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Nominal type system
In computer science, a nominal or nominative type system (or name-based type system) is a major class of type system, in which compatibility and equivalence of data types is determined by explicit declarations and/or the name of the types. Nominal systems are used to determine if types are equivalent, as well as if a type is a subtype of another. It contrasts 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 are never considered compatible, even if they have identical field declarations.
However, C++ also allows a typedef 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 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 to an existing type.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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