Donnie Jones donnie at darthik.com
Fri May 9 14:34:47 EDT 2008

```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.
>
> _______________________________________________