GHCi runtime linker found a duplicate definition error

[Aerosol]

Today I started to play with attoparsec package and tried to write simple parser. I wrote really toy data type like this:

module SimpleParser where

import qualified Data.ByteString.Char8 as BS
import Data.Attoparsec.ByteString

data Str = End deriving (Show, Eq)

I tried to type:

• *SimpleParser> End
  

in ghci and will get End as I expected. Instead End I got following error:

GHCi runtime linker: fatal error: I found a duplicate definition for symbol
   _hs_bytestring_long_long_uint_hex
whilst processing object file
   /home/alex/.cabal/lib/x86_64-linux-ghc-7.6.3/bytestring-0.10.4.0/HSbytestring-0.10.4.0.o
This could be caused by:
   * Loading two different object files which export the same symbol
   * Specifying the same object file twice on the GHCi command line
   * An incorrect `package.conf' entry, causing some object to be
     loaded twice.
GHCi cannot safely continue in this situation.  Exiting now.  Sorry.

As we can see in error output GHCi runtime linker found duplicate definition of _hs_bytestring_long_long_uint_hex. Let's see what's wrong bytestring package with:

cabal info bytestring

It will give us output like this:

* bytestring       (library)
    Synopsis:      Fast, compact, strict and lazy byte strings with a list
                   interface
    Versions available: 0.9.0.4, 0.9.1.9, 0.9.1.10, 0.9.2.0, 0.9.2.1, 0.10.0.1,
                        0.10.0.2, 0.10.2.0, 0.10.4.0 (and 14 others)
    Versions installed: 0.10.4.0, 0.10.2.0
    Homepage:      https://github.com/haskell/bytestring
    Bug reports:   https://github.com/haskell/bytestring/issues
    Description:   An efficient compact, immutable byte string type (both strict
                   and lazy) suitable for binary or 8-bit character data.
..............................................
..............................................
..............................................

We can note that there two installed versions of bytestring package. Let's remove old version with:

• sudo ghc-pkg unregister --force bytestring-0.10.0.2
  

That's all. After this error with GHCi runtime linker found a duplicate definition error...... will disappear.

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