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

[Sebastien Zany]

More specifically (assuming I understood the statement correctly):

Suppose I have two base functors F1 and F2 and folds for each: fold1 :: (F1
a -> a) -> (μF1 -> a) and fold2 :: (F2 a -> a) -> (μF2 -> a).

Now suppose I have two algebras f :: F1 μF2 -> μF2 and g :: F2 A -> A.

When is the composition (fold2 g) . (fold1 f) :: μF1 -> A a catamorphism?

On Thu, Aug 23, 2012 at 10:11 PM, Sebastien Zany <
sebastien at chaoticresearch.com> wrote:

> From page 3 of
> http://research.microsoft.com/en-us/um/people/emeijer/Papers/meijer94more.pdf
> :
>
>  it is not true in general that catamorphisms are closed under composition
>
>
> When is this true?
>
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