User:Ezrakilty/Draft of Anamorphism as pattern of function definition

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In functional programming, an anamorphism is a function defined according to a certain pattern, which builds up results of an type one constructor at a time from a seed. The term comes from Greek ἀνά (upwards) + morphism (from Greek μορφή, or form, shape) and the concept has its grounding in Category theory.

The concept of anamorphism generalizes the list-producing unfold functions to arbitrary algebraic data types. In fact, the terms 'anamorphism' and "unfold" (as a noun) are often used interchangeably. Anamorphisms, which are co-recursive, are dual to catamorphisms, which are recursive, just as unfolds are dual to folds.

Examples

Anamorphisms on Lists

An anamorphism on lists builds a (potentially infinite) list from a seed value and a construction function.

In terms of a predicate finished :: b -> Bool and a function step :: b -> (a,b), a list anamorphism f has the following definition (in Haskell syntax):

f x | finished x = []
      | otherwise = a : f y
           where (a, y) = step x

Such a function takes a seed value, x and checks whether it is "finished" in which case it simply returns the empty list. If not, it uses the step operation to generate a "next" list element and a new seed; it generates the rest of the list by recursing with the new seed.

This pattern can be abstracted into a higher-order function definition, as followss:

-- The type signature of 'ana':
ana :: (b->(a,b)) -> (b->Bool)-> b -> [a]   
-- Its definition:
ana step finished x = f x
  where
      f x | finished x = []
            | otherwise = a : f y
        where (a, y) = step x

Anamorphisms on other data structures

An anamorphism can be defined for any recursive type, according to a generic pattern. For example, the unfold for the tree data structure

data Tree a = Leaf a | Branch Tree a Tree

is as follows

ana :: (b->Either a (b,a,b)) -> b -> Tree a
ana unspool x = case unspool x of
                  Left a -> Leaf a
                  Right (l,x,r) -> Branch (ana unspool l) x (ana unspool r)

History

One of the first publications to introduce the notion of an anamorphism in the context of programming was the paper Functional Programming with Bananas, Lenses, Envelopes and Barbed Wire,^[1] by Erik Meijer et al., which was in the context of the Squiggol programming language.

Applications

Functions like zip and iterate are examples of anamorphisms. zip takes a pair of lists, say ['a','b','c'] and [1,2,3] and returns a list of pairs [('a',1),('b',2),('c',3)]. Iterate takes a thing, x, and a function, f, from such things to such things, and returns the infinite list that comes from repeated application of f, i.e. the list [x, (f x), (f (f x)), (f (f (f x))), ...].

zip (a:as) (b:bs) = if (as==[]) || (bs ==[])   -- || means 'or'
                     then [(a,b)]
                     else (a,b):(zip as bs) 

iterate f x = x:(iterate f (fx))

To prove this, we can implement both using our generic unfold, ana, using a simple recursive routine:

zip2 = ana g p
   where
   p (as,bs) = (as==[]) || (bs ==[]) 
   g ((a:as), (b:bs)) = ((a,b),(as,bs))

iterate2 f = ana (\a->(a,f a)) (\x->False)

Ruby comes with an explicit unfold operation; so, for example:

10.unfold { |n| n-1 unless n == 1 }.inspect 
       => [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

10.class.unfold(&:superclass).inspect 
       => [Fixnum, Integer, Numeric, Object]

In a language like Haskell, by contrast, even the abstract functions fold, unfold and ana are merely defined terms, as we have seen from the definitions given above.

Anamorphisms in category theory

In category theory, anamorphisms are the categorical dual of catamorphisms.

Notation

A notation for ana f found in the literature is $[\!(f)\!]$ . The brackets used are known as lens brackets, after which anamorphisms are sometimes referred to as lenses.