[Functional Programming] Free monad application

Let's say we have a command line application:

const { Task } = require("../../libs/types");
const { save, all } = require("../../libs/db");
const { last } = require("ramda");

const AuthorTable = "Authors";
const Author = (name) => ({ name });
const PostTable = "Posts";
const Post = (title, body) => ({ title, body });

const readline = require("readline").createInterface({
  input: process.stdin,
  output: process.stdout,
});

const formatPost = (post) => `${post.title}:
${post.body}`;
const print = (s) =>
  Task((rej, res) => {
    res(console.log(s));
  });

const getInput = (q) =>
  Task((rej, res) => readline.question(q, (i) => res(i.trim())));

const menu = () =>
  getInput("where do you want to go today? (createAuthor, write, latest)").map(
    (route) => router[route]
  );

const createAuthor = () =>
  getInput("Name? ")
    .map((name) => Author(name))
    .chain((author) => save(AuthorTable, author))
    .map(() => menu);

const latest = () =>
  all(PostTable)
    .map((posts) => last(posts))
    .map(formatPost)
    .chain(print)
    .map(() => menu);

const write = () =>
  getInput("Title: ")
    .chain((title) => getInput("Body: ").map((body) => Post({ title, body })))
    .chain((post) => save(PostTable, post))
    .map(() => latest);

const start = () =>
  all(AuthorTable).map((authors) => (authors.length ? menu : createAuthor));

const router = { menu, createAuthor, latest, write };

const runApp = (f) => f().fork(console.error, runApp);

runApp(start);

All the actions are in 'Task' data type. such as 'getInput', 'print' dealing with console and stdout/stdin.

Those actions is not easy to test. 

Free monad can help in those cases. We can mock those actions as data type, then we are able to test those data type. In the end, we just interpret into real functions.

const { Task } = require("../../libs/types");
const { save, all } = require("../../libs/db");
const { last } = require("ramda");
const { liftF } = require("../../libs/free");
const { taggedSum } = require("daggy");

const AuthorTable = "Authors";
const Author = (name) => ({ name });
const PostTable = "Posts";
const Post = (title, body) => ({ title, body });

/** real actions */
const readline = require("readline").createInterface({
  input: process.stdin,
  output: process.stdout,
});
const formatPost = (post) => `${post.title}
${post.body}`;
const writeOutput = (s) =>
  Task((rej, res) => {
    console.log(s);
    res(s);
  });
const getInput = (q) =>
  Task((rej, res) => readline.question(q, (i) => res(i.trim())));

/** Mock data type */
const Console = taggedSum("Console", { Question: ["q"], Print: ["s"] });
const Db = taggedSum("Db", {
  Save: ["table", "record"],
  All: ["table", "query"],
});

/**Lift into free monads */
const print = (s) => liftF(Console.Print(s));
const question = (s) => liftF(Console.Question(s));
const dbAll = (table, query) => liftF(Db.All(table, query));
const dbSave = (table, record) => liftF(Db.Save(table, record));

/**App logics */
const menu = () =>
  question("where do you want to go today? (createAuthor, write, latest)").map(
    (route) => router[route]
  );

const createAuthor = () =>
  question("Name? ")
    .map((name) => Author(name))
    .chain((author) => dbSave(AuthorTable, author))
    .map(() => menu);

const latest = () =>
  dbAll(PostTable)
    .map((posts) => last(posts))
    .map(formatPost)
    .chain(print)
    .map(() => menu);

const write = () =>
  question("Title: ")
    .chain((title) => question("Body: ").map((body) => Post(title, body)))
    .chain((post) => dbSave(PostTable, post))
    .map(() => latest);

const start = () =>
  dbAll(AuthorTable).map((authors) => (authors.length ? menu : createAuthor));

const router = { menu, createAuthor, latest, write };

/** interpret */
const dbToTask = (x) =>
  x.cata({
    Save: save,
    All: all,
  });
const consoleToTask = (x) =>
  x.cata({
    Question: getInput,
    Print: writeOutput,
  });
const interpret = (x) => (x.table ? dbToTask(x) : consoleToTask(x));

const runApp = (f) =>
  f().foldMap(interpret, Task.of).fork(console.error, runApp);

runApp(start);