sml - building a lexical analyser using ml-lex -

i need create new instance of lexer tied standard input stream.
however, when type in

val lexer = makelexer( fn n => inputline( stdin ) ); 

i error don't understand:

stdin:1.5-11.13 error: operator , operand don't agree [tycon mismatch]   operator domain: int -> string   operand:         int -> string option   in expression: 

(makelexer function name present in source code)

inputline returns string option, , guess string expected.

what want either have makelexer take string option, so:

fun makelexer  none    = <whatever want when stream empty>   | makelexer (some s) = <the normal body makelexer, working on string s> 

or change line to:

val lexer = makelexer( fn n => valof ( inputline( stdin ) ) ); 

valof takes option type , unpacks it.

note that, since inputline returns none when stream empty, it's better idea use first approach, rather second.