How to check a string against a symbols whitelist a functional way in Scala?

Question

I need to guarantee a string only contains allowed symbols. Now I do it this way:

def isCorrect(s: String, allowedChars: String): Boolean = {
  s.distinct.foreach(c => {
    if (!allowedChars.contains(c))
      return false
  })

  true
}

Needless to say this does not look too pretty. Is there a better, more functional way to do this?

Answer 1

For the record, you can make this a little more generic by not limiting yourself to strings of characters, and a little more functional (in my view) by switching the order of arguments and using two argument lists. Here's how I'd write it:

def isCorrect[A](allowed: Set[A])(s: Seq[A]) = s forall allowed

Now you can treat this method as a function and "partially apply" it to create more specialized functions:

val isDigits = isCorrect("0123456789".toSet) _
val isAs = isCorrect(Set('A')) _

Which allows you to do the following:

scala> isDigits("218903")
res1: Boolean = true

scala> isAs("218903")
res2: Boolean = false

scala> isDigits("AAAAAAA")
res3: Boolean = false

scala> isAs("AAAAAAA")
res4: Boolean = true

Or you could still just use something like isCorrect("abcdr".toSet)("abracadabra").