[Haskell-cafe] strange behavior of GHC 7.8.2

[Kazu Yamamoto (=?iso-2022-jp?B?GyRCOzNLXE9CSScbKEI=?=)]

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