[Haskell-cafe] Re: Shootout summary

[Matthias Neubauer]

dons at cse.unsw.edu.au (Donald Bruce Stewart) writes:

>> > Fannkuch entry by Bertram Felgenhauer
>> > Mandelbrot entry
>> 
>> I've done some benchmarking of the current entries for fannkuch and
>> mandelbrot, and have proposed final entries for these two tests.

Using >>= of the list monad in the current Fannkuch proposal
(permutations) hides some costly ++ applications that can be also
optimized away:

Instead of writting
 
  permutations l = foldr perm' [l] [2..length l] 
      where perm' n l = l >>= take n . iterate (rotate n)
 
saying something like 

  permutations l = foldr perm' [l] [2..length l]
    
  perm' n  = foldr (takeIter n (rotate n)) []

  takeIter :: Int -> (a -> a) -> a -> [a] -> [a]
  takeIter 0 f x rest = rest 
  takeIter n f x rest = x : takeIter (n-1) f (f x) rest

gains us another 5% or so.

-Matthias

-- 
Matthias Neubauer                                       |
Universität Freiburg, Institut für Informatik           | tel +49 761 203 8060
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