Free Monad example, Flow and fp-ts, npm install fp-ts

free.js

// @flow

import type { Free } from 'fp-ts/lib/Free'
import { liftF } from 'fp-ts/lib/Free'
import { Identity, identity } from 'fp-ts/lib/Identity'

// Creating set of instructions (AST)

export class Write<A> {
  +_tag: 'Write' = 'Write';
  +message: string;
  +more: A;
  constructor(message: string, more: A) {
    this.message = message
    this.more = more
  }
}

export class Read<A> {
  +_tag: 'Read' = 'Read';
  +more: (input: string) => A;
  constructor(more: (input: string) => A) {
    this.more = more
  }
}

export type ConsoleT = <U, L, A>(x: [U, L, A]) => Console<A>

export type Console<A> = Write<A> | Read<A>

export type ConsoleF<A> = Free<ConsoleT, A>

// Creating the DSL  
  
export const write = (message: string): ConsoleF<void> => liftF(new Write(message, undefined))
export const read = (): ConsoleF<string> => liftF(new Read(a => a))

// A program

const program = write("What's your name?")
  .chain(() => read())
  .chain(name => write(`Hello ${name}!`))

// Running the program

export function exaustive(x: empty): empty {
  return x;
}

export function identityInterpreter<A>(fa: Console<A>): Identity<A> {
  switch (fa._tag) {
    case 'Write':
      console.log(fa.message)
      return identity.of(fa.more)
    case 'Read':
      return identity.of(fa.more('Giulio'))
  }
  return exaustive(fa)
}

program.foldFree(identity)(identityInterpreter)
/*
What's your name?
Hello Giulio!
*/

// Another Interpreter

import { IO, io } from 'fp-ts/lib/IO'

export const prompt: IO<string> = new IO(() => window.prompt(''))
export const log = (message: string): IO<void> => new IO(() => window.alert(message))

export function ioInterpreter<A>(fa: Console<A>): IO<A> {
  switch (fa._tag) {
    case 'Write':
      const a = fa.more
      return log(fa.message).map(() => a)
    case 'Read':
      return prompt.map(fa.more)
  }
  return exaustive(fa)
}

program.foldFree(io)(ioInterpreter).run()
/*
  An alert, a prompt and another alert with the name you inserted will be displayed in the browser
*/

WARNING: Deprecated

• It looks like it does not work on the current version of fp-ts.
• Free does not exist in fp-ts but in fp-ts-contrib now.