get_pos_len_exn, and check_pos_len_exn are intended to be used by functions that take a sequence (array, string, bigstring, ...) and an optional pos and len specifying a subrange of the sequence. Such functions should call get_pos_len with the length of the sequence and the optional pos and len, and it will return the pos and len specifying the range, where the default pos is zero and the default len is to go to the end of the sequence.

It should be the case that:

  pos >= 0 && len >= 0 && pos + len <= total_length

Note that this allows pos = total_length and len = 0, i.e., an empty subrange at the end of the sequence.

get_pos_len_exn returns (pos', len') specifying a subrange where:

  pos' = match pos with None -> 0 | Some i -> i
  len' = match len with None -> total_length - pos' | Some i -> i

check_pos_len_exn ~pos ~len ~total_length raises unless pos >= 0 && len >= 0 && pos + len <= total_length.

Like get_pos_len_exn. Returns an Or_error.t.