// Tutorial //

Flow Generic Constraints

Published on July 6, 2017
    Default avatar
    By Matthew Garcia
    Developer and author at DigitalOcean.
    Flow Generic Constraints

    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);
      }
    }
    

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    About the authors
    Default avatar
    Developer and author at DigitalOcean.

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