Tail Recursion In Haskell


Recursion in its simplest form can be understood as a function that calls itself.

fact x = x * fact (x - 1)

In the above function, fact(x) is equal to x times the value of fact(x-1). fact can be described as infinitely recursive; it will never complete because it doesn't have a base case. A base case could be something like fact 1 = 1, where the function no longer recurses and will be capable of terminating.

fact 0 = 1
fact x = x * fact (x - 1)

If x is larger than 0, fact will eventually terminate, and the factorial of that number will be returned.

Tail Recursion

The term tail recursion refers to a form of recursion in which the final operation of a function is a call to the function itself.

fact2 x =
  tailFact x 1
  where tailFact 0 a = a
        tailFact n a = tailFact (n - 1) (n * a)

The fact2 function wraps a call to tailFact a function that's tail recursive. The first argument n in tailFact tells the function we want the nth factorial. Second, a, is an accumulator that maintains the values of the previous multiplication operations.

Why use Tail Recursion?

So even the simple examples make it obvious, tail recursion can come with some added complexity. What good is it other than to confuse other readers of your code? The answer has to do with how most programming languages handle function calls. Take this small example:

foo x = 10 + x
bar = 3 * foo 4

Say your program is in function bar and it reaches the call to foo. It then proceeds to execute the code at the memory address of the foo function. When foo completes, how does your program know where to go back to? The call stack, of course.

Generally, the call stack is a structure in memory that tracks the current depth of a running program. As functions call other functions, the number of locations on the stack grows. You can think of it as digital breadcrumbs.

As with any memory structure, there is a limit to how large the call stack can grow. It's large enough to not worry about most of the time. However, when doing recursion, the number of items on the stack can rise quickly. Running out of room can result in a stack overflow, which will likely terminate your program or at least not give you the result you expected.

Assuming a language's compiler can optimize for it, Tail recursion can help overcome this issue. Because there are no hanging operations left in the function, the same stack frame can be reused. That's why an accumulator was needed in the tailFact function; it eliminates having to multiply after the recursive call.

Another Example: Fibonacci Numbers

A popular place for using recursion is calculating Fibonacci numbers. They are part of a sequence as follows: 1,2,3,5,8,13,21... Starting at 1, each term of the Fibonacci sequence is the sum of the two numbers preceding it. A simple recursive solution in Haskell is as follows:

fibs 0 = 1
fibs 1 = 1
fibs n = fibs (n - 1) + fibs (n - 2)

Notice that the fibs function needs to call itself twice to calculate the nth Fibonacci. The number of recursive calls grows exponentially where the first two calls will each make two of their own, and so on.

Using tail recursion, while slightly more complex, will prevent the exponential growth of function calls.

fibs2 = tailFibs 0 1 0 

tailFibs prev1 prev2 start end 
          | start == end = next
          | otherwise = tailFibs next prev1 (start + 1) end
          where next = prev1 + prev2

The workhorse in this solution is tailFibs, takes four arguments, three are accumulators of some sort. prev1 and prev2 are the previous first and second terms respectively. Start is the index of the currently calculated term, and end is passed through on each call for the base condition.

In this instance, tail recursion turns out to be much more performant over the simpler implementation. Attempting to get even the 100th term shows a significant difference between them.

Use Appropriately

Being able to approach problems with the power of tail recursion can help conquer in a way that feels natural, without mutable state, and without worry of stack overflows. It's part of what makes functional languages like Haskell shine.

Tail recursion, while useful, is best used for algorithms that are recursive in nature and likely to wind up going very deep. Otherwise, you may wind up with something far more complex and fragile than necessary.

28 Jun 2010