Re: Proposal: Add filterM or filterA to Data.Sequence

On Tue, Dec 30, 2014 at 6:22 PM, Milan Straka wrote:

Hi all,

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-----Original message----- From: David Feuer Sent: 30 Dec 2014, 18:03

There won't be any such numbers in this case, because access to the details of the sequence structure does not seem to help implement this function.

sorry for not making clear -- I was interested in difference between the mentioned | filterMSeq f = fmap fromList . filterM f . toList and your foldr-based implementation. I can imagine that one can use filterMSeq in place instead of filterM, so that is why I am interested between filterMSeq and the specialized implementation (which is just a plain foldr + <|).

Oh, I really don't know which is better (and that's true also for unfoldr and unfoldl--is the current implementation better or worse than using fromList?).

Note that it is not obvious what type does filterM has (unlike e.g. zip), now that we have Applicative and also filterM is changing in base. From this point of view, it makes sense to use fmap fromList . filterM f . toList because then we do not have inconsistent filterM-s.

I have no problem with having filterM in Data.Sequence be more general than filterM in Control.Monad for old versions. I think both unfold(r/l) and filterM actually make a potential point in my favor—if there are multiple implementations that seem reasonable, having whichever turns out to be fastest in the library will save users time benchmarking them. David