[Haskell-cafe] Order of Evaluation

[Albert Y. C. Lai]

Lennart Augustsson wrote:
> Even so, it's instructive to study how the normal order reduction of 
> this expression would proceed under the assumption that all 4 elements 
> will be used.

I think it's useful to try normal order until weak head normal form.

Not all steps are shown. Definitions of take, map, zipWith are taken 
from the Haskell 98 Report. Whenever you see me expanding a parameter, 
it is because some function's pattern matching forces it.

(take 4 . map (>0)) (f s t)
take 4 (map (>0) (f s t))
take 4 (map (>0) (zipWith (-) s t))
take 4 (map (>0) (zipWith (-) (2:t) (3:s)))
take 4 (map (>0) ( 2-3 : zipWith (-) t s ))
take 4 ( 2-3>0 : map (>0) (zipWith (-) t s) )
2-3>0 : take (4-1) (map (>0) (zipWith (-) t s))