Proposal: add "writer" Monad instance (,) o (Ticket #1951)

[Conal Elliott]

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)


Suggested review period: two weeks (ending Dec 16).

Comments?

  - Conal
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