Re: Proposal: Add "writer" Monad instance (, ) o to Control.Monad.Instances

FWIW, I think I used "o" for "output". I'd go with (->) i and (,) o . On Fri, Sep 24, 2010 at 2:44 AM, Thomas Schilling wrote:

+1

I'd also prefer the variable to be called 'w' like for the writer Monad. But that's a bikeshed, so I don't mind either way.

On 23 September 2010 16:58, 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

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