[ moved to haskell-café ]
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 ----------------------------------------------------------------------
Oops, I didn't notice that nco1 was recursive. I should have said the expression (cis wn) is evaluated once for each call to nco1. It is not evaluated once for each element of the list in map's second argument, which is what I thought was being asked.
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)] ----------------------------------------------------------------------
This difference is due to the different buffering behaviour on stdout/stderr between compiled and interpreted programs. GHCi sets these handles to unbuffered by default, which tends to be more useful for an interpreter. Cheers, Simon