[Haskell-cafe] When is a composition of catamorphisms a catamorphism?

wren ng thornton wren at freegeek.org
Tue Aug 28 05:35:17 CEST 2012

On 8/26/12 9:10 PM, Sebastien Zany wrote:
> Thanks Wren. That was my guess too, but it seems not necessary:
> http://stackoverflow.com/questions/12103309/when-is-a-composition-of-catamorphisms-a-catamorphism

Well, sure. I was meaning in the general case. If you have the right 
kind of distributivity property (as colah suggests) then things will 
work out for the particular case. But, having the right kind of 
distributivity property typically amounts to being a natural 
transformation in some appropriately related category; so that just 
defers the question to whether an appropriately related category always 
exists, and whether we can formalize what "appropriately related" means.

Live well,

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