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

[Thomas Schilling]

+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 <jake.mcarthur at gmail.com> 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
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