On Thu, Sep 23, 2010 at 12:58 PM, Jake McArthur wrote:

...

where the return definition relies on the Applicative instance of ((,) o). Written out explicitly,

       return a = (mempty,a)

I didn't realize we had an applicative instance for ((,) a). That being the case, +1 for this proposal. I would just like to change ((,) o) to ((,) a) to be consistent with the Applicative instance [1]. [1] http://haskell.org/ghc/docs/6.12.1/html/libraries/base-4.2.0.0/Control-Appli... Cheers! =) -- Felipe.

+1, especially since the Applicative instance is already there. On Sep 23, 2010, at 11:58 AM, Jake McArthur wrote:

This proposal [1] was originally submitted to Trac by Conal Elliott, but it was apparently abandoned and closed after some time. I've picked it back up and written out a patch. Here is the text of the proposal:

...

I'd like to have a (,)-style writer instance alongside the (->)-based reader instance for Monad in Control.Monad.Instances.

Here's the current reader:

instance Monad ((->) r) where return = const f >>= k = \ r -> k (f r) r

and my proposed writer:

instance Monoid o => Monad ((,) o) where return = pure (o,a) >>= f = (o `mappend` o', a') where (o',a') = f a

where the return definition relies on the Applicative instance of ((,) o). Written out explicitly,

return a = (mempty,a)

Control.Monad.Instances will also need two new imports:

import Data.Monoid (Monoid(..)) import Control.Applicative (pure)

- Jake

[1] http://hackage.haskell.org/trac/ghc/ticket/1951 _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries