I missed something. What is the argument for having MLton.TextIO.mkstemp
return only the string? This would be very confusing since the file has been
made. Also I don't understand the suffix. I think that the way to go is
either
string -> string * TextIO.outstream
or
string -> string * TextIO.instream * TextIO.outstream
where the string argument is just like in the C mkstemp, but without the
trailing "XXXXXX".