[Haskell-beginners] List operations

[Alex Rozenshteyn]

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 <es at ertes.de> wrote:

> Daniel Fischer <daniel.is.fischer at googlemail.com> 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
>
>
> --
> nightmare = unsafePerformIO (getWrongWife >>= sex)
> http://ertes.de/
>
>
>
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-- 
          Alex R
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