Hi,
sorry my english is not the best :(
2007/11/5, gitulyar
Please help me. I'm new in Haskell programming, but wrote some things in Scheme. I make so function:
fib 1 = 1 fib 2 = 2 fib n = fib (n-1) + fib (n-2)
And when I call "fib 30" it works about 5 seconds. As for me it's really TOO SLOW.
Because the scheme is Inefficient If you define fib like this: dfib 0 = (1,1) dfib n = let (a,b) = dfib (n-1) in (b, b+a) -- dfib n = (fib n, fib (n+1)) this explote lazy evaluation fib n = fst (dfib n) With this definition the lazy evaluation calculate only one fib 1, one fib 2......etc.
Tell me please if I have something missed, maybe some compiler (interpretaitor) options (I use ghc 6.6.1).
The scheme is bad, no ghci.
P.S. As I understand function "fib n" should be calculated one time. For example if I call "fib 30" compiler builds tree in which call function "fib 28" 2 times and so on. But as for lazy calculation principle it should be calculated just ones and then it's value is used for all other calls of this function with the same argument. But it seems that this principle doesn't work in this algorithm.
If you have this: mult:Int->Int mult x = x + x + x ------------------------------------------------------------------- mult (fib 20) = <Definition> (fib 20) + (fib 20) + (fib 20) = < By lazy evaluation, this is equal..> x + x + x where x = fib 20 ------------------------------------------------------------------- In this case fib 20 calculate only the first call, no three times. But fib 20 fib 20 = < Definition> fib 19 + fib 18 Then the calulate of fib 19 and fib 18 individualy