Tutorial

Generic Functions in Flow

Published on June 29, 2017
    author

    Matthew Garcia

    Generic Functions in Flow

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    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');
    

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    About the authors
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    Matthew Garcia

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