# [Haskell-cafe] list comprehansion performance has hug different

Junior White efiish at gmail.com
Tue Jan 29 10:25:49 CET 2013

```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 (λ(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 (λ(colDist,q) -> abs (try - q) == colDist)
\$ zip [1..] qs

the performance difference is: (set :set +s in ghci)
*Main> length (queens1 8)
92
(287.85 secs, 66177031160 bytes)
*Main> length (queens2 8)
92
(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?
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