Re: Re: block-based programming...

[flyingmoose]

I agree with your post up until the point about tail recursion. Tail recursion is *easily* eliminated by reducing the recursion to a simple loop. This is much friendlier on your stack, and much more efficient! Other forms of recursion are harder to eliminate, but the refactoring of tail recursion is well known.

That being said, tail recursion often serves as as decent way to design an algorithm originally, but it should be eliminated when you get the chance.

In his case, assume the program is a "C" program implemented as a state machine that will run for years. Tail recursion would chew up a tremendous amount of stack and lead to program termination. Refactored as a simple while loop, the program would run forever.

This is one (not so rare) example where the reality of how a computer works oversteps the theory of basic "computer science". What appears to be logically equivalent is not once you put it on real hardware.

edit: darn, it looks like I'm turning into a CS professor now... run away! run away!

[Abigail-II]

This is one (not so rare) example where the reality of how a computer works oversteps the theory of basic "computer science". What appears to be logically equivalent is not once you put it on real hardware.

What has "real hardware" to do with it? All you need is a compiler that's able to detect tail recursion, and eliminate it, instead of putting the burden on the programmer. Yes, such compilers exist. Ironically, they are more common in environments that are more concerned with the 'theory of basic "computer science"' than in environments that use 'real hardware'.

Abigail

[flyingmoose]

So can you give me a list of languages that do/don't eliminate tail recursion automatically? Do you have that list memorized? What gcc versions? MSVC? How about java (some JVM's can, some can't). Does Fortran? What about pascal? COBOL? Python (doesn't)? Where are the efficiency tradeoffs in those various language? Is tail recursion ever faster? (never). How does Perl handle it? What about Ruby? One really can't keep this straight -- especially if you don't confine yourself to a very narrow field of languages. All the more reason to be explicit.

This also goes back to my earlier comments about being blunt and forthright with code. Tail-recursion isn't natural in this case. In math, it's natural, but where a loop applies, use the loop. So why do it? What is the value there?

Again, like the other argument, our neurons are wired differently and I doubt we will reach agreement.

[Abigail-II]

Oh, now you're suddenly concerned about languages. What has 'real hardware' (whatever that means) to do with it? And why would I have to memorize which compilers eliminate tail recursion? That's as useful as to memorize which compilers or languages exist. But rest assured that when I use a compiler and make use of tail recursion, I know whether it will eliminate it or not. Which is all I need to know.

This also goes back to my earlier comments about being blunt and forthright with code. Tail-recursion isn't natural in this case.

If you want to discuss specific cases, then be specific. I didn't argue that tail recursion was ok in this example. Tail recursion was bad in this example. But you extended it to make some general remarks about tail recursion, and that's where I disagree.

Abigail