newtype deriving clause ceases to work in HEAD
Hi, The following code compiles with GHC 6.4.2, but does not typecheck with GHC HEAD pulled on Sunday. module CompilerMonad where import Control.Monad import Control.Monad.Reader import Control.Monad.Error newtype CompilerError = CE String deriving Error newtype CM r a = CM (ReaderT r (ErrorT CompilerError (Either CompilerError)) a) deriving (Monad, MonadReader a, MonadError CompilerError) Compiling the code gives: $ /home/mboes/src/ghc/compiler/stage2/ghc-inplace /tmp/CompilerMonad.hs /tmp/CompilerMonad.hs:10:8: Can't make a derived instance of `MonadReader a (CM r)' (even with cunning newtype deriving: the eta-reduction property does not hold) When deriving instances for `CM' whereas with 6.4.2 $ ghc-6.4.2 --make /tmp/CompilerMonad.hs Chasing modules from: /tmp/CompilerMonad.hs Compiling CompilerMonad ( /tmp/CompilerMonad.hs, /tmp/CompilerMonad.o ) As the above code seemed to be doing what I meant for it to do in 6.4.2, and the docs for HEAD don't seem to have changed, I'm curious as to why this doesn't work in HEAD anymore, and indeed, if this is perhaps a regression? Thanks, Mathieu
It's actually a software engineering choice, in this case a good one. The instance that 6.4.1 derives looks like this: instance MonadReader a (ReaderT r (ErrorT CompilerError (Either CompilerError))) => MonadReader a (CM r) The instance declaration is ok, but you'll never be able to use it, because the instance in Control.Monad.Reader looks like: instance (Monad m) => MonadReader r (ReaderT r m) where You intended to write deriving (MonadReader r) GHC 6.4.1 accepts your declaration but generates a useless insatance decl. GHC 6.6 will still let you generate the same instance, but only if you use another type variable, for instance deriving (MonadReader x) GHC 6.6's behaviour is defined by the appropriate section of the user manual, which you got when you pulled the HEAD. In short, a false alarm, I believe, though it took me a few minutes to puzzle out! Simon | -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Mathieu Boespflug | Sent: 29 November 2006 00:25 | To: glasgow-haskell-users@haskell.org | Subject: newtype deriving clause ceases to work in HEAD | | Hi, | | The following code compiles with GHC 6.4.2, but does not typecheck | with GHC HEAD pulled on Sunday. | | module CompilerMonad where | | import Control.Monad | import Control.Monad.Reader | import Control.Monad.Error | | newtype CompilerError = CE String deriving Error | | newtype CM r a = CM (ReaderT r (ErrorT CompilerError (Either | CompilerError)) a) | deriving (Monad, MonadReader a, MonadError CompilerError) | | Compiling the code gives: | | $ /home/mboes/src/ghc/compiler/stage2/ghc-inplace /tmp/CompilerMonad.hs | | /tmp/CompilerMonad.hs:10:8: | Can't make a derived instance of `MonadReader a (CM r)' | (even with cunning newtype deriving: | the eta-reduction property does not hold) | When deriving instances for `CM' | | whereas with 6.4.2 | | $ ghc-6.4.2 --make /tmp/CompilerMonad.hs | Chasing modules from: /tmp/CompilerMonad.hs | Compiling CompilerMonad ( /tmp/CompilerMonad.hs, /tmp/CompilerMonad.o ) | | As the above code seemed to be doing what I meant for it to do in | 6.4.2, and the docs for HEAD don't seem to have changed, I'm curious | as to why this doesn't work in HEAD anymore, and indeed, if this is | perhaps a regression? | | Thanks, | | Mathieu | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
GHC 6.4.1 accepts your declaration but generates a useless insatance decl. GHC 6.6 will still let you generate the same instance, but only if you use another type variable, for instance deriving (MonadReader x)
GHC 6.6's behaviour is defined by the appropriate section of the user manual, which you got when you pulled the HEAD.
In short, a false alarm, I believe, though it took me a few minutes to puzzle out!
Ah! Well spotted! Sorry for the bother. Something I hadn't noticed after dozens of minutes staring blankly after some late night coding. I was rather puzzled given that I had written quite a bit of typechecked code on top of the erroneous CM monad declaration. Thanks for your help Simon, Mathieu
| -----Original Message----- | From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users- | bounces@haskell.org] On Behalf Of Mathieu Boespflug | Sent: 29 November 2006 00:25 | To: glasgow-haskell-users@haskell.org | Subject: newtype deriving clause ceases to work in HEAD | | Hi, | | The following code compiles with GHC 6.4.2, but does not typecheck | with GHC HEAD pulled on Sunday. | | module CompilerMonad where | | import Control.Monad | import Control.Monad.Reader | import Control.Monad.Error | | newtype CompilerError = CE String deriving Error | | newtype CM r a = CM (ReaderT r (ErrorT CompilerError (Either | CompilerError)) a) | deriving (Monad, MonadReader a, MonadError CompilerError) | | Compiling the code gives: | | $ /home/mboes/src/ghc/compiler/stage2/ghc-inplace /tmp/CompilerMonad.hs | | /tmp/CompilerMonad.hs:10:8: | Can't make a derived instance of `MonadReader a (CM r)' | (even with cunning newtype deriving: | the eta-reduction property does not hold) | When deriving instances for `CM' | | whereas with 6.4.2 | | $ ghc-6.4.2 --make /tmp/CompilerMonad.hs | Chasing modules from: /tmp/CompilerMonad.hs | Compiling CompilerMonad ( /tmp/CompilerMonad.hs, /tmp/CompilerMonad.o ) | | As the above code seemed to be doing what I meant for it to do in | 6.4.2, and the docs for HEAD don't seem to have changed, I'm curious | as to why this doesn't work in HEAD anymore, and indeed, if this is | perhaps a regression? | | Thanks, | | Mathieu | _______________________________________________ | Glasgow-haskell-users mailing list | Glasgow-haskell-users@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
participants (2)
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Mathieu Boespflug -
Simon Peyton-Jones