[Haskell-cafe] Order of Evaluation

[Donnie Jones]

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 at 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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