computer science Computer science is the study of computation, information, and automation. Computer science spans Theoretical computer science, theoretical disciplines (such as algorithms, theory of computation, and information theory) to Applied science, ...

, a

type system In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a ''type'' (for example, integer, floating point, string) to every '' term'' (a word, phrase, or other set of symbols). Usu ...

is nominal (also called nominative or name-based) if compatibility and equivalence of

data type In computer science and computer programming, a data type (or simply type) is a collection or grouping of data values, usually specified by a set of possible values, a set of allowed operations on these values, and/or a representation of these ...

s is determined by explicit declarations and/or the name of the types. Nominal systems are used to determine whether types are equivalent, as well as whether a type is a subtype of another. Nominal type systems contrast with ''structural systems'', where comparisons are based on the structure of the types in question and do not require explicit declarations.

Nominal typing

Nominal typing means that two variables are type-compatible if and only if their declarations name the same type. For example, in C, two struct types with different names in the same translation unit are never considered compatible, even if they have identical field declarations. However, C also allows a

typedef 

typedef is a  reserved keyword in the programming languages  C,  C++, and Objective-C. It is used to create an additional name (''alias'') for another data type, but does not create a new type, except in the obscure case of a  qualified typedef of ...

declaration, which introduces an alias for an existing type. These are merely syntactical and do not differentiate the type from its alias for the purpose of type checking. This feature, present in many languages, can result in a loss of type safety when (for example) the same primitive integer type is used in two semantically distinct ways.

Haskell Haskell () is a general-purpose, statically typed, purely functional programming language with type inference and lazy evaluation. Designed for teaching, research, and industrial applications, Haskell pioneered several programming language ...

provides the C-style syntactic alias in the form of the type declaration, as well as the newtype declaration that does introduce a new, distinct type,

isomorphic In mathematics, an isomorphism is a structure-preserving mapping or morphism between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between the ...

to an existing type.

Nominal subtyping

In a similar fashion, nominal subtyping means that one type is a subtype of another if and only if it is explicitly declared to be so in its definition. Nominally-typed languages typically enforce the requirement that declared subtypes be structurally compatible (though Eiffel allows non-compatible subtypes to be declared). However, subtypes which are structurally compatible "by accident", but not declared as subtypes, are not considered to be subtypes. C++, C#,

Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...

, Kotlin,

Objective-C Objective-C is a high-level general-purpose, object-oriented programming language that adds Smalltalk-style message passing (messaging) to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was ...

Delphi Delphi (; ), in legend previously called Pytho (Πυθώ), was an ancient sacred precinct and the seat of Pythia, the major oracle who was consulted about important decisions throughout the ancient Classical antiquity, classical world. The A ...

Swift Swift or SWIFT most commonly refers to: * SWIFT, an international organization facilitating transactions between banks ** SWIFT code * Swift (programming language) * Swift (bird), a family of birds It may also refer to: Organizations * SWIF ...

, Julia and

Rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH) ...

all primarily use both nominal typing and nominal subtyping. Some nominally-subtyped languages, such as Java and C#, allow classes to be declared ''final'' (or ''sealed'' in C# terminology), indicating that no further subtyping is permitted.

Comparison

Nominal typing is useful at preventing accidental type equivalence, which allows better type-safety than structural typing. The cost is a reduced flexibility, as, for example, nominal typing does not allow new super-types to be created without modification of the existing subtypes.

References

Sources

External links