Nominal Typing

Definitions

1. A type system where compatibility is determined by explicit declarations (class name, implements/extends) rather than structural similarity. Two types with identical fields are NOT compatible unless one explicitly extends/implements the other. Used by Java, C#, Swift, and Rust.

   In plain English: A type system where types must be explicitly declared as related — same shape alone doesn't make them compatible.

   Example: "In Java, Meters and Feet are different types even if both wrap an int — nominal typing prevents mixing them up."

Related Terms

• Algebraic Data Type
• Annotation
• Class
• Generics
• Horn Clause
• Algebraic Data Type
• Covariance
• Dependent Type
• Effect System