[Haskell-cafe] strange behavior of GHC 7.8.2

[Yuras Shumovich]

I'm 90% sure you have MonadIO and RWST imported from different versions of
transformers. Try to load the code into ghci and check ':i MonadIO' and ':i
RWST'. If you'll see fully qualified (including package name and version)
names somewhere, then that is the issue.
See also
http://stackoverflow.com/questions/11068272/acid-state-monadstate-instance-for-update
 07.07.2014 6:19 пользователь "Kazu Yamamoto" <kazu at iij.ad.jp> написал:

> Hi cafe,
>
> I noticed that strange behavior of GHC 7.8.2. Consider the following
> example which requires the "ghc" package:
>
> ----
> {-# LANGUAGE GeneralizedNewtypeDeriving #-}
>
> module A where
>
> import GHC
> import MonadUtils
> import Control.Monad.Trans.RWS.Lazy (RWST(..))
>
> newtype M a = M (RWST () () () IO a)
>               deriving (Functor,Applicative,Monad,MonadIO)
> ----
>
> The "ghc" library depends on "transformers" 0.3.0.0. If "transformers"
> *0.4.1.0* is NOT installed, GHCi can handle the code above well:
>
> ----
> % ghci -package ghc A.hs
> ...
> Loading package transformers-0.3.0.0 ... linking ... done.
> ...
> Ok, modules loaded: A.
> [*A]
> >
> ----
>
> However, if "transformers" *0.4.1.0* is installed, an error happens:
>
> ----
> % cabal install transformers
> % ghci -package ghc A.hs
> ...
> Loading package transformers-0.3.0.0 ... linking ... done.
> ...
> [1 of 1] Compiling A                ( A.hs, interpreted )
>
> A.hs:10:49:
>     No instance for (MonadIO (RWST () () () IO))
>       arising from the 'deriving' clause of a data type declaration
>     Possible fix:
>       use a standalone 'deriving instance' declaration,
>         so you can specify the instance context yourself
>     When deriving the instance for (MonadIO M)
> Failed, modules loaded: none.
> [Prelude]
> >
> ----
>
> A you can see, "transformers" 0.3.0.0 is certainly linked.
> How can I interpret this behavior?
>
> If this is a bug of GHC 7.8.2, I will file this to GHC's trac.
>
> P.S.
>
> We noticed this because "doctest" of ghc-mod fails only for GHC 7.8.2.
>
> --Kazu
>
>
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