Re: [Haskell-cafe] strict, lazy, non-strict, eager

On Sun, Dec 25, 2011 at 12:14 AM, Eugene Kirpichov wrote:

On Sat, Dec 24, 2011 at 10:49 PM, Dan Doel wrote:

...

I think it's good to be clear on all these specifics, and people could do with a better recognition of the difference between (non-)strict and (lazy)eager (hint: you can have an eager, non-strict language).

Can you elaborate? That's apparently my blind spot.

A while back, there was a paper on something called (I believe) optimistic evaluation. The strategy goes like this: when you evaluate 'f x', first you start evaluating 'x'. If that takes too long, or you encounter an exception, you (re)thunk it, and continue evaluating the body of f lazy style, in case you don't really need x. This is arguably eager, since you reduce arguments to functions immediately if possible. And it has some advantages over lazy evaluation in common cases. For instance, it avoids foldl building up a huge nested thunk that would cause a stack overflow. But it isn't strict, because const 5 undefined is still 5. You can also imagine sparking on every function application, so that arguments automatically get reduced in parallel, and as soon as possible. I think that's been determined to not be very effective, though. -- Dan