[Haskell-cafe] Strange No instance error with cabal install

Roel van Dijk vandijk.roel at gmail.com
Tue Sep 20 13:55:02 CEST 2011

I believe this is because of aeson depending on *any* version of
deepseq. This was very recently fixed in the development version:


2011/9/20 Rune Harder Bak <rune at bak.dk>:
> Sometimes when one of our developers (using Arch-linux) tries to cabal
> install packages he gets
> a "no instance for" error.
> Code that compiles fine on my computer, and other computers I try it
> on. Even some packages on hackage
> (trying to install aeson-native for instance gives Data/Aeson/Types.hs:196:22:
>    No instance for (NFData Object)
>      arising from a use of `rnf'
>    Possible fix: add an instance declaration for (NFData Object)
>    In the expression: rnf o
>    In an equation for `rnf': rnf (Object o) = rnf o
>    In the instance declaration for `NFData Value')
> I tried removing the .ghc and .cabal folder, even reinstalled ghc-7.04
> and cabal-install (most recent), but the problem persisted (might not
> have been a clean install though, I'm no Arch expert)
> I know this is kind of vauge, and not in she shape of filling a bug
> report, heck I can't even reproduce the error on my computer.
> But perhaps somebody else has run in to this strange behavior before?
> Best,
> Rune
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