tail

simple tail call optimization and stack safety for Java

View the Project on GitHub kag0/tail

com.github.kag0.tail

simple tail call optimization for Java

enables infinitely deep tail recursive calls without throwing a StackOverflowError

no transitive dependencies

Install

add the jitpack repository

<repositories>
...
  <repository>
    <id>jitpack.io</id>
    <url>https://jitpack.io</url>
  </repository>
...
</repositories>

add the dependency

<dependencies>
...
  <dependency>
    <groupId>com.github.kag0</groupId>
    <artifactId>tail</artifactId>
    <version>Tag</version>
  </dependency>
...
</dependencies>

Use

import com.github.kag0.tail.Tail;
import static com.github.kag0.tail.Tail.*;

Tail<Void> infiniteLoop(int i) {
  System.out.println("Loop " + i + ", stack still intact!");
  return call(() -> infiniteLoop(i + 1));
}

infiniteLoop(0).evaluate();

Example: tail optimizing factorial computation

un-optimized first version

let’s start with a simple recursive method to compute the nth factorial. this code will throw a StackOverflowError for large values of n.

long factorial(long n) {
  if(n == 1) return 1;
  else return n * factorial(n - 1);
}

move the recursive call into the tail position

the tail position is just another way of saying “the last thing you do before the return”.

long factorial(long fact, long n) {
  if(n.equals(1)) return fact;
  return factorial(fact * n, n - 1);
}

this may require a slight refactor, usually to add an additional parameter to accumulate progress.

wrap the return type in Tail

this will enforce that the recursive call is in the tail position.

Tail<Long> factorial(long fact, long n)

wrap base cases with done

if(n.equals(0)) return done(fact);

wrap recursive calls with call

return call(() -> factorial(fact * n, n - 1));

profit

call .evaluate() on the invocation of your method.

factorial(1, Long.MAX_VALUE).evaluate();

recursive methods no longer blow the stack.
note that if you skip the ‘move the recursive call into the tail position’ step, the code will not compile because the method is not tail recursive and therefore not stack safe. thanks to Tail that is covered by type safety.

making safe recursive calls outside the tail position

in addition to making tail recursion safe, we can also use trampolining to enable recursive methods that would otherwise be tricky to make tail recursive.

to do this, just use .flatMap to chain two calls together.
for example

Tail<Integer> ackermann(int m, int n) {
  if(m == 0) 
    return done(n + 1);
  if(m > 0 && n == 0) 
    return call(() -> ack(m - 1, 1));
  if(m > 0 && n > 0) 
    return call(() -> ack(m, n - 1)).flatMap(nn -> ack(m - 1, nn));
  throw new IllegalArgumentException();
}

Benchmarks