Better, cleaner, briefer, more idiomatic use of Scala's Console

option.scala

import scala.annotation.tailrec

import java.io.EOFException
import java.lang.NumberFormatException

object Sandbox extends App {
  override def main(args: Array[String]) = {
    print("Enter an int, and I'll compute the factorial: ")

    val done = for {
      userInt <- safeIntRead()
      valid <- Some(userInt) if userInt > 0
      done <- Some(fact(valid))
    } yield done

    println( done getOrElse "could not compute" )
  }

  def safeIntRead() = {
    try{ Option(readInt) }
    catch {
      case _ => None
    }
  } 

  def fact(x: BigInt) = {
    require(x >= 0)

    @tailrec
    def factRec(x: BigInt, acc: BigInt): BigInt = {
      if (x > 0) factRec(x - 1, acc * x)
      else acc
    }
 
    factRec(x, 1)
  }
}