I am trying to read a string from stdin and flush it out to stdout but I can't find a Standard ML way

Question

NOTE: I'm totally Newbie in Standard ML. I merely have basic F# knowledge.

This is a good ol' code in C

#include <stdio.h>

int main()
{
  char str[100]; // size whatever you want

  scanf("%s", str);
  printf("%s\n", str);
  return 0;
}

now, I want to make a Standard ML-version-equivalent of this code. so I tried this:

val str = valOf (TextIO.inputLine TextIO.stdIn)
val _ = print str

but my SML/NJ says this:

uncaught exception Option
 raised at: smlnj/init/pre-perv.sml:21.28-21.34

I googled it, and I also searched this site, but I cannot find any solution which doesn't cause error.

does anyone knows it?

EDIT: I tried this code:

fun main =
    let val str = valOf (TextIO.inputLine TextIO.stdIn)
    in
        case str
            of NONE => print "NONE\n"
            | _     => print str
    end

but it also makes error:

stdIn:1.6-1.10 Error: can't find function arguments in clause
stdIn:4.9-6.33 Error: case object and rules don't agree [tycon mismatch]
  rule domain: 'Z option
  object: string
  in expression:
    (case str
      of NONE => print "NONE\n"
       | _ => print str)

Answer 1

This answer was pretty much given in the next-most recent question tagged sml: How to read string from user keyboard in SML language? -- you can just replace the user keyboard with stdin, since stdin is how you interact with the keyboard using a terminal.

So you have two problems with this code:

fun main =
    let val str = valOf (TextIO.inputLine TextIO.stdIn)
    in
        case str
            of NONE => print "NONE\n"
            | _     => print str
    end

One problem is that if you write fun main then it has to take arguments, e.g. fun main () = .... The () part does not represent "nothing" but rather exactly one thing, being the unit value.

The other problem is eagerness. The Option.valOf function will crash when there is no value, and it will do this before you reach the case-of, making the case-of rather pointless. So what you can do instead is:

fun main () =
    case TextIO.inputLine TextIO.stdIn of
         SOME s => print s
       | NONE => print "NONE\n"

Using the standard library this can be shortened to:

fun main () =
    print (Option.getOpt (TextIO.inputLine TextIO.stdIn, "NONE\n"))

I encourage you to read How to read string from user keyboard in SML language?