Flow type inference using the "decoders" library

decoder-inference.js

// @flow
import { guard, object, string, number } from 'decoders'
import type { Decoder } from 'decoders'

// This is the type-level "function" that pulls the generic out of the decoder
type ExtractDecoderType = <T>(d: Decoder<T>) => T

// Now we define our run-time decoder, without defining the type first...
const person = object({
  name: string,
  age: number
})

// And here is the "magic": We extract the inferred type out of the Decoder<T>
// using the amazing $Call utility function and the "typeof" type-level operator
type Person = $Call<ExtractDecoderType, typeof person>

// Now we can make our actual decoder
const decodePerson = guard(person)

// And here's our test function
function hello (p: Person): string {
  return `Hello, ${p.name}`
}

// Time to try it out.
//
// The run-time behaviour now matches the type-level behaviour, yay!
const a = decodePerson({
  name: 'test',
  age: 1
})

// Look ma! 100% Flow coverage!
console.log(hello(a))