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1 Recursion patterns
Meijer, Fokkinga & Patterson identified in the paper Functional programming with bananas, lenses, envelopes and barbed wire a number of generic patterns for recursive programming that they had observed, catalogued and systematized. The aim of that paper is to establish a number of rules for modifying and rewriting expressions involving these generic recursion patterns.
As it turns out, these patterns are instances of the same phenomenon we saw last lecture: where the recursion comes from specifying a different algebra, and then take a uniquely existing morphism induced by initiality (or, as we shall see, finality).
Before we go through the recursion patterns, we need to establish a few pieces of theoretical language, dualizing the Eilenberg-Moore algebra constructions from the last lecture.
1.1 Coalgebras for endofunctors
Definition If is an endofunctor, then a P-coalgebra on A is a morphism .
A morphism of coalgebras: is some such that the diagram
Just as with algebras, we get a category of coalgebras. And the interesting objects here are the final coalgebras. Just as with algebras, we have
Lemma (Lambek) If is a final coalgebra, it is an isomorphism.
Finally, one thing that makes us care highly about these entities: in an appropriate category (such as ω − CPO), initial algebras and final coalgebras coincide, with the correspondence given by inverting the algebra/coalgebra morphism. In Haskell not quite true (specifically, the final coalgebra for the lists functor gives us streams...).
Onwards to recursion schemes!
We shall define a few specific morphisms we'll use repeatedly. This notation, introduced here, occurs all over the place in these corners of the literature, and are good to be aware of in general:
- If is an initial algebra for T, we denote a = inA.
- If is a final coalgebra for T, we denote a = outA.
- We write μf for the fixed point operator
mu f = x where x = f x
- MFP write (fΔg) for
Delta f g = \x -> (f x, g x)
- MFP write for
(Nabla f g) (Left x) = f x (Nabla f g) (Right x) = g x
These two last constructions are directly motivated by the maps induced from the universal properties of products and coproducts.
We shall write and (f + g) for the Δ and constructions, respectively.
We note that in the situation considered by MFP, inital algebras and final coalgebras coincide, and thus inA,outA are the pair of isomorphic maps induced by either the initial algebra- or the final coalgebra-structure.
A catamorphism is the uniquely existing morphism from an initial algebra to a different algebra. We have to define maps down to the return value type for each of the constructors of the complex data type we're recursing over, and the catamorphism will deconstruct the structure (trees, lists, ...) and do a generalized fold over the structure at hand before returning the final value.
The intuition is that for catamorphisms we start essentially structured, and dismantle the structure.
Example: the length function from last lecture. This is the catamorphism for the functor given by the maps
u :: Int u = 0 m :: (A, Int) -> Int m (a, n) = n+1
cata :: (F a b -> b) -> T a -> b cata phi = mu (\x -> phi . fmap x . outT)
We can reframe the example above as a catamorphism by observing that here,
data F a b = Nil | Cons a b deriving (Eq, Show) type T a = [a] instance Functor (F a) where fmap _ Nil = Nil fmap f (Cons n a) = Cons n (f a) outT :: T a -> F a (T a) outT  = Nil outT (a:as) = Cons a as lphi :: F a Int -> Int lphi Nil = 0 lphi (Cons a n) = n + 1 l = cata lphi
An anamorphism is the categorical dual to the catamorphism. It is the canonical morphism from a coalgebra to the final coalgebra for that endofunctor.
Here, we start unstructured, and erect a structure, induced by the coalgebra structures involved.
Example: we can write a recursive function
first :: Int -> [Int] first 1 =  first n = n : first (n - 1)
This is an anamorphism from the coalgebra for on generated by the two maps
c 0 = Left () c n = Right (n, n-1)
and we observe that we can chase through the diagram
to conclude that therefore
f 0 =  f n = n : f (n - 1)
which is exactly the recursion we wrote to begin with.
MFP define the anamorphism by a fixpoint as well, namely:
ana psi = mu (\x -> inT . fmap x . psi)
We can, again, recast our illustration above into a structural anamorphism, by:
-- Reuse mu, F, T from above inT :: F a (T a) -> T a inT Nil =  inT (Cons a as) = a:as fpsi :: Int -> F Int Int fpsi 0 = Nil fpsi n = Cons n (n-1)
2 Further reading
Terminology in the literature: in and out, inl, inr.
- Write a fold for the data type and demonstrate how this can be written as a catamorphism by giving the algebra it maps to.data T a = L a | B a a | C a a a
- The integers have a partial order induced by the divisibility relation. We can thus take any integer and arrange all its divisors in a tree by having an edge if d | n and d doesn't divide any other divisor of n. Write an anamorphic function that will generate this tree for a given starting integer. Demonstrate how this function is an anamorphism by giving the algebra it maps from.