On 17 Jul 2009, at 12:41, Cristiano Paris wrote:
Thank you all for your answers and sorry for the delay I'm writing this message but before replying, I wanted to be sure to understand your arguments! Now, I'm starting to get into this "tying the knot" thing and understand why the Haskell version of fib ties the knot while my Python version does not. It seems all related to the thunk thing, i.e. in the Haskell version the subsequent calls to fib are not actual calls because they all refers to the same thunk, which is evaluated "on demand". Now, to confirm my hypothesis, I wrote a slight different version of fib, like follows: fib' n = 1:1:(fib' n) `plus` (tail $ fib' n) where plus = zipWith (+) i.e. I inserted a fictious argument n in the definition of fib'. Now, if I try "take 30 $ fib' 100", it takes significntly longer than "take 30 fib": specifically, the latter is instantaneous, while the former takes about 5 seconds to complete on my MacBook Pro. Is this an evidence that the "tying the knot" process is going on in the first version?
That's correct
More, I've read that a fully lazy language would memoize all functions by default: in this case, even fib' would have been tying the knot. But this is not the case of Haskell. Am I wrong?
Memoization is not a feature of lazyness. If you can do it in such a way that you don't waste significant amount of RAM, then it may be a nice optimisation, and an alternative evaluation strategy, but it would not be lazy.
Bob _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
