On Fri, Jul 17, 2009 at 12:41 PM, Cristiano Paris wrote:

... 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?

BTW, after a -O2 compilation, fib' is apparently as fast a fib. Cristiano