library submission: Data.Either.unzipEithers :: [Either a b] -> ([a], [b])

[Jeremy Shaw]

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.