The Comonad.Reader

As you may recall, every functor in Haskell is strong, in the sense that if you provided an instance of Monad for that functor the following definition would satisfy the requirements mentioned here:

 
strength :: Functor f => a -> f b -> f (a,b)
strength = fmap . (,)
 

In an earlier post about the cofree comonad and the expression problem, I used a typeclass defining a form of duality that enables you to let two functors annihilate each other, letting one select the path whenever the other offered up multiple options. To have a shared set of conventions with the material in Zipping and Unzipping Functors, I have since remodeled that class slightly:

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I've had a few people ask me questions about Adjunctions since my recent post and a request for some more introductory material, so I figured I would take a couple of short posts to tie Adjunctions to some other concepts.

Representable Functors

A covariant functor is said to be representable by an object if it is naturally isomorphic to .

We can translate that into Haskell, letting play the role of with:

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This post is a bit of a departure from my recent norm. It contains no category theory whatsoever. None. I promise.

Now that I've bored away the math folks, I'll point out that this also isn't a guide to better horticulture. Great, there goes the rest of you.

Instead, I want to talk about Bloom filters, Bloom joins for distributed databases and some novel extensions to them that let you trade in resources that we have in abundance for ones that are scarce, which I've been using for the last few months and which I have never before seen before in print. Primarily because I guess they have little to do with the strengths of Bloom filters.

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