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

[Conal Elliott]

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
<nominolo at googlemail.com>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 <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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> >
>
>
>
> --
> Push the envelope. Watch it bend.
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