Tutorial

Flow Generic Constraints

Published on July 6, 2017

By Matthew Garcia

There are cases in Flow where you’ll want to create a generic function or class, but want to limit what can be passed as a type argument.

Where Generic Constraints are Useful

Let’s say you have a class that wraps a Map, but makes a shallow copy before inserting:

class ShallowMap<T> {
  map: Map<string, T>;

  constructor() {
    this.map = new Map();
  }

  get(key: string) {
    return this.map.get(key);
  }

  set(key: string, value: T) {
    // Make a shallow copy.  Flow will give an error on this line.
    const copy = {...value};
    this.map.set(key, copy);
  }
}

Flow won’t let you do this, since, while objects can be spread, other types can’t:

// This works.
const spreadEmptyObject = {...Object.create(null)};
const spreadObjectWithProperty = {...{id: 2}};
// This doesn't.
const spreadNumber = {...0};
const spreadString = {...''};

And there’s nothing saying T has to be an object.

Adding Something that Says `T` Has to Be An Object

It’s pretty simple; just add a colon and the constraining type after the generic type declaration:

// `T` has to be an object.
class ShallowMap<T: Object> {
  map: Map<string, T>;

  constructor() {
    this.map = new Map();
  }

  get(key: string) {
    return this.map.get(key);
  }

  set(key: string, value: T) {
    // Flow is okay with this line now.
    const copy = {...value};
    this.map.set(key, copy);
  }
}

More Specific Constraints

Constraints can be more specific, allowing more complex interactions with generic types, since they fit some criteria. For example, a generic map that uses a property of T to determine the key:

type Keyable = {
  key: string,
};

// `T` has a string property called `key`.
class AutoKeyMap<T: {key: string}> {
  map: Map<string, T>;

  constructor() {
    this.map = new Map();
  }

  get(key: string) {
    return this.map.get(key);
  }

  set(value: T) {
    // Since `T` has a string property `key`, we can access it.
    const key = value.key;
    this.map.set(key, value);
  }
}

Thanks for learning with the DigitalOcean Community. Check out our offerings for compute, storage, networking, and managed databases.

Learn more about our products

About the author(s)

See author profile

Category:

Tutorial

Tags:

While we believe that this content benefits our community, we have not yet thoroughly reviewed it. If you have any suggestions for improvements, please let us know by clicking the “report an issue“ button at the bottom of the tutorial.