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
