tailrec

Learn recursion safety with tailrec.

What does tailrec do?

tailrec makes a recursive function tail-recursive; the compiler turns it into a loop. This avoids stack overflow on large inputs.

Example: tailrec fun sumTo(n: Long, acc: Long = 0L): Long = if (n <= 0L) acc else sumTo(n - 1L, acc + n). Here the recursive call is the last step of the function; that is a tail call. Without tailrec, Kotlin does not convert it; large n risks stack overflow. Functions that are not tail calls produce a warning with tailrec.

Recursion with an accumulator

Many recursive functions can be rewritten as tail calls by adding an accumulator parameter. The accumulator carries the intermediate result.

Example: tailrec fun factorial(n: Long, acc: Long = 1L): Long = if (n <= 1L) acc else factorial(n - 1L, acc * n). Here acc holds the running product; each step lowers n and grows acc. Call: factorial(5L). Result is 120. This pattern scales safely to big inputs.

Exercise Write tailrec sumTo

Define tailrec fun sumTo(n: Long, acc: Long = 0L): Long. If n <= 0L return acc; otherwise call sumTo(n - 1L, acc + n). In main, write println(sumTo(10L)).

Show example solution

tailrec fun sumTo(n: Long, acc: Long = 0L): Long =
    if (n <= 0L) acc else sumTo(n - 1L, acc + n)
fun main() {
    println(sumTo(10L))
}

Test yourself

When does tailrec fail?
- When the recursive call is the last step
- When Long is used
- When there is work after the recursive call
- When an accumulator parameter is used
Show answer

When there is work after the recursive call — tailrec only works on real tail calls.

Add the tailrec keyword

____ fun sumTo(n: Long, acc: Long = 0L): Long =
    if (n <= 0L) acc else sumTo(n - 1L, acc + n)

Show answer

tailrec fun sumTo(n: Long, acc: Long = 0L): Long =
    if (n <= 0L) acc else sumTo(n - 1L, acc + n)

Match tailrec parts

tailrec

Loop conversion marker
acc

Accumulator
n - 1L

Decreasing input
if (n <= 0L)

Base case

Next lesson: Type-safe builders

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