用于将可选字符串解析为int

Question

Say I have the following for comprehension:

val validatedInput = for {
    stringID <- parseToInt(optionalInputID)
} yield (stringID)

where optionalInputID is an input parameter of type Option[String] . I want to be able to convert an Option[String] into just a String, if of course there is an option present. As far as I'm aware, you cannot case match inside a for comprehension.

Some details have been omitted, such as other for comprehension items. Therefore I would like to know if it's possible to do this inside the for comprehension. If not, then what's a suitable alternative? Can I do it outside of the for comprehension?

Answer 1

Simply add it to the for comprehension :

val validatedInput = for {
  inputID <- optionalInputID
  stringID <- parseToInt(inputID)
} yield (stringID)

It will work only if parseToInt has type of Option . If it returns something of Try , you can't do it - because you can't mix Try and Option in the same for-comprehension.

If parseToInt returns Try , you can do the following:

val validatedInput = for {
  inputID <- optionalInputID
  stringID <- parseToInt(inputID).toOption
} yield (stringID)

Answer 2

I want to be able to convert an Option[String] into just a String.

Therefore I would like to know if it's possible to do this inside the for comprehension

In Scala, for-comprehension desugars into a combinitation of map , flatMap , filter , none of which allows to extract the value out of the Option .

If not, then what's a suitable alternative? Can I do it outside of the for comprehension?

To do so you can use one of get (unsafe), or it safer version getOrElse , or fold :

val validatedInput: Option[String] = Some("myString")

scala>validatedInput.get
// res1: String = "myString"

scala>validatedInput.getOrElse("empty")
// res2: String = "myString"

scala>validatedInput.fold("empty")(identity)
// res3: String = "myString"