What's wrong with this call to `fold`, which cause...

# getting-started

[Ellen Spertus]

What's wrong with this call to fold, which causes IntelliJ to give me a very odd suggestion?

Untitled.kt

[Ellen Spertus]

IntelliJ suggests (on line 3) changing List<Int> to List<Int>. (Spoiler: It doesn't help.)

[Ellen Spertus]

The problem turned out to be due to the placement of the plus signs. If I move them from the starts of lines 5 and 6 to the ends of lines 4 and 5, the code works.

[Stephan Schroeder]

interesting, this might be one of those case where the Kotlin compiler gets confused (= the expression is ambiguous) due to the language not using semicolons!?? 🤔

[Michael de Kaste]

interesting, if you remove the type ( just have fold have "acc, num -> ") then it works

[Michael de Kaste]

return numList.fold(emptyList()) { acc, num ->
    acc.filter { it <= num } +
        num +
        acc.filter { it > num }
}

[Stephan Schroeder]

@Michael de Kaste it's probably not removing the types but putting the pluses at the end of the line that made it work.

[phldavies]

It is indeed because of the location of the pluses. By having them at the start of the lines you're actually invoking the unaryPlus operator, i.e.

acc.filter { it <= num }
+ listOf(num)
+ acc.filter { it > num }

is equivalent to

acc.filter { it <= num }
listOf(num).unaryPlus()
acc.filter { it > num }.unaryPlus()