Hi Nicholas,

If you use a where statement, GHC only has to compute the computation group year once, because it can be shared.

year1 [y1,y2,y3,y4x] = y1
year2 [y1,y2,y3,y4x] = y2
year3 [y1,y2,y3,y4x] = y3
year4x [y1,y2,y3,y4x] = y4x
cow_group_sum year = sum (group year)

      --look here--
group 1 = [1,0,0,0]
group (year+1) = [year4x gy, year1 gy,year2 gy,year4x gy + year3 gy]
        where gy = group year

main = print (cow_group_sum 50)

This works a lot faster.

I don't understand why GHC doesn't pick this up.

Greets,

Edgar
On Mon, Nov 8, 2010 at 2:08 PM, nicholas.ulysses <nicholas.ulysses@gmail.com> wrote:
It's my code to do some recursive things, focus on function 'group'.

---- cow.hs
 year1 [y1,y2,y3,y4x] = y1
 year2 [y1,y2,y3,y4x] = y2
 year3 [y1,y2,y3,y4x] = y3
 year4x [y1,y2,y3,y4x] = y4x
 cow_group_sum year = sum (group year)

 --look here--
 group 1 = [1,0,0,0]
 group (year+1) = [(year4x (group year)), (year1 (group year)),
(year2 (group year)), ((year4x (group year)) + (year3 (group year)))]


 main = print (cow_group_sum 50)
---- end

Every time 'group' was called, the (group year) function was called 4
times . when call (group 30), it takes tens seconds to finish.

I have tried 'ghc -O3', but it's still 'lazy' on this thing.

Map-Reduce seems to be the answer to the question. But I want to make
sure that ghc CAN or CANNOT do the Optimize for Dynamic Programming?


----------------
xingbo wu
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