[Haskell-cafe] Re: Different choice operations in a continuation monad
sebf at informatik.uni-kiel.de
Fri Jun 18 12:22:35 EDT 2010
Edward Kmett wrote:
> Sebastian Fischer wrote:
> Heinrich Apfelmus wrote:
> newtype CMaybe a = CMaybe (forall r. (a -> Maybe r) -> Maybe r)
> Yes, with this type `orElse` has the same type as `mplus`, which is
> very nice.
> This type is the same as "Codensity Maybe" using category-extras
> which is a 'bit bigger than Maybe'. (To see why, figure out how
> Codensity Reader is isomorphic to State!) This is the wiggle room
> you're using to get the distributive operator.
I encounter the Codensity type constructor every now and then. I used
it to Reinvent Haskell Backtracking, learned about implementing a
state monad with a reader monad wrapped in Codensity when
implementing Lazy Nondeterministic Programming and Janis Voigtländer
also used it to improve the asymptotics of free monads.
I wonder whether for every monad `m` and `a :: Codensity m a`
getCodensity a f = getCodensity a return >>= f
Is this true? Why (not)?
> Another encoding of Maybe is through Yoneda Endo
Interesting. I did not yet encounter Yoneda in my own code but will
watch out in the future.
Underestimating the novelty of the future is a time-honored tradition.
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