Simon Marlow wrote:
The original version should also evaluate the expression 'cis wn' only once: [...]
Nice theory, but GHC's interpreter and compiler behave differently: -- Main.hs ----------------------------------------------------------- module Main where import Data.Complex ( Complex ) import qualified Data.Complex ( cis ) import Debug.Trace cis :: (RealFloat a) => a -> Complex a cis x = trace "cis" $ Data.Complex.cis x nco1, nco2 :: RealFloat a => a -> [ Complex a ] nco1 wn = 1 : map ((*) (cis wn)) (nco1 wn) nco2 wn = iterate (cis wn *) 1 main :: IO () main = do let test f = print . take 5 . f $ (pi/2 :: Double) test nco1 test nco2 ---------------------------------------------------------------------- panne@jeanluc:~> ghci Main.hs ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 5.05, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Compiling Main ( Main.hs, interpreted ) Ok, modules loaded: Main. *Main> main [1.0 :+ 0.0,cis 6.123031769111886e-17 :+ 1.0,cis (-1.0) :+ 1.2246063538223773e-16,cis (-1.836909530733566e-16) :+ (-1.0),cis 1.0 :+ (-2.4492127076447545e-16)] [1.0 :+ 0.0,cis 6.123031769111886e-17 :+ 1.0,(-1.0) :+ 1.2246063538223773e-16,(-1.836909530733566e-16) :+ (-1.0),1.0 :+ (-2.4492127076447545e-16)] *Main> :quit Leaving GHCi. panne@jeanluc:~> ghc -Wall -O Main.hs && ./a.out cis cis cis cis [1.0 :+ 0.0,6.123031769111886e-17 :+ 1.0,(-1.0) :+ 1.2246063538223773e-16,(-1.836909530733566e-16) :+ (-1.0),1.0 :+ (-2.4492127076447545e-16)] cis [1.0 :+ 0.0,6.123031769111886e-17 :+ 1.0,(-1.0) :+ 1.2246063538223773e-16,(-1.836909530733566e-16) :+ (-1.0),1.0 :+ (-2.4492127076447545e-16)] ---------------------------------------------------------------------- ??? Cheers, S.