Hi Cafe,

I have two programs for the same problem "Eight queens problem",

My two grograms only has little difference, but the performance, this is my solution:

-- solution 1------------------------------------------------------------

queens1 :: Int -> [[Int]]

queens1 n = map reverse $ queens' n

where queens' 0 = [[]]

queens' k = [q:qs | q <- [1..n], qs <- queens' (k-1), isSafe q qs]

isSafe try qs = not (try `elem` qs || sameDiag try qs)

sameDiag try qs = any (ë(colDist, q) -> abs (try - q) == colDist) $ zip [1..] qs

-- solution 2--------------------------------------------------------------

queens2 :: Int -> [[Int]]

queens2 n = map reverse $ queens' n

where queens' 0 = [[]]

queens' k = [q:qs | qs <- queens' (k-1), q <- [1..n], isSafe q qs]

isSafe try qs = not (try `elem` qs || sameDiag try qs)

sameDiag try qs = any (ë(colDist,q) -> abs (try - q) == colDist) $ zip [1..] qs

the performance difference is: (set :set +s in ghci)

*Main> length (queens1 8)

(287.85 secs, 66177031160 bytes)

*Main> length (queens2 8)

(0.07 secs, 17047968 bytes)

*Main>

The only different in the two program is in the first is "q <- [1..n], qs <- queens' (k-1)," and the second is "qs <- queens' (k-1), q <- [1..n]".

Does sequence in list comprehansion matter? And why?