F# tail recursion and why not write a while loop?

Question

I'm learning F# (new to functional programming in general though used functional aspects of C# for years but let's face it, that's pretty different) and one of the things that I've read is that the F# compiler identifies tail recursion and compiles it into a while loop (see http://thevalerios.net/matt/2009/01/recursion-in-f-and-the-tail-recursion-police/).

What I don't understand is why you would write a recursive function instead of a while loop if that's what it's going to turn into anyway. Especially considering that you need to do some extra work to make your function recursive.

I have a feeling someone might say that the while loop is not particularly functional and you want to act all functional and whatnot so you use recursion but then why is it sufficient for the compiler to turn it into a while loop?

Can someone explain this to me?

Answer 1

You could use the same argument for any transformation that the compiler performs. For instance, when you're using C#, do you ever use lambda expressions or anonymous delegates? If the compiler is just going to turn those into classes and (non-anonymous) delegates, then why not just use those constructions yourself? Likewise, do you ever use iterator blocks? If the compiler is just going to turn those into state machines which explicitly implement IEnumerable<T>, then why not just write that code yourself? Or if the C# compiler is just going to emit IL anyway, why bother writing C# instead of IL in the first place? And so on.

One obvious answer to all of these questions is that we want to write code which allows us to express ourselves clearly. Likewise, there are many algorithms which are naturally recursive, and so writing recursive functions will often lead to a clear expression of those algorithms. In particular, it is arguably easier to reason about the termination of a recursive algorithm than a while loop in many cases (e.g. is there a clear base case, and does each recursive call make the problem "smaller"?).

However, since we're writing code and not mathematics papers, it's also nice to have software which meets certain real-world performance criteria (such as the ability to handle large inputs without overflowing the stack). Therefore, the fact that tail recursion is converted into the equivalent of while loops is critical for being able to use recursive formulations of algorithms.

Answer 2

A recursive function is often the most natural way to work with certain data structures (such as trees and F# lists). If the compiler wants to transform my natural, intuitive code into an awkward while loop for performance reasons that's fine, but why would I want to write that myself?

Also, Brian's answer to a related question is relevant here. Higher-order functions can often replace both loops and recursive functions in your code.