[Haskell-cafe] I really don't understand this
jm at alliot.org
Tue Jan 23 19:39:40 UTC 2018
Thank you very much for your answer, but I still don't get it (I might
be not bright enough :-))
I rewrote the program to suppress all syntactic sugar; for me, the value
of the first argument of >>= is never used, so I can't see why it
changes anything to put acc as the first argument or anything else (such
as return false for example...).
I would really appreciate a pointer to a chapter of any manual or
introduction to Haskell which would be able to explain why with acc as
first argument of (>==) the program runs at least 2 hours (I stopped it
after 2 hours) and with (return False) it takes less than one second,
while the actual value of the first argument of (>>=) is meaningless.
Thanks again for answering
import qualified Data.IntMultiSet as IMS
b :: IMS.IntMultiSet
b = IMS.fromList ([1, 2, 3, 4])
search2 :: IMS.IntMultiSet -> Int -> IO Bool
search2 mynumbers nb = ins mynumbers (return False)
ins numbers acc =
(\x acc1 ->
let numbers2 = IMS.delete x numbers
(\y acc2 ->
let numbers3 = IMS.delete y numbers2
res = x + y
in if res == nb
then (return True)
else ins (IMS.insert res numbers3) acc2)
main = do
v <- search2 b 999999999
Le 23/01/2018 à 16:03, Li-yao Xia a écrit :
> Bonjour Jean-Marc,
> "acc" contains the computation of the whole search up to a point.
> Calling it once (as part of the fold) makes it grow linearly in the
> size of the search space, but calling it twice (once more as "v <-
> acc") makes it grow exponentially.
> On 01/23/2018 08:35 AM, Jean-Marc Alliot wrote:
>> I apologize if my question is stupid, but here is a simple Haskell
>> program which never stops.
>> However, if I comment the line (which is apparently useless):
>> The program works like a charm...
>> I am using GHC 8.0.2 (debian sid) and multiset 0.3.3. If anyone has
>> an idea, I would gladly hear it.
>> Thanks in advance
>> PS: don't try to understand what the program is doing; it is just the
>> reduction to a few lines of a much larger code; I have tried to find
>> a smaller subset which is not working "properly".
>> PPS: The program can also be downloaded from:
>> import qualified Data.IntMultiSet as IMS
>> b :: IMS.IntMultiSet
>> b = IMS.fromList ([1, 2, 3, 4])
>> search :: IMS.IntMultiSet -> Int -> IO Bool
>> search mynumbers nb = ins mynumbers (return False)
>> ins numbers acc = do
>> v <- acc
>> (\x acc1 ->
>> let numbers2 = IMS.delete x numbers
>> in IMS.fold
>> (\y acc2 ->
>> let numbers3 = IMS.delete y numbers2
>> res = x + y
>> in if res == nb
>> then (return True)
>> else ins (IMS.insert res numbers3) acc2)
>> main = do
>> v <- search b 999999999
>> print v
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