Nominal & Structural Typing

Differences between nominal and structural typing

An important attribute of every type system is whether they are structural or nominal, they can even be mixed within a single type system. So it’s important to know the difference.

A type is something like a string, a boolean, an object, or a class. They have names and they have structures. Primitives like strings or booleans have a very simple structure and only go by one name.

More complex types like object or classes have more complex structures. They each get their own name even if they sometimes have the same structure overall.

A static type checker uses either the names or the structure of the types in order to compare them against other types. Checking against the name is nominal typing and checking against the structure is structural typing.

Nominal typing

Languages like C++, Java, and Swift have primarily nominal type systems.

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class Foo { method(input: string) { /* ... */ } }
class Bar { method(input: string) { /* ... */ } }

let foo: Foo = new Bar(); // Error!

Here you can see a pseudo-example of a nominal type system erroring out when you’re trying to put a Bar where a Foo is required because they have different names.

Structural typing

Languages like OCaml, Haskell, and Elm have primarily structural type systems.

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class Foo { method(input: string) { /* ... */ } }
class Bar { method(input: string) { /* ... */ } }

let foo: Foo = new Bar(); // Works!

Here you can see a pseudo-example of a structural type system passing when you’re trying to put a Bar where a Foo is required because their structure is exactly the same.

But as soon as you change the shape it will start to cause errors.

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class Foo { method(input: string) { /* ... */ } }
class Bar { method(input: number) { /* ... */ } }

let foo: Foo = new Bar(); // Error!

It can get a little bit more complicated than this.

We’ve demonstrated both nominal and structure typing of classes, but there are also other complex types like objects and functions which can also be either nominal or structural. Even further, they can be different within the same type system (most of the languages listed before has features of both).

For example, Flow uses structural typing for objects and functions, but nominal typing for classes.

Functions are structurally typed

When comparing a function type with a function it must have the same structure in order to be considered valid.

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// @flow
type FuncType = (input: string) => void;
function func(input: string) { /* ... */ }
let test: FuncType = func; // Works!
Objects are structurally typed

When comparing an object type with an object it must have the same structure in order to be considered valid.

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type ObjType = { property: string };
let obj = { property: "value" };
let test: ObjType = obj;
Classes are nominally typed

When you have two classes with the same structure, they still are not considered equivalent because Flow uses nominal typing for classes.

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// @flow
class Foo { method(input: string) { /* ... */ } }
class Bar { method(input: string) { /* ... */ } }
let test: Foo = new Bar(); // Error!
Cannot assign `new Bar()` to `test` because `Bar` [1] is incompatible with `Foo` [2].

If you wanted to use a class structurally you could do that by mixing them with objects as interfaces:

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type Interface = {
  method(value: string): void;
};

class Foo { method(input: string) { /* ... */ } }
class Bar { method(input: string) { /* ... */ } }

let test: Interface = new Foo(); // Okay.
let test: Interface = new Bar(); // Okay.

Mixing nominal and structural typing

The design decision in Flow around mixing nominal and structural typing was chosen based on how objects, functions, and classes are already used in JavaScript.

The JavaScript language is a bunch of object-oriented ideas and functional ideas mixed together. Developer’s usage of JavaScript tends to be mixed as well. Classes (or constructor functions) being the more object-oriented side and functions (as lambdas) and objects tend to be more on the functional side, developers use both simultaneously.

When someone writes a class, they are declaring a thing. This thing might have the same structure as something else but they still serve different purposes. Imagine two component classes that both have render() methods, these components could still have totally different purposes, but in a structural type system they’d be considered exactly the same.

Flow chooses what is natural for JavaScript, and should behave the way you expect it to.

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