#3709: Data.Either.partitionEithers is not lazy enough
----------------------------------+-----------------------------------------
Reporter: daniel.is.fischer | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries/base
Version: 6.10.4 | Keywords:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Data.Either.partitionEithers gives a stack overflow for long or infinite
lists.
Change
{{{
partitionEithers :: [Either a b] -> ([a],[b])
partitionEithers = foldr (either left right) ([],[])
where
left a (l, r) = (a:l, r)
right a (l, r) = (l, a:r)
}}}
to
{{{
partitionEithers :: [Either a b] -> ([a],[b])
partitionEithers = foldr (either left right) ([],[])
where
left a ~(l, r) = (a:l, r)
right a ~(l, r) = (l, a:r)
}}}
to make it lazy enough.
Example:
{{{
Data.Either> let fun k = case gcd k 15 of { 3 -> Left "Fizz"; 5 -> Left
"Buzz"; 15 -> Left "FizzBuzz"; _ -> Right k }
Data.Either> take 10 . snd . partitionEithers $ map fun [1 .. 2*10^6 ::
Integer]
*** Exception: stack overflow
Data.Either> take 10 . snd . lpartitionEithers $ map fun [1 :: Integer ..
]
[1,2,4,7,8,11,13,14,16,17]
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3709>
GHC <http://www.haskell.org/ghc/>
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