More Hylomorphisms in Haskell

9 04 2009

If you lost yourself in this post, I advise you to start in catamorphisms, then anamorphisms and then hylomorphisms.

Like I said before (in those posts) when you write an hylomorphism over a particular data type, that means just that the intermediate structure is that data type.

In fact that data will never be stored into that intermediate type C or D. Because we glue the ana and cata together into a single recursive pattern. A and E could be some data type your function need. With this post I will try to show you more hylomorphisms over some different data types to show you the power of this field.

Leaf Tree’s

The data type that we going to discuss here is the LTree. In Haskell we can represent LTree as:

data LTree a = Leaf a | Fork (LTree a, LTree a)

Is just like a binary tree, but the information is just in the leaf’s. Even more: a leaf tree is a tree that only have leaf’s, no information on the nodes. This is an example of a leaf tree:

To represent all the hylomorphisms over Ltree we draw the following diagram:

The example I’m going to give is making the fibonacci function using a hylomorphism over this data type. If you remember the method I used before, I’m going to start by the anamorphism [(h)]. Before that I’m going to specify the strategy to define factorial. I’m going to use the diagram’s again, remember that type 1 is equivalent to Haskell ( ):

As you can see I’m going to use Ltree~1 as my intermediate structure, and I’ve already define the names of my gen functions add to the catamorphism and fibd to the anamorphism. The strategy I prefer, is do all the hard work in the anamorphism, so here the gen fibd for the anamorphism is:

fibd n | n < 2     = i1   ()
       | otherwise = i2   (n-1,n-2)

This function combined with the anamorphism, going to generate leaf tree’s with n leaf’s, being n the result of that fib.

Then we just have to write the gen add for the catamorphism. This function (combined with the catamorphism) counts the number of leafs that a leaf tree have.

add = either (const 1) plus
    where plus = uncurry (+)

The final function, the fibonacci function is the hylomorphism of those two defined before:

fib =  hyloLTree add fibd

Here is all the auxiliary functions you need to run this example:

inLTree = either Leaf Fork

outLTree :: LTree a -> Either a (LTree a,LTree a)
outLTree (Leaf a)     = i1   a
outLTree (Fork (t1,t2)) = i2    (t1,t2)

cataLTree a = a . (recLTree (cataLTree a)) . outLTree

anaLTree f = inLTree . (recLTree (anaLTree f) ) . f

hyloLTree a c = cataLTree a . anaLTree c

baseLTree g f = g -|- (f >< f)

recLTree f = baseLTree id f


The lists that I’m going to talk here, are the Haskell lists, wired into the compiler, but is a definition exist, it will be:

data [a] = [ ] | a : [a]

So, our diagram to represent the hylomorphism over this data type is:

The function I’m going to define as a hylomorphism is the factorial function. So, we know that our domain and co-domain is Integers, so now we can make a more specific diagram to represent our solution:

As you can see I’m going to use [Integer] to represent my intermediate data, and I’ve already define the names of my gen functions mul to the catamorphism and nats to the anamorphism. Another time, that I do all the work with the anamorphism, letting the catamorphism with little things to do (just multiply). I’m start to show you the catamorphism first:

mul = either (const 1) mul'
    where mul' = uncurry (*)

As you can see the only thing it does is multiply all the elements of a list, and multiply by 1 when reach the [] empty list.

In the other side, the anamorphism is generating a list of all the elements, starting in n (the element we want to calculate the factorial) until 1.

nats = (id -|- (split succ id)) . outNat

And finally we combine this together with our hylo, that defines the factorial function:

fac = hylo mul nats

Here is all the code you need to run this example:

inl = either (const []) (uncurry (:))

out []    = i1 ()
out (a:x) = i2(a,x)

cata g   = g . rec (cata g) . out   

ana h    = inl . (rec (ana h) ) . h

hylo g h = cata g . ana h

rec f    = id -|- id >< f

Binary Tree’s

Here, I’m going to show you the hanoi problem solved with one hylomorphism, first let’s take a look at the Btree structure:

data BTree a = Empty | Node(a, (BTree a, BTree a))

So, our generic diagram representing one hylomorphism over BTree is:

There is a well-known inductive solution to the problem given by the pseudocode below. In this solution we make use of the fact that the given problem is symmetrical with respect to all three poles. Thus it is undesirable to name the individual poles. Instead we visualize the poles as being arranged in a circle; the problem is to move the tower of disks from one pole to the next pole in a specified direction around the circle. The code defines H_n.d to be a sequence of pairs (k, d) where n is the number of disks, k is a disk number and d are directions. Disks are numbered from 0 onwards, disk 0 being the smallest. Directions are boolean values, true representing a clockwise movement and false an anti-clockwise movement. The pair (k, d) means move the disk numbered k from its current position in the direction d.

excerpt from R. Backhouse, M. Fokkinga / Information Processing Letters 77 (2001) 71–76

So, here, I will have a diagram like that, b type stands for Bool and i type for Integer:

I’m going to show all the solution here, because the description of the problem is in this quote, and in the paper:

hanoi = hyloBTree f h

f = either (const []) join
    where join(x,(l,r))=l++[x]++r

h(d,0) = Left ()
h(d,n+1) = Right ((n,d),((not d,n),(not d,n)))

And here it is, all the code you need to run this example:

inBTree :: Either () (b,(BTree b,BTree b)) -> BTree b
inBTree = either (const Empty) Node

outBTree :: BTree a -> Either () (a,(BTree a,BTree a))
outBTree Empty              = Left ()
outBTree (Node (a,(t1,t2))) = Right(a,(t1,t2))

baseBTree f g = id -|- (f >< g))

cataBTree g = g . (recBTree (cataBTree g)) . outBTree

anaBTree g = inBTree . (recBTree (anaBTree g) ) . g

hyloBTree h g = cataBTree h . anaBTree g

recBTree f = baseBTree id f


Maybe in the future I will talk more about that subject.

Hylomorphisms in Haskell

9 04 2009

If you miss something in this post, I suggest you to start in Catamorphisms and Anamorphisms.

A Hylomorphism is just the composition of one catamorphism and then one anamorphism.
hylo~f~h~=~cata~f~\circ~ana~h, replacing that by the proper notation we have: [|f,h|]~=~(|f|)~\circ~[(h)]

In this post I will use the structure of a binary tree:

data BTree a = Empty | Node(a, (BTree a, BTree a))

I will use the tuples to don’t have to write uncurry’s. As I will show you, when we say that we are making a hylomorphism on a particular data type T, what we are trying to say is that the intermediate structure of our combination of catamorphism and anamorphism is that data type T. This is the structure throw our morphism will communicate with each other.


So, here I will solve the Quicksort algorithm with a hylomorphism over BTree.

The intermediate structure being a BTree doesn’t mean that my function will receive BTree. My qSort function works over lists. So the first thing to do, is draw the respective anamorphism from [a] to BTree~a:

My strategy here is to do all the work in the anamorphism, so, I need a function h with type:
h : [a] \rightarrow 1 + a \times [a] \times [a], or in Haskell h :: [a] \rightarrow Either () (a, ([a], [a]))

That function is qsep:

qsep :: [a] -> Either () (a, ([a], [a]))
qsep []    = Left ()
qsep (h:t) = Right (h,(s,l))
    where (s,l) = part (<h) t

part:: (a -> Bool) -> [a] -> ([a], [a])
part p []                = ([],[])
part p (h:t) | p h       = let (s,l) = part p t in (h:s,l)
             | otherwise = let (s,l) = part p t in (s,h:l)

This code is very simple, in qsep I chose a pivotal element (first one), and filter the bigger to one side, and the other ones to the other, just like the algorithm. The function that do all that job is part, it process all the list finding the elements that satisfy the condition p, to put them in the left side of the tuple, and the others into the right side.

This function by it self don’t do almost anything, it is only a simple part of the algorithm.


Next step is to see the diagram for catamorphisms from BTree~a to [a]:

As I said before, the heavy duty is on the side of the anamorphism, so here, the catamorphism will be very very simple. In fact it is.

inord :: Either a (a, ([a], [a])) -> [a]
inord = either (const []) join
    where join(x,(l,r))=l++[x]++r

That right! The only thing that the catamorphism do is a inorder passage over the structures a + a \times [a] \times [a], which is very simple, as as shown by the code.


The first thing is to draw the diagram, now for the hylomorphism, the composition of the cata with the ana:

Once having made the two most important parts of the function (the ana and cata), the hylo is very simple to do. You just have to make a function hyloBTree:

hyloBTree h g = cataBTree h . anaBTree g

And our function qSort bacame:

qSort :: Ord a => [a] -> [a]
qSort = hyloBTree inord qsep

And that’s it, now I’m going to show you the all code that you need to put all the things together and working.

inBTree :: Either () (b,(BTree b,BTree b)) -> BTree b
inBTree = either (const Empty) Node

outBTree :: BTree a -> Either () (a,(BTree a,BTree a))
outBTree Empty              = Left ()
outBTree (Node (a,(t1,t2))) = Right(a,(t1,t2))

baseBTree f g = id -|- (f >< g))

cataBTree g = g . (recBTree (cataBTree g)) . outBTree

anaBTree g = inBTree . (recBTree (anaBTree g) ) . g

hyloBTree h g = cataBTree h . anaBTree g

recBTree f = baseBTree id f


If you need more explanations feel free to contact me.

Catamorphisms in Haskell

19 12 2007

Lately I have had many works to do at university, therefore I’m sorry for the non regularity of my post’s.
By the way, I also liked to announce that this is my first conscientious post at Planet Haskell.

Why Catamorphisms and Point-Free

Catamorphisms is the way that we can explain in one function how recursive patterns works in some data type.
Here I will use Point-Free notation because what matters here is to show the data flow and the composition of functions.
Point-Free style is used to think in data flow terms, and very useful to program verification, applying formalism to our code.

In point-free style of programming programs are expressed as combinations of simpler functions. This notation is known as write functions without their arguments. Pointwise is the normal form how we write a function.

Couple of examples of point-free notation:

sum = foldr (+) 0 -- point-free
sum l = foldr (+) 0 l -- pointwise


f = (*10).(+2) -- point-free
f n = (n+2)*10 -- pointwise


First of all to define a function, for example f, I say:


I will assume that you are familiarized with infix notation, const, either, uncurry and composition \circ function.


In Haskell we have this definition for lists:

data [a] = [] | a : [a]

Let’s create the same, but more convenient. Consider the following isomorphic type for lists:

data List a = Empty | Node(a,List a) deriving Show

To represent [1,2,3] we wrote Node(1,Node(2,Node(3,Empty))).

As you can see, to construct a (List a) we have two options, Empty or Node. Formally we represent the constructor Empty as 1. And we use (+) to say that our two possibilities are 1 or Node. We could see Node as a the following function:

Node :: (a,List a) -> List a

So typologically we have 1 + (a,List~a). We use (\times) to define that two things occurs in parallel, like tuples do, so we can redefine it: 1 + (a \times~List~a)

Now we can say that (List~a) is isomorphic to (1 + a \times~List~a).
This is something to say that (List~a) and (1 + a \times~List~a) keep the same information without any change or any loss.

Catamorphisms as composition of functions

Let A, B, C, D be Inductive data types (sets) and out, cata, rec functions.

We will write cata(g)_{List} using the composition of out, cata, rec functions. That way we are breaking our problem in small ones. So, in the end we will have the following definition for cata(g)_{List}:

cata(g)_{List} = g \circ rec_{List} \circ out_{List}

The function that we want is cata(g), and that function is over (List~a) so we have:

cata :: (D -> C) -> List a -> C

Type A is (List~a). Maybe this isn’t clear yet, let’s start with function out


The function outList is responsible to create the isomorphism between (1 + a \times~List~a) and (List~a), so the code could be something like this:

outList :: List a -> Either () (a,List a)
outList Empty    = Left ()
outList (Node t) = Right t

In Haskell we represent the type 1 as (), (+) as Either and (\times) as (,).

So, type B is (1 + a \times~List~a).

function g

The function g is also known as *gen*, here is where we said the step that pattern do. Imagine that we want to insert all the values of (List~a) into [a]:

-- pointwise
g :: Either () (a,[a]) -> [a]
g (Left()) = []
g (Right(a,h)) = a:h

-- pointfree
g = either (const []) (uncurry (:))

We represent cata(g) as (| g |).
Now we can be more specific with our graphic:


Here we have to get a function rec that transform 1 + (a \times~List~a) into 1 + (a \times~[a]). That function, general rec, will be:

recg f g h = f -|- (g ><  g) x = ((f . fst) x , (g . snd) x)

With that function we can say exactly what to do with type 1, a, and List~a in domain of rec.
So we want something like this:

rec g = recG id id g

like that we said that (1 + (a \times~\_)) will be the same in the counter domain (1 + (a \times~\_)) of rec. Now we need a function that receive a List~a and give us a [a]
Yes, that function is (| g |)! So, the final graphic became:


Finally we gonna to define the function cata(g):

cata g = outList . rec (cata g) . g

where g is our *gen* function,

g = either (const []) (uncurry (:))

And our objective:

List2HaskellList = cata $ either (const []) (uncurry (:))

More catamorphisms

Imagine we have the following data type:

data NList a where
    Leaf  :: a -> NList a
    NNode :: a -> [NList a] -> NList a

out, rec, and cata became:

out (Leaf a) = Left a
out (NNode a l) = Right (a,l)

Using the previous definitions of (-|-) and (><)

rec f = id -|- (id >< map f)
cata g = g . rec (cata g) . out

Imagging that g has type:

g :: Either a (a,[[a]]) -> [a]

And the graphic for this cata became:


I’ve talked about cata’s without any formalism, the idea was to explain to someone who didn’t know.

I will talk more about catamorphisms and how to calculate programs with them.
In the future I will like to talk about anamorphisms too. And Later on I will talk about point-free over non recursive functions.

foldr the magic function

20 11 2007


In Haskell we essentially use recursion, this mean defining a function using itself definition.
A recursive function is, for example the factorial:

in haskell we define it like that:

fact 0 = 1
fact n = n * fact (n-1)

As you can see Haskell is authentic mathematic, pure functional language.

My point here is to show you the magic of the function foldr in Haskell, so I will assume that you already know Haskell, and maybe are accustomed to use foldr function.


Seeing the magic

The definition of the function length could be something like this:

length [] = 0
length (h:t) = 1 + length t

The definition of the function sum could be something like this:

sum [] = 0
sum (h:t) = h + sum t

And the definition of the function mult could be something like this:

mult [] = 1
mult (h:t) = h * mult t

As you can see in all three definitions we have a pattern and we can generalize it:

functionName [] = stopCase
functionName (h:t) = constant operator (functionName t)

We always need the stopping case of the function!

And the same happens in other functions like reverse:

reverse [] = []
reverse (h:t) = reverse t ++ [h]

In this case we have:

functionName [] = stopCase
functionName (h:t) = (functionName t) operator constant

So, we will always use the function applied to the tail of the list!

If we use some of lambda notation we can see one more pattern, the real one:

sum [] = 0
sum (h:t) = (a b -> a + b) h (sum t)

reverse [] = []
reverse (h:t) = (a b -> b ++ [a]) h (reverse t)

Our lambdas functions have arity 2, because those are the parts in which we can see the lists (head + tail).

Now we can generalize even more, considering function have arity 2:

functionName [] = stopCase
functionName (h:t) = function h (functionName t)

well, that’s not right for Haskell. Because it doesn’t know what function represents, that’s because the definition of functionName doesn’t receive that argument. And the same happens with stopCase.

functionName function stopCase [] = stopCase
functionName function stopCase (h:t)
     = function h (functionName function stopCase t)

Et voilá.

In fact the last definition is the true one for foldr. Easy right?

Functions with foldr’s

Well… Now we can redefine previous functions just with foldr’s:

length l = foldr (a b -> 1 + b) 0 l
sum l = foldr (+) 0 l
mult l = foldr (*) 1 l
reverse l = foldr (a b -> b ++ [a]) [] l

And all the functions over list’s…

and l = foldr (&&) True l
or l = foldr (||) False l

More magic

Foldr is more magical than this, it is one catamorphism! A Catamorphism over this data type:

data [a] = [] | a : [a]

Haskell List’s!

That means that foldr functions “knows” the recursive pattern over List’s.

Foldr also allows you to use Pointfree notation, witch I like. Formal explanation.

length = foldr (a b -> 1 + b) 0

More foldr’s

We can also write foldr’s functions to every data type that we create, and “explain” how recursive patterns works for our new data type:

data BTree a where
    Empty :: BTree a
    Node :: a -> BTree a -> BTree a -> BTree a

foldrBTree :: (a -> b -> b -> b) -> b -> BTree a -> b
foldrBTree op k Empty = k
foldrBTree op k (Node a l r) = op a (foldrBTree op k l) (foldrBTree op k r)

Now if we have a Binary Tree and want to know how many nodes it has, we just have to do this:

countNodes = foldrBTree (\a nodesLeft nodesRight ->a+nodesLeft+nodesRight) 0

If we want to know how many Empty’s our tree has:

countEmpty = foldrBTree (\_ nodesLeft nodesRight -> nodesLeft + nodesRight) 1

and so on…

Fold’s are really a genuine help for your code.
Later on I will talk more about catamorphisms…

Haskell history

27 10 2007

Haskell is so far one of my favorite programming languages, I don’t know if you noticed (lolz). Because it is ease of proof, no side effects, strongly typed, polymorphic, powerful, elegant and so much more.

Yesterday I was searching if there were any report talking about Haskell authors, other than Simon Peyton Jones or Paul Hudak. What I found out much more than I expected. I’ve found the History of Haskell paper. The paper that was published at 3rd History of Programming Language Conference in June 2007.

I start reading immediately, it was fascinating… The paper is a bit long (55 pages in two columns style), but deserves attention. The history is told for those who made History, Simon Peyton Jones, Paul Hudak, John Hughes and Philip Wadler, the main authors.

Haskell principles, ideology, implementations, tools – Haskell world, are in this paper. This document is excellent for those who want to learn Haskell or just know it history.


Haskell Wikipedia entrance