library submission: Data.Either.unzipEithers :: [Either a b] -> ([a], [b])
http://hackage.haskell.org/trac/ghc/ticket/1695 This function is like unzip for a list of Eithers instead of tuples. According to google code search, I am at least the 5th person to implement this function: http://www.google.com/codesearch?hl=en&lr=&q=%22%5BEither+a+b%5D+-%3E+%28%5Ba%5D%2C%5Bb%5D%29%22&btnG=Search I opted for the name unzipEithers because the function is similar to two existing functions: unzip and catMaybes. It seems less similar to partition, because partition does not desconstruct the values, it just divides them into to two groups. I have no particular attachment to the name however. I modeled the implementation after unzip and partition. Hopefully I made effective use of the irrefutable patterns and laziness, but someone else should verify. j.
Oops, just realized this is a dup: http://hackage.haskell.org/trac/ghc/ticket/974 But, perhaps we can make some progress ? j. At Fri, 14 Sep 2007 13:12:41 -0700, Jeremy Shaw wrote:
http://hackage.haskell.org/trac/ghc/ticket/1695
This function is like unzip for a list of Eithers instead of tuples.
According to google code search, I am at least the 5th person to implement this function:
I opted for the name unzipEithers because the function is similar to two existing functions: unzip and catMaybes. It seems less similar to partition, because partition does not desconstruct the values, it just divides them into to two groups. I have no particular attachment to the name however.
I modeled the implementation after unzip and partition. Hopefully I made effective use of the irrefutable patterns and laziness, but someone else should verify.
j. _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries
On Fri, Sep 14, 2007 at 01:12:41PM -0700, Jeremy Shaw wrote:
You haven't suggested a deadline. As the GHC release and ICFP et al are coming up, how about 15 October to give people a chance to comment?
This function is like unzip for a list of Eithers instead of tuples.
Seems fine to me. Thanks Ian
Folks On 17 Sep 2007, at 12:57, Ian Lynagh wrote:
On Fri, Sep 14, 2007 at 01:12:41PM -0700, Jeremy Shaw wrote:
You haven't suggested a deadline. As the GHC release and ICFP et al are coming up, how about 15 October to give people a chance to comment?
This function is like unzip for a list of Eithers instead of tuples.
Seems fine to me.
For your general amusement, a generalisation:
separate :: (Traversable t, Applicative a, Monoid (a x), Monoid (a y)) => t (Either x y) -> (a x, a y) separate = foldMap (either (pure &&& mempty) (mempty &&& pure))
I don't know whether the applicative/monoid(*) output generalisation is especially useful, but the shift to any traversable input might come in handy. Then again, one could always turn the traversable thing into a list first, using (foldMap pure). General question: how often does a one-liner need to show up in common usage to get into the library? You can really go a long way in one line! (*) foldMap needs a Monoid, so Alternative is not a suitable alternative; as ever, it would be nice to be able to demand (forall x. Monoid (a x)) or some such. All the best Conor
On Mon, Sep 17, 2007 at 01:51:34PM +0100, Conor McBride wrote:
For your general amusement, a generalisation:
separate :: (Traversable t, Applicative a, Monoid (a x), Monoid (a y)) => t (Either x y) -> (a x, a y) separate = foldMap (either (pure &&& mempty) (mempty &&& pure))
Or even newtype Wrap f a = Wrap { unWrap :: f a } instance Alternative f => Monoid (Wrap f a) where mempty = Wrap empty Wrap x `mappend` Wrap y = Wrap (x <|> y) separate :: (Foldable t, Alternative f) => t (Either x y) -> (f x, f y) separate = (unWrap *** unWrap) . foldMap (either ((Wrap . pure) &&& mempty) (mempty &&& (Wrap . pure)))
At Mon, 17 Sep 2007 12:57:46 +0100, Ian Lynagh wrote:
On Fri, Sep 14, 2007 at 01:12:41PM -0700, Jeremy Shaw wrote:
You haven't suggested a deadline. As the GHC release and ICFP et al are coming up, how about 15 October to give people a chance to comment?
October 15 sounds good to me. j.
participants (4)
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Conor McBride -
Ian Lynagh -
Jeremy Shaw -
Ross Paterson