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 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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