It is indeed intended for monads that have no Functor instance. While one could argue that every monad should have a Functor instance, this is not the case in practice. If, on the other hand, Monad was defined as class Functor m => Monad m then all would be fine and we would be able to use the current implementation for `void`. A Functor constraint on Monad is an entirely different discussion though. Jurriën On 16 Jan 2012, at 16:22, Erik Hesselink wrote:

2012/1/16 Simon Hengel :

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voidM :: Monad m => m a -> m () voidM m = m >> return ()

Is voidM meant to be used with instance of Monad that are no monads, and hence have no Functor instance (like HtmlM[1])?

You could easily implement a Functor instance for this type, since it would mirror the instance for `Const a` [2], i.e. only change the type parameter, and otherwise be the identity function.

Erik

[1] http://hackage.haskell.org/packages/archive/blaze-html/0.4.3.1/doc/html/Text... [2] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Control-App...

On Mon, Jan 16, 2012 at 10:05:25AM -0600, Antoine Latter wrote:

2012/1/16 Jurriën Stutterheim :

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It is indeed intended for monads that have no Functor instance. While one could argue that every monad should have a Functor instance, this is not the case in practice. If, on the other hand, Monad was defined as

 class Functor m => Monad m

then all would be fine and we would be able to use the current implementation for `void`. A Functor constraint on Monad is an entirely different discussion though.

We would be better off adding Functor instances for the Monads in question.

IIRC, Functor rather than Monad was chosen for void's constraint as all Monads could/should also have Functor instances. The ticket was http://hackage.haskell.org/trac/ghc/ticket/3292 and it links to the discussion. Thanks Ian

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voidM :: Monad m => m a -> m () voidM m = m >> return ()

Is voidM meant to be used with instance of Monad that are no monads, and hence have no Functor instance (like HtmlM[1])?

You could easily implement a Functor instance for this type, since it would mirror the instance for `Const a` [2], i.e. only change the type parameter, and otherwise be the identity function.

Yes, you are right. As long as we do not use bind we are fine. Cheers, Simon