[Haskell-cafe] list comprehansion performance has hug different

[Junior White]

So this is a problem in lazy evaluation language, it will not appear in
python or erlang, am i right?


On Tue, Jan 29, 2013 at 5:54 PM, Junior White <efiish at gmail.com> wrote:

> 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.kazak at gmail.com>wrote:
>
>> Junior White <efiish at 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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