This is a compendium of some of the terminology that is used in the documentation for Bow and in literature about Functional Programming. It is expected to grow over time as we make progress in the documentation of the library. If there is a term that you would like to see in this list, please, open an issue and we will consider adding it here.


Ad-hoc polymorphism

Ad-hoc polymorphism is a type of polymorphism achieved by having functions with the same name but different signatures and/or different number of arguments. In Functional Programming, ad-hoc polymorphism is achieved with type classes that act as constraints added to type parameters in parametrically polymorphic types or functions. Ad-hoc polymorphism is also known as function overloading.



A function is a piece of code that has the following properties:

  • Total: it must provide an output for every input.
  • Deterministic: for a given input, the function always returns the same output.
  • Pure: the evaluation of the function does not cause any other effects besides computing the output.

Notice that Swift does not enforce any of these properties in their definition. Swift functions that have these properties are generally referred as pure functions, whereas the ones that do not have some of them are called impure functions.

Functional Programming

Functional Programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids changing state and mutable data.


Instance (of a type class)

An instance of a type class is the implementation of such type class for a given type. They are usually done in Swift using the extension functionality in the language.



A kind is a group of types. As values are grouped into types, types are grouped into kinds. The notion of kind lets us conceptualize type constructors that receive a number of type arguments and abstract over them. This generalization is known as higher kinded types and enable generic polymorphic programs. Swift does not support this feature natively, but it is simulated in Bow.



Memoization is an optimization technique that transforms expensive computational function calls into searches in a lookup table by caching previous calls.


Parametric polymorphism

Parametric polymorphism is a type of polymorphism that allows specifying generic types to create functions and/or other types. These generic types are called type parameters.


A procedure is a Swift function that is not total, deterministic and/or pure, as required in the mathematical definition of function. Generally, it is referred to as an impure function.


Referential Transparency

Referential transparency is a property of a function that allows it to be replaced by the result of its execution without altering the overall behavior of the program.



A type is a set of values that helps us conceptualize data and restrict the possible values that functions can accept as input or provide as output. Types can be finite (like Bool, with two values: true and false) or infinite (like String).

Type class

A type class is a group of functions that operate on generic type parameters and is governed by algebraic laws. They are usually represented in Swift as protocols with associated types or self requirements.

Type constructor

A type constructor is a type that receives other types and produces new types. Examples of type constructors are Array<Element> or Optional<Wrapped>, where providing types for Element or Wrapper types yields new types like Array<Int> or Optional<String>.

Type parameter

A type parameter is a placeholder in a type constructor or generic function that let us provide different types and generate a family of types or functions. For instance, in Array<Element>, Element is a type parameter for Array and replacing with concrete types yields a family of new types.



A witness is an intermediate class that is used to simulate Higher Kinded Types in Bow. As a convention, they are prefixed by For. As an example, consider the Option<A> data type in Bow. This data type extends Kind<ForOption, A> in order to enable HKT support. ForOption is the witness for Option.