Re: [Haskell-beginners] List operations

You're not applying the operation 1000000 times, you're traversing to the 1000000th node in the list and applying the operation once. Laziness never evaluates the other thunks. On Thu, May 19, 2011 at 12:15 PM, Ertugrul Soeylemez wrote:

Daniel Fischer wrote:

...

However, since you're asking about cost, which indicates that you care for performance, the above would be better written as

[x*x*x | x <- list]

unless you depend on the small differences in the outcome [I'm not quite sure how many bits may be affected, not many, typically none or one]. Functions like (**), exp, log, sin, cos, ... are slow, very slow. If the exponent is a small (positive) integer, specifically giving a sequence of multiplication steps is much faster, also using (^) instead of (**) is faster for small exponents (but slower than an explicit multiplication sequence).

Neither does this really match my intuition, nor can I confirm it with an experiment. Applying (** 3) a million times to a Double takes a second and gives me the expected Infinity. Applying (^3) or (\x -> x*x*x) a million times to the same value, well, I didn't want to wait for it to finish.

The experiment was ran with the following codes in GHCi:

iterate (** 3) 1.000000001 !! 1000000 iterate (^3) 1.000000001 !! 1000000 iterate (\x -> x*x*x) 1.000000001 !! 1000000

Note that exponentiation is a cheap operation on Double.

Greets Ertugrul

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