I'm trying to write a Kotlin contract so I can get...

# announcements

[Marcin Wisniowski]

I'm trying to write a Kotlin contract so I can get smart casts in code calling my function, but this syntax doesn't work, what's the correct way to achieve this?

[Shawn]

I haven’t really worked with contracts yet, but wouldn’t the correct implies predicate be closer to objects is Array<Any>

[Shawn]

also if I’m reading this right, are you trying to use this for a sort of multi-let situation where you’re checking if multiple objects are null?

[Marcin Wisniowski]

Correct, is there something existing for this situation?

[Marcin Wisniowski]

This doesn't work either

[Shawn]

right, that makes sense

[Shawn]

Honestly I’m not sure so I can’t say definitely either way, but I don’t believe there’s a way to do it within the current constraints of the contracts and type system

[Shawn]

considering that vararg objects is really just a bunch of syntax sugar for variadics that results in getting an Array<T> passed in without having to make callers write function(arrayOf(...))

[Shawn]

there isn’t really a good way to relay to the type system via a function that all variables you called this notNull() function with can now be safely smart-casted to non-null types

[Marcin Wisniowski]

So is there any way to improve this if statement?

[Marcin Wisniowski]

I'm considering keeping my notNull() function without the contract, and just using !! inside which is guaranteed to be safe, but still doesn't feel right.

[Shawn]

I’m really not certain, someone else who knows the contracts system is probably gonna have to step in to give you a definitive answer, sorry 😕

[Marcin Wisniowski]

Ok thank you for your help. 🙂

[Shawn]

If you find yourself doing this kind of thing a lot, you could consider factoring the logic out into a small handful of utility methods

[Shawn]

I’ve never actually used it myself before, but I do have this saved in my “kotlin junk drawer” of sorts

inline fun <A, B, C, R> where(a: A?, b: B?, c: C?, block: (A, B, C) -> R?): R? = if (
    a != null &&
    b != null &&
    c != null
  ) block(a, b, c) else null

[Shawn]

contrived example:

fun frobnicate(i: Int?, s: String?, c: Char?) {
  where(i, s, c) { int, string, char ->
    println(int.toString() + string + char)
  }
}

[Marcin Wisniowski]

That would work, but the calling code now has to rename (or shadow) all the variables in the lambda. 😕

[Shawn]

you’re absolutely right, it’s not the most convenient

[Shawn]

tradeoffs, I suppose 😕

[karelpeeters]

I guess you could copy/paste or even generate code for this for 1 to 10 parameters, that should work.