Tutorial

Generic Functions in Flow

Published on June 29, 2017

By Matthew Garcia

One of the best concepts Flow borrows from Object-Oriented programs is the concept of generics. There are many cases where generics are essential to refining type checks.

Where are Generics Useful?

Let’s say you have a function memoize. With Flow types, it might look something like this:

function memoize(func: (key: any) => any): (key: any) => any {
  const registry = new Map();
  return function(key: any): any {
    let value = registry.get(key);
    if (typeof value === 'undefined') {
      value = func(key);
      registry.set(key, value);
    }
    return value;
  };
}

The problem is that it will swallow the specifics of func:

// Type is (val: number) => boolean
function isPrime(val: number): boolean {
  // Implementation...
}
// Type is (key: any) => any
const memoizedIsPrime = memoize(isPrime);
// This gives an error, since `Sentinel` is not a number.
isPrime('Sentinel');
// This does not give an error, since 'Sentinel' is any.
memoizedIsPrime('Sentinel');

Making it Generic

It’s as simple as declaring types in chevrons before the parameters and using those as types:

// We're using `K` and `V` here for convention, but you can name them pretty much anything.
export default function memoize<K, V>(func: (key: K) => V): (key: K) => V {
  const registry = new Map();
  return function(key: K): V {
    let value = registry.get(key);
    if (typeof value === 'undefined') {
      value = func(key);
      registry.set(key, value);
    }
    return value;
  };
}

Flow will infer the rest:

// Type is (val: number) => boolean
function isPrime(val: number): boolean {
  // Implementation...
}
// Type is (key: K) => V.  Flow infers that `K` is number and `V` is boolean.
const memoizedIsPrime = memoize(isPrime);
// This gives an error, since `Sentinel` is not a number.
isPrime('Sentinel');
// This gives an error, since 'Sentinel' is a 'K' (number).
memoizedIsPrime('Sentinel');

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.