strange behavior of GHC 7.8.2
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
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-f...
07.07.2014 6:19 пользователь "Kazu Yamamoto"
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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Hi Yuras, Thank you for your reply.
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-f...
Yes. Probably MonadIO belongs to transformers 0.3.0.0 while RWST does to transformers 0.4.1.0. My question is why this happens. I think that GHCi should use RWST provided by transformers 0.3.0.0. --Kazu
On Jul 7, 2014, at 9:58 AM, Kazu Yamamoto (山本和彦)
Hi Yuras,
Thank you for your reply.
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-f...
Yes. Probably MonadIO belongs to transformers 0.3.0.0 while RWST does to transformers 0.4.1.0.
My question is why this happens. I think that GHCi should use RWST provided by transformers 0.3.0.0.
I think GHC always picks the package with the highest version number, and does _not_ resolve to the package that gives the least amount of type errors. The "Loading package transformers-0.3.0.0" is not there due to your own use of transformers. It's being loaded because it is a dependency of 'ghc-7.8.2'. Also note that this behaviour is not specific to 7.8.*, it is also present in 7.6.3. The only thing you can to with both version of transformers installed is to just do:
ghci -package ghc -hide-package transformers-0.4.1.0 A.hs
-- Christiaan
Also note that this behaviour is not specific to 7.8.*, it is also present in 7.6.3. The only thing you can to with both version of transformers installed is to just do:
ghci -package ghc -hide-package transformers-0.4.1.0 A.hs
Or, perhaps better:
ghci -package ghc -package transformer-0.3.0.0 A.hs
Which works for ghc 7.8.2, but for some reason not for ghc 7.6.3. Maybe because in 7.8.2, transformers-0.3.0.0 is in the global package database (bundled with GHC), while that is not the case for 7.6.3? -- Christiaan
Christiaan, Thank you for your reply.
I think GHC always picks the package with the highest version number, and does _not_ resolve to the package that gives the least amount of type errors. The "Loading package transformers-0.3.0.0" is not there due to your own use of transformers. It's being loaded because it is a dependency of 'ghc-7.8.2'.
I have reached the same conclusion. "-v" reveals this: ---- % ghci -v A.hs -package ghc ... hiding package transformers-0.3.0.0 to avoid conflict with later version transformers-0.4.1.0 ... ----
Also note that this behaviour is not specific to 7.8.*, it is also present in 7.6.3. The only thing you can to with both version of transformers installed is to just do:
ghci -package ghc -hide-package transformers-0.4.1.0 A.hs
Thanks. "ghci A.hs -package ghc -package transformers-0.3.0.0" works for me. --Kazu
participants (3)
-
Christiaan Baaij -
Kazu Yamamoto -
Yuras Shumovich