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

Doaitse Swierstra doaitse at swierstra.net
Wed Jan 30 10:51:41 CET 2013

```From the conclusion that both programs compute the same result it can be concluded that  the fact that you have made use of a list comprehension has forced  you to make a choice which should not matter, i.e. the order in which to place the generators. This should be apparent from your code.

My approach is such a situation is to "define your own generator" (assuming here that isSafe needs both its parameters):

pl `x` ql = [ (p,q) | p <-pl, q <- ql]

queens3 n =  map reverse \$ queens' n
where queens' 0       = [[]]
queens' k       = [q:qs | (qs, q) <- queens' (k-1) `x` [1..n], isSafe q qs]
isSafe   try qs = not (try `elem` qs || sameDiag try qs)
sameDiag try qs = any (\(colDist,q) -> abs (try - q) == colDist) \$ zip [1..] qs

Of course you can make more refined versions of `x`, which perform all kinds of fair enumeration, but that is not the main point here. It is the fact that the parameters to `x` are only evaluated once which matters here.

Doaitse

On Jan 29, 2013, at 10:25 , Junior White <efiish at gmail.com> wrote:

> 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?
> _______________________________________________