Hello,

I'm quite new to Haskell, but this is my understanding... Please correct me if I am wrong, as there is a good chance I am.  ;)

### Begin Code ###
module Main where

main =
  putStrLn (show( (take 4 . map (> 0)) (f s t) ))
  where
    s = 2 : t
    t = 3 : s
    f = zipWith (-)
{-
 - Output:
 - *Main> main
 - [False,True,False,True]
 -}
   
{- 
 - (take 4 . map (> 0)) (f s t)
 - Evaluates the list for take until 4 elements have been reached.
 - Below I replaced (f s t) with the values to make the evaluation
 - explicit.
 -
 - Evaluation:
 -
 - map (> 0) (zipWith (-) [2 ..] [3 ..])
 - False -- 1st element for take.
 -
 - map (> 0) (zipWith (-) [3 ..] [2 ..])
 - True -- 2nd element for take.
 -
 - map (> 0) (zipWith (-) [2 ..] [3 ..])
 - False -- 3rd element for take.
 -
 - map (> 0) (zipWith (-) [3 ..] [2 ..])
 - True -- 4th element for take.
 -}

-- EOF.
### End Code ###

Hope that helps.
__
Donnie Jones


On Fri, May 9, 2008 at 1:52 PM, PR Stanley <prstanley@ntlworld.com> wrote:
Hi
 (take 4 . map (>0)) (f s t)
 where
       s = 2 : t
       t = 3 : s
 f = zipWith (-)
What would be the order of evaluation for the above code? How would I illustrate the evaluation step-by-step?
I'm guessing that  the code necessitates lazy evaluation and as such it starts with take then it applies f which in turn applies s and t and zipWith until the first element satisfies the predicate in map and This is repeated 4 times
What does the list think?
Many thanks,
Paul
P.S. I'm not done with induction. I'm just letting it rst for a bit.

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