pure generators

[Pelle]

On 05/13/2010 01:22 PM, bearophile wrote:
> Pelle:
>
>> It is stored as a list, and is in effect memoized.<
>
> In practice I think Haskell doesn't memoize things there. I think it optimizes most things away. The main point of pure generators is indeed that they give the compiler enough guarantees (just as pure functions) that allow it to perform those optimizations safely. The point of my original post is that I'd like a way to give the D compiler such semantic information, mostly for optimization purposes (but there are other purposes, similarly to the purposes of pure functions).

Yes, it does. Try it!

Prelude> let fibs = 1 : 1 : zipWith (+) fibs (tail fibs)
Prelude> fibs !! 80000

... some time passes, a huge number appears.

Prelude> fibs !! 80000

... almost instantly, the same number.

Edit the 80000 to a large enough number to slow your system down.

Note that all numbers below the large number chosen is very fast as 
well, once the large calculation is done.