On Wed, Jan 30, 2013 at 5:51 PM, Doaitse Swierstra <doai...@swierstra.net>wrote:
> 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.
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