Hi all, I am trying to do something and I want to ...

# announcements

[Ivan Brko]

Hi all, I am trying to do something and I want to check what would be a idiomatic way to do it:
I will simplify the problem here:

Let's say I have the following function

fun getSomeString(): String?

and I have a function that wraps a String (not String?) in some Response

fun getResponse(msg: String): Response

then I have a third function in which I need to return a Response and I am calling the getSomeString, if it returns a String I need to feed it to getResponse, but if it returns null I need to return some ErrorResponse object.

Now, I wanted to make this a one liner, but I'm having problems with thinking how to do it
It would be great if I could do something like this:

fun GetResponse: Response = 
  when(getSomeString()) {
    is String message -> getResponse(message)
    else -> ErrorResponse()
  }

but I can't do this is String message. In my real use-case this is not a String but a data class with a lot of fields, and I could match on all those fields and then recreate the object on the right side of -> but that just doesn't seem right.
How would this usually be done in Kotlin? I am used to some other languages where this approach of naming the String would be done in is String message ->...

[Björn Mayer]

From what I understand from your question, you want to make a safe call:

getSomeString()?.let { getResponse(it)} ?: ErrorResponse()

[Jakub Pi]

You can use the elvis operator or a ?.let construction if you are only worried about the null check.

[Ivan Brko]

Hi, thanks both for the answer. I come from languages with strong pattern matching so I immediately went in that direction with when

[Ivan Brko]

I'll do it like that. Do you happen to know if it can somehow be done with when?

[araqnid]

you can declare the variable in the when expression, that is when (val message = getSomeString()), and then the is String clause will permit a smart cast

[aipok]

You can revert when statement

fun GetResponse: Response = 
  when(val message = getSomeString()) {
    null -> ErrorResponse()
    else -> getResponse(message)
  }

This should smartcast message in else case

[Ivan Brko]

thanks for the answer, I knew something like that should work!

[aipok]

But personally for null else case I would prefer to use ?.let{ } ?:

[Ivan Brko]

yeah, it seems like it is more idiomatic in kotlin so i'll go with that, just wanted to know how when handles these situations

[aipok]

or even better would be if else statement

[Jakub Pi]

Alternatively wrap the return in an Option<>

[aipok]

fun GetResponse: Response { 
  val message = getSomeString()
  return if(message != null) {
    getResponse(message)
  } else { 
    ErrorResponse()
  }
}

[Kroppeb]

?.let{ }?: is an anti-pattern imho. The docs tells us that we should prefer if else but I like that you can assign inside a when, something that isn't possible with a if

[aipok]

Yes, thats why I prefer if else over when then possible.

[aipok]

When could generate weird output in bitecode. While if else is straight forward.

[Kroppeb]

I would expect a when with a single case and a an else to become converted to a simple if.

[aipok]

totally agree 😄

[Ivan Brko]

is this something people debate on or is ?.let{} ?: not considered idiomatic code in general? would you see it in popular libraries and so?

I agree that if/else solution is easy to read, but I wanted to see how you would to this with fun ... = instead of fun ... : <type> {return x} as I find the former syntax to be nicer. That's why I like the when solution, as it is easy to read and the problem can be solved as a single expression

[aipok]

I would still prefer to use second variant with if else since I will be sure in what will be generated in bytecode. But if I would not care about that, I would prefer to use when over ?.let

[Kroppeb]

?.let{} ?: is not semantically equivalent with if(it != null) else

[Kroppeb]

Could cause bugs if a different function decides to have a nullable return.

[psh]

Couple of other things you might like to play around with - especially if this is repeated code -

Factory method

fun getResponse() = Response.from(getSomeString())

where I wrapped the if / else into the response class

class Response(private val someData: String) {
    companion object {
        fun from(data: String?) = if (data == null)
            ErrorResponse()
        else
            Response(data)
    }
}

Extension Method

fun getResponse() = getSomeString().toResponse()

and then pulled the if / else into an extension method

fun String?.toResponse() : Response = if (this == null)
    ErrorResponse()
else
    Response(this)