Re: [Haskell-cafe] I really don't understand this

23 Jan 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)
   where
     ins numbers acc =
       (>>=)
         acc
         (\_ ->
            IMS.fold
              (\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)
                       acc1
                       numbers2)
              acc
              numbers)

main = do
   v <- search2 b 999999999
   print v

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.
Cordialement,
Li-yao
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):
v<-acc
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:
http://www.alliot.fr/tmp/example.hs
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)
   where
     ins numbers acc = do
       v <- acc
       IMS.fold
         (\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)
                  acc1
                  numbers2)
         acc
         numbers
main = do
   v <- search b 999999999
   print v
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