[Haskell-cafe] Re: howto tuple fold to do n-ary cross product?
Luke Palmer
lrpalmer at gmail.com
Sun Nov 30 18:18:59 EST 2008
On Sun, Nov 30, 2008 at 3:13 PM, Martijn van Steenbergen
<martijn at van.steenbergen.nl> wrote:
> Luke Palmer wrote:
>>
>> The other nice one problem is allowing the argument itself to be
>> infinite (you have to require all of the lists to be nonempty).
>
> I think the requirement has to be a lot stronger for that to work.
>
> If every sublist has two elements, the answer is 2^infinity lists which is
> uncountable.
Good catch. If there are infinitely many finite lists, you can
construct a searchable set of results:
import Data.Searchable -- from infinite-search
finiteList :: [a] -> Set a
finiteList = foldr1 union . map singleton
cross :: Eq a => [[a]] -> Set a
cross = sequence . map finiteList
ghci> let cantor = cross (repeat [True,False])
ghci> fmap (take 10) $ search cantor $ \xs -> not (any (xs !!) [3..6])
Just [True,True,True,False,False,False,False,True,True,True]
Which is pretty much unrelated to what we were talking about. But
it's cool to show of Martin Escardo's neat stuff.
Luke
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