There was a proposal to add it, but it went stale. I favor adding it.
On Thu, 25 Jul 2013, Nathan van Doorn wrote:
This is a relatively simple instance, so I was surprised that I couldn't find it in base.
My proposal is to add the instance to Control.Monad.Instances, undeprecating the module.
Wasn't it already added? Nonetheless, I strongly suggest to use Control.Monad.Trans.Writer instead. To me, using a pair as a monad looks like an error not like a feature.
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