type inference on empty arrays kotlin

Question

Let's say I have a piece of code like:

fun temp2 (li : MutableList<Int>):Int {
    if (li.isEmpty()) return 0
    val pos=li.filter { it>0 }
    val neg=li.filter { it<0 }

    if (pos.isEmpty() && neg.isNotEmpty()){

        // this gives compiling error because Required: Int, Found: Int?
        // But I just checked one line higher that neg is Not Empty, so there (at least I guess) 
       // no possible way to have an NPE?
        //return neg.max()
          return neg.max()!! //this works fine
    }

Is there any particular reason why compiler doesn't infer that .max() can only yield an Int, and thus this should not be an error, or am I missing something? Kotlin's documentation proudly points out about Smart Casts, and I think this is a quite similar and easy inference to make?

s1m0nw1 · Accepted Answer · 2018-01-29 10:07:56Z

4

That can’t be handled by smart casting, you're using a max() extension function which always returns a nullable type, Int? in your case:

public fun <T : Comparable<T>> Iterable<T>.max(): T?

The compiler does what this method signature suggests: it makes you handle the possible null. How should the compiler know wether max works as intended? It might be implemented wrongly.

The following, on the other hand, works thanks to smart casting:

val maxNeg: Int? = li.filter { it < 0 }.max()
if (maxNeg != null) {
    return maxNeg //Can be used as Int
}

edited Jan 29, 2018 at 10:07

answered Jan 27, 2018 at 15:55

s1m0nw1

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3 Comments

Denis Over a year ago

I understand it, but that's my point, there is no null possible :/

s1m0nw1 Over a year ago

@dgan you’re right, it can’t be null. Have a look at the description of smart casting again and you will notice that the examples are different: kotlinlang.org/docs/reference/typecasts.html

s1m0nw1 Over a year ago

@dgan: the compiler can’t be sure that max works as intended. It simply sees the return type which might be null

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