Thanks again! I understand now. I'll be careful when the next time I use list comprehension.
On Tue, Jan 29, 2013 at 5:48 PM, Artyom Kazak <artyom.ka...@gmail.com>wrote: > Junior White <efi...@gmail.com> писал(а) в своём письме Tue, 29 Jan 2013 > 12:40:08 +0300: > > Hi Artyom, >> Thanks! But I don't understand why in the first case "queens' (k-1)" is >> being recomputed n times? >> > > Because your list comprehension is just a syntactic sugar for > > concatMap (\q -> > concatMap (\qs -> if isSafe q qs then [q:qs] else []) > (queens' (k-1))) > [1..n] > > Here `queens' (k-1)` does not depend on `qs`, and therefore it *could* be > floated out of the lambda: > > let queens = queens' (k-1) > in > concatMap (\q -> > concatMap (\qs -> if isSafe q qs then [q:qs] else []) > queens) > [1..n] > > But it is an unsafe optimisation. Suppose that the `queens` list is very > big. If we apply this optimisation, it will be retained in memory during > the whole evaluation, which may be not desirable. That's why GHC leaves > this to you. >
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