Re: [Haskell-cafe] Derving NFData via Generics from a type that has a vector doesn't work. (was trying to understand out of memory exceptions)

16 Apr 2013


      -- + Roman,

-- hey Roman,

-- seems like i cant use deepseq or Generic derive of NFData on data types
containing vectors.  The following code tries to use deepseq to force
evaluation of a type containing vectors, but when the code is running it
seems to not work as expected (blows up in memory).  any ideas?


{-# LANGUAGE DeriveGeneric #-}
import Control.DeepSeq
import System.IO
import GHC.Generics (Generic)
import qualified Data.Vector as V
import qualified Data.ByteString.Lazy.Char8 as BL

scanl' :: NFData a => (a -> b -> a) -> a -> [b] -> [a]
scanl' f q ls =  q : (case ls of
                        []   -> []
                        x:xs -> let q' = f q x
                                in q' `deepseq` scanl' f q' xs)

-- this runs without blowing up
-- main = print $ last $ scanl' (+) (0::Int) [0..]

data Simple = Simple (V.Vector Double)
            deriving (Show, Generic)

instance NFData Simple

--this blows up
main = do
   let initial = Simple $ V.fromList (take 100 $ repeat 0)
       sumvs (Simple a) (Simple b) = Simple $ V.zipWith (+) a b
   print $ last $ scanl' sumvs initial $ repeat $ initial


On Tue, Apr 16, 2013 at 12:36 PM, anatoly yakovenko
wrote:
...
This compiles but the process runs out of memory, so it seams that NFData
derivation isn't doing its job.
On Apr 16, 2013, at 12:15 PM, José Pedro Magalhães  wrote:
...
What is the error that you get?
Cheers,
Pedro
On Tue, Apr 16, 2013 at 8:07 PM, Anatoly Yakovenko <
aeyakovenko@gmail.com> wrote:
-- ok, something in deriving NFData using Generics in a type that has a
Vector in it.
{-# LANGUAGE DeriveGeneric #-}
import Control.DeepSeq
import System.IO
import GHC.Generics (Generic)
import qualified Data.Vector as V
import qualified Data.ByteString.Lazy.Char8 as BL
scanl' :: NFData a => (a -> b -> a) -> a -> [b] -> [a]
scanl' f q ls =  q : (case ls of
                        []   -> []
                        x:xs -> let q' = f q x
                                in q' `deepseq` scanl' f q' xs)
-- this runs without blowing up
-- main = print $ last $ scanl' (+) (0::Int) [0..]
data Simple = Simple (V.Vector Double)
            deriving (Show, Generic)
instance NFData Simple
--this blows up
main = do
   let initial = Simple $ V.fromList (take 100 $ repeat 0)
       sumvs (Simple a) (Simple b) = Simple $ V.zipWith (+) a b
   print $ last $ scanl' sumvs initial $ repeat $ initial
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