Haskell's algebraic data types

Question

I\'m trying to fully understand all of Haskell\'s concepts.

In what ways are algebraic data types similar to generic types, e.g., in C# and Java? And how are they di

Answer 1

old question, but no one's mentioned nullability, which is an important aspect of Algebraic Data Types, perhaps the most important aspect. Since each value most be one of alternatives, exhaustive case-based pattern matching is possible.

Answer 2

Haskell's algebraic data types are named such since they correspond to an initial algebra in category theory, giving us some laws, some operations and some symbols to manipulate. We may even use algebraic notation for describing regular data structures, where:

• + represents sum types (disjoint unions, e.g. Either).
• • represents product types (e.g. structs or tuples)
• X for the singleton type (e.g. data X a = X a)
• 1 for the unit type ()
• and μ for the least fixed point (e.g. recursive types), usually implicit.

with some additional notation:

• X² for X•X

In fact, you might say (following Brent Yorgey) that a Haskell data type is regular if it can be expressed in terms of 1, X, +, •, and a least fixed point.

With this notation, we can concisely describe many regular data structures:

• Units: data () = ()

  1

• Options: data Maybe a = Nothing | Just a

  1 + X

• Lists: data [a] = [] | a : [a]

  L = 1+X•L

• Binary trees: data BTree a = Empty | Node a (BTree a) (BTree a)

  B = 1 + X•B²

Other operations hold (taken from Brent Yorgey's paper, listed in the references):

• Expansion: unfolding the fix point can be helpful for thinking about lists. L = 1 + X + X² + X³ + ... (that is, lists are either empty, or they have one element, or two elements, or three, or ...)

• Composition, ◦, given types F and G, the composition F ◦ G is a type which builds “F-structures made out of G-structures” (e.g. R = X • (L ◦ R) ,where L is lists, is a rose tree.

• Differentiation, the derivative of a data type D (given as D') is the type of D-structures with a single “hole”, that is, a distinguished location not containing any data. That amazingly satisfy the same rules as for differentiation in calculus:

  1′ = 0

  X′ = 1

  (F + G)′ = F' + G′

  (F • G)′ = F • G′ + F′ • G

  (F ◦ G)′ = (F′ ◦ G) • G′

---

References:

• Species and Functors and Types, Oh My!, Brent A. Yorgey, Haskell’10, September 30, 2010, Baltimore, Maryland, USA
• Clowns to the left of me, jokers to the right (Dissecting Data Structures), Conor McBride POPL 2008

Answer 3

For me, the concept of Haskell's algebraic data types always looked like polymorphism in OO-languages like C#.

Look at the example from http://en.wikipedia.org/wiki/Algebraic_data_types:

data Tree = Empty 
          | Leaf Int 
          | Node Tree Tree

This could be implemented in C# as a TreeNode base class, with a derived Leaf class and a derived TreeNodeWithChildren class, and if you want even a derived EmptyNode class.

(OK I know, nobody would ever do that, but at least you could do it.)

Answer 4

"Algebraic Data Types" in Haskell support full parametric polymorphism, which is the more technically correct name for generics, as a simple example the list data type:

 data List a = Cons a (List a) | Nil

Is equivalent (as much as is possible, and ignoring non-strict evaluation, etc) to

 class List<a> {
     class Cons : List<a> {
         a head;
         List<a> tail;
     }
     class Nil : List<a> {}
 }

Of course Haskell's type system allows more ... interesting use of type parameters but this is just a simple example. With regards to the "Algebraic Type" name, i've honestly never been entirely sure of the exact reason for them being named that, but have assumed that it's due the mathematical underpinnings of the type system. I believe that the reason boils down to the theoretical definition of an ADT being the "product of a set of constructors", however it's been a couple of years since i escaped university so i can no longer remember the specifics.

[Edit: Thanks to Chris Conway for pointing out my foolish error, ADT are of course sum types, the constructors providing the product/tuple of fields]

Answer 5

A simple reason why they are called algebraic; there are both sum (logical disjunction) and product (logical conjunction) types. A sum type is a discriminated union, e.g:

data Bool = False | True

A product type is a type with multiple parameters:

data Pair a b = Pair a b

In O'Caml "product" is made more explicit:

type 'a 'b pair = Pair of 'a * 'b

Answer 6

Haskell's datatypes are called "algebraic" because of their connection to categorical initial algebras. But that way lies madness.

@olliej: ADTs are actually "sum" types. Tuples are products.