Question

I tried to run code from http://gregorycollins.net/posts/2011/10/01/cufp2011/index.html#(43) in GHCi, but got “Floating point exception, and GHCi quits.

{-# LANGUAGE OverloadedStrings #-}

import           Control.Applicative
import           Data.Aeson
import           Data.Attoparsec (parseOnly)
import           Data.ByteString.Char8  (ByteString)
import qualified Data.ByteString.Char8 as S
import qualified Data.ByteString.Lazy.Char8 as L


------------------------------------------------------------------------------
example1 :: ByteString -> Either String Coord
example1 bs = parseOnly json bs >>= convert
  where
    convert value = case fromJSON value of
                      (Error e)   -> Left e
                      (Success a) -> Right a

example2 :: Coord -> ByteString
example2 c = S.concat $ L.toChunks $ encode c


------------------------------------------------------------------------------
data Coord = Coord { _x :: Double, _y :: Double }
  deriving (Show, Eq)

instance ToJSON Coord where
    toJSON (Coord x y) = object ["x" .= x, "y" .= y]


instance FromJSON Coord where
    parseJSON (Object v) = Coord    <$>
                           v .: "x" <*>
                           v .: "y"

    -- A non-Object value is of the wrong type, so use mzero to fail.
    parseJSON _          = empty

---

λ> :l JsonExample.hs 
[1 of 1] Compiling Main             ( JsonExample.hs, interpreted )
Ok, modules loaded: Main.
λ> example2 $ Coord 1 1
"Floating point exception

Answer 1

The occuring FPE is a GHC bug, see http://hackage.haskell.org/trac/ghc/ticket/5386 for more details.

You can try to use the http://hackage.haskell.org/package/aeson-native package instead (it’s API compatible to aeson), which avoids using the C++ library that causes this issue (by using blaze-textual-native), or you can try recompiling blaze-textual (and then also recompile aeson so that it picks up the recompiled blaze-textual) in native mode by

cabal install -fnative

(see https://github.com/mailrank/blaze-textual/blob/master/README.markdown for more details)