Why do I get 'A function is marked as tail-recursi...

# getting-started

[Adam S]

Why do I get 'A function is marked as tail-recursive but no tail calls are found' and 'Recursive call is not a tail call'? Is there a way to make this function work with tailrec?

class Container(
    val name: String,
    val parent: Container?
)

// WARNING A function is marked as tail-recursive but no tail calls are found
tailrec fun Container.rootParent() : Container {
  return parent?.rootParent() // WARNING Recursive call is not a tail call
    ?: this
}

playground

[Klitos Kyriacou]

This version doesn't warn about it not having tail recursion:

tailrec fun Container.rootParent() : Container {
    if (parent == null) return this
    return parent.rootParent()
}

[Klitos Kyriacou]

Your original version is directly equivalent to this, which also warns about tail recursion, even though it knows (and warns about) the condition rp == null always being false.

tailrec fun Container.rootParent() : Container {
    if (parent == null) return this
    val rp = parent.rootParent()
    if (rp == null) return this
    return rp
}

[Klitos Kyriacou]

Interestingly, this also issues the warning:

tailrec fun Container.rootParent() : Container {
    if (parent == null) return this
    val rp = parent.rootParent()
    return rp
}

But if you remove the val rp and return directly, it doesn't give the warning. Seems like a limitation in the compiler's capabilities.

[Adam S]

nice, thanks!

[Daniel Pitts]

This also appears to work:

tailrec fun Container.rootParent():Container = if (parent == null) this else parent.rootParent()

Ironically, IntelliJ tells me that it can be replaced with parent?.rootParent() ?: this, but then the warning happens.

[Daniel Pitts]

Also, avoiding tailrec, this would work:

fun Container.rootParent() = generateSequence(this) { it.parent }.last()