Chasing your tail

[pozorvlak]

I have a question for the CS types out there: what exactly is the point of tail-call elimination?

Before you go into great detail about stack overflows, rest assured that I understand that bit. But why is tail-recursive code considered such a good thing? I mean, you write your code in a non-obvious way so that the optimiser can turn it into the loop that you meant to write in the first place. Why not just write the loop yourself? It has the same effect, expresses your intent more clearly, and doesn't require the mental gymnastics to make your code tail-recursive, or to decode it when reading.

I can see why you need it if you're determined to make your language stateless (though actually, that's a related question: how is recursive code with accumulators any safer than iterative code with local state?), but why is it needed in languages like Lisp, which already have perfectly good looping constructs?

Don't get me wrong: recursion is clearly a Good Thing. But all the really interesting uses of recursion, it seems to me, are things like recursion on trees or other data structures, which are not in general tail-recursive.

In other hacking news, I spent much of my evening making a set of spice racks out of chopsticks, duct tape and strawberry boxes. My kitchen is now a fraction tidier.

Comments

Continuation Passing Style

[ryani.livejournal.com]

Tail-call elimination is nice when the tail is known statically, but it is, as you said, just an optimization at that point.

It becomes much more interesting (and important!) when you don't statically know what the tail-call will be.

For example, consider embedding a Prolog-like logic language into Haskell. The easiest way I've seen to implement backtracking is by passing a "success" and "failure" continuation around. (see http://okmij.org/ftp/papers/LogicT.pdf)

-- success continuation
type SK ans a = a -> FK ans -> ans
-- failure continuation
type FK ans = ans

-- LogicT is a monad transformer, where m is the type of the
-- monad to be transformed.  For example, LogicT IO a could 
-- include side effects as part of the computation of a
-- SFKT = "success/failure continuation thread"
newtype LogicT m a = SFKT (forall ans. SK (m ans) a -> FK (m ans) -> m ans)

See the paper for the rest of the details on how this is a Monad and how to use it to implement MonadPlus and a set of other interesting operations like "fair disjunction".

However, these continuation-passing styles rely heavily on the fact that tail-calling a continuation function runs in constant stack space.

Re: Continuation Passing Style

[pozorvlak.livejournal.com]

Cool! In some sense, I suppose this is a more general version of drj11's answer below...

[david jones (from livejournal.com)]

State machines.

[pozorvlak.livejournal.com]

Aha! I knew there was a good answer somewhere.

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