In computer science, a type class is a type system construct that supports ad hoc polymorphism. This is achieved by adding constraints to type variables in parametrically polymorphic types. Such a constraint typically involves a type class
T and a type variable
a, and means that
a can only be instantiated to a type whose members support the overloaded operations associated with
Type classes first appeared in the Haskell programming language, and were originally conceived as a way of implementing overloaded arithmetic and equality operators in a principled fashion. In contrast with the "eqtypes" of Standard ML, overloading the equality operator through the use of type classes in Haskell does not require extensive modification of the compiler frontend or the underlying type system.
Since their creation, many other applications of type classes have been discovered.
The programmer defines a type class by specifying a set of function or constant names, together with their respective types, that must exist for every type that belongs to the class. In Haskell, types can be parameterized; a type class
Eq intended to contain types that admit equality would be declared in the following way:
class Eq a where (==) :: a -> a -> Bool (/=) :: a -> a -> Bool
This declaration may be read as stating a "type
a belongs to type class
Eq if there are functions named
(/=), of the appropriate types, defined on it." A programmer could then define a function
elem (which determines if an element is in a list) in the following way:
elem :: (Eq a) => a -> [a] -> Bool elem y  = False elem y (x:xs) = (x == y) || elem y xs
elem has the type
a -> [a] -> Bool with the context
(Eq a), which constrains the types which
a can range over to those
a which belong to the
Eq type class. (Note: Haskell
=> can be called a 'class constraint'.)
A programmer can make any type
t a member of a given type class
C by using an instance declaration that defines implementations of all of
C's methods for the particular type
t. For instance, if a programmer defines a new data type
t, they may then make this new type an instance of
Eq by providing an equality function over values of type
t in whatever way they see fit. Once they have done this, they may use the function
[t], that is, lists of elements of type
Note that type classes are different from classes in object-oriented programming languages. In particular,
Eq is not a type: there is no such thing as a value of type
Type classes are closely related to parametric polymorphism. For example, note that the type of
elem as specified above would be the parametrically polymorphic type
a -> [a] -> Bool were it not for the type class constraint "
(Eq a) =>".
A type class need not take a type variable of kind , but can take one of any kind. These type classes with higher kinds are sometimes called constructor classes (the constructors referred to are type constructors such as Maybe, rather than data constructors such as Just). An example is the monad class:
class Monad m where (>>=) :: m a -> (a -> m b) -> m b return :: a -> m a
The fact that m is applied to a type variable indicates that it has kind * -> *, i.e. it takes a type and returns a type.
Multi-parameter type classes
Type classes permit multiple type parameters, and so type classes can be seen as relations on types. For example, in the GHC standard library, the class
IArray expresses a general immutable array interface. In this class, the type class constraint
IArray a e means that
a is an array type that contains elements of type
e. (This restriction on polymorphism is used to implement unboxed array types, for example.)
Like multimethods, multi-parameter type classes support calling different implementations of a method depending on the types of multiple arguments, and indeed return types. They are more efficient than multimethods because they do not involve searching for the method to call on every call at runtime: the method to call is stored in the dictionary of the type class instance, just as with single-parameter type classes.
Haskell code that uses multi-parameter type classes is not portable, as this feature is not part of the Haskell 98 standard. The popular Haskell implementations, GHC and Hugs, support multi-parameter type classes.
In Haskell, type classes have been refined to allow the programmer to declare functional dependencies between type parameters—a concept inspired from relational database theory. That is, the programmer can assert that a given assignment of some subset of the type parameters uniquely determines the remaining type parameters. For example, general monads
m which carry a state parameter of type
s satisfy the type class constraint
MonadState s m. In this constraint, there is a functional dependency
m -> s. This means that for a given monad, the state type accessible from this interface is uniquely determined. This aids the compiler in type inference, as well as aiding the programmer in type-directed programming.
Type classes and implicit parameters
|This section does not cite any sources. (January 2012)|
Type classes and implicit parameters are very similar in nature, although not quite the same. A polymorphic function with a type class constraint such as:
sum :: Num a => [a] -> a
can be intuitively treated as a function that implicitly accepts an instance of
sum_ :: Num_ a -> [a] -> a
Num_ a is essentially a record that contains the instance definition of
Num a. (This is in fact how type classes are implemented under the hood by the Glasgow Haskell Compiler.)
However, there is a crucial difference: implicit parameters are more flexible – you can pass different instances of
Num Int. In contrast, type classes enforce the so-called coherence property, which requires that there should only be one unique choice of instance for any given type. The coherence property makes type classes somewhat antimodular, which is why orphan instances (instances that are defined in a module that neither contains the class nor the type of interest) are strongly discouraged. On the other hand, coherence adds an additional level of safety to the language, providing the programmer a guarantee that two disjoint parts of the same code will share the same instance.
As an example, an ordered set (of type
Set a) requires a total ordering on the elements (of type
a) in order to function. This can be evidenced by a constraint
Ord a, which defines a comparison operator on the elements. However, there can be numerous ways to impose a total order. Since set algorithms are generally intolerant of changes in the ordering once a set has been constructed, passing an incompatible instance of
Ord a to functions that operate on the set may lead to incorrect results (or crashes). Thus, enforcing coherence of
Ord a in this particular scenario is crucial.
Instances (or "dictionaries") in Scala type classes are just ordinary values in the language, rather than a completely separate kind of entity. While these instances are by default supplied by finding appropriate instances in scope to be used as the implicit actual parameters for explicitly-declared implicit formal parameters, the fact that they are ordinary values means that they can be supplied explicitly, to resolve ambiguity. As a result, Scalar type classes do not satisfy the coherence property and are effectively a syntactic sugar for implicit parameters.
Other approaches to operator overloading
In Standard ML, the mechanism of "equality types" corresponds roughly to Haskell's built-in type class
Eq, but all equality operators are derived automatically by the compiler. The programmer's control of the process is limited to designating which type components in a structure are equality types and which type variables in a polymorphic type range over equality types.
SML's and OCaml's modules and functors can play a role similar to that of Haskell's type classes, the principal difference being the role of type inference, which makes type classes suitable for ad hoc polymorphism. The object oriented subset of OCaml is yet another approach which is somewhat comparable to the one of type classes.
- Polymorphism (computer science) (other kinds of polymorphism)
- Haskell programming language (the language in which type classes were first designed)
- Operator overloading (one application of type classes)
- Monad (functional programming) (
Monadis an example of a type class)
- Concepts (C++) (a similar idea mooted, but not yet part of the language)
- Rust (programming language)
- "Type classes, first proposed during the design of the Haskell programming language, ..." —John Garrett Morris (2013), "Type Classes and Instance Chains: A Relational Approach"
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- Mark Jones. Type Classes with Functional Dependencies. From Proc. 9th European Symposium on Programming. March, 2000.
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- E. Kmett, Type Classes vs. the World, Boston Haskell Meetup. https://www.youtube.com/watch?v=hIZxTQP1ifo
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