Also playing around with contracts, and I feel lik...

# eap

[Jeff Gulbronson]

Also playing around with contracts, and I feel like I’m missing something

@ExperimentalContracts
fun String?.trueIfNotNullOrEmpty(): Boolean {
  contract {
    returns(true) implies (this@trueIfNotNullOrEmpty != null)
  }
  return !isNullOrEmpty()
}

@ExperimentalContracts
fun foo() {
  val maybeNull : String? = "foo"
  val isNotNull = maybeNull.trueIfNotNullOrEmpty()
  if (isNotNull) {
    val definitelyNotNull : String = maybeNull
  }
}

Intellij is saying I can’t assign maybeNull to definitelyNotNull, because it’s String vs String?. My contract seems right, it’s really just the inverse of what isNullOrEmpty uses. Any idea where I went wrong? Sorry in advance if it’s blindingly obvious.

[dsavvinov]

Actually that won't work irrelevant of contracts. You can change maybeNull.trueIfNotNullOrEmpty() into plain maybeNull != null, and you still won't get smartcast.

Here's an issue for that: https://youtrack.jetbrains.com/issue/KT-25747

[Jeff Gulbronson]

Ah of course, I forgot that maybeNull != null wouldn’t work.

Interestingly I changed the contract to

contract {
    returns(true) implies (this@trueIfNotNullOrEmpty is String)
  }

And still no good, though I would’ve expected this one to work. I suppose contracts are new so I’m not 100% sure what should work and what shouldn’t.

[dsavvinov]

Sorry, am I right that you've changed only contract declaration? In such case, that won't work either -- the issue here is local Boolean isNull. Compiler has to check that this boolean is initialized to something special, and keep in mind that if isNull == true then maybeNull != null (or maybeNull is String, actual kind of smartcast is irrelevant). We don't do that at the moment

(sorry if I got you wrong -- if you've inlined isNull variable and still don't get smartcast, then the issue is somewhere else. If you'll show complete code, we can continue digging here :) )

[Jeff Gulbronson]

Ah, very cool! Inlining worked, it was able to smart cast correctly.

[Jeff Gulbronson]

Interesting extracting the variable caused it to break, even though everything is a val

[Jeff Gulbronson]

Thank you for the help!