48));
         i <- i + 1
       | _ -> valid <- false
     done;
     acc