Re: Add voidM to Control.Monad

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...