On Wed, Jan 30, 2013 at 5:51 PM, Doaitse Swierstra
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.
Thanks for your reply! I must learn more to fully understand what's going on inside the list comprehension. But when I frist learn Haskell, it says sequence doesn't matter, but now it is a big matter, can compiler do some thing for us? I think this behavior is not friendly to newbies like me, I will take a very long time to work through it.