[Haskell-cafe] Strange discrepancy between Emacs interpreter and command-line on windows

Don Stewart dons at galois.com
Wed Jun 9 17:06:24 EDT 2010

> Hello,
> I have a strange issue which sprang today out of nowhere. When I load
> a certain file using bytestring package in Ghci using emacs, I got the
> following error:
> Couldn't match expected type `Data.ByteString.Internal.ByteString'
>            against inferred type
> `bytestring-'
>     In the second argument of `hPutStrLn', namely `(fromString msgs)'
>     In a stmt of a 'do' expression: hPutStrLn cnx (fromString msgs)
> The same file compiles fine on the command-line. I have a single
> installation of haskell platform, using ghc 6.10.4, on windows XP.

Looks like you might have two versions of bytestring installed (ghc-pkg
list bytestring) and are attempting to combine them. The diamond
dependency problem.

    * delete the old version (ghc-pkg unregister)
    * recompile any libs that depend on it.

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