building a lexical analyser using ml-lex

I need to create a new instance of a lexer tied to the standard input stream.

However, when I type in

val lexer = makeLexer( fn n => inputLine( stdIn ) );

I get an error开发者_StackOverflow社区 that I don't understand:

stdIn:1.5-11.13 Error: operator and operand don't agree [tycon mismatch]
  operator domain: int -> string
  operand:         int -> string option
  in expression:

(makeLexer is a function name present in my source code)

inputLine returns a string option, and my guess is a string is expected.

What you want to do is either have makeLexer take a string option, like so:

fun makeLexer  NONE    = <whatever you want to do when stream is empty>
  | makeLexer (SOME s) = <the normal body makeLexer, working on the string s>

or change your line to:

val lexer = makeLexer( fn n => valOf ( inputLine( stdIn ) ) );

valOf takes an option type and unpacks it.

Note that, since inputLine returns NONE when the stream is empty, it's probably a better idea to use the first approach, rather than the second.

An example of how to make an interactive stream is given on page 38 (or 32 in the paper) of the User's Guide to ML-Lex and ML-Yacc

The example code could be simpler by using inputLine. So I would use the example given by Sebastian, keeping in mind that inputLine might return NONE using stdIn atleast if the user presses CTRL-D.

val lexer =
let 
  fun input f =
      case TextIO.inputLine f of
        SOME s => s
      | NONE => raise Fail "Implement proper error handling."
in 
  Mlex.makeLexer (fn (n:int) => input TextIO.stdIn)
end

Also the calculator example on page 40 (34 in the paper) shows how to use this in a whole

In general the user guide contains some nice examples and explanations.