[JS Compose] 3. Use chain for composable error handling with nested Eithers (flatMap)

We refactor a function that uses try/catch to a single composed expression using Either. We then introduce the chain function to deal with nested Eithers resulting from two try/catch calls.

For example we have this code using try & catch:

const getPort = () => {
  try {
    const file = fs.readFileSync('config.json');
    const c = JSON.parse(file);
    return c.port;
  } catch(e) {
    return 3000;
  }
}

And now, we want to use Either to rewirte the code:

// SETUP: fake fs
//==========
const fs = {
  readFileSync: name => {
    if(name === 'config.json') {
      return JSON.stringify({port: 8888})
    } else {
      throw('missing file!')
    }
  }
}

//=================

const Right = x => ({
    map: f => Right(f(x)),
    fold: (f, g) => g(x),
    toString: () => `Right(${x})`
  });

const Left = x => ({
  map: f => Left(x),
  fold: (f, g) => f(x),
  toString: () => `Left(${x})`
});

const fromNullable = x => 
  x != null ? Right(x): Left(null);

const tryCatch = f => {
  try {
    return Right(f());
  } catch(e) {
    return Left(e);
  }
}

//=========================
const getPort = () =>
  tryCatch(() => fs.readFileSync('config.json'))
  .map(f => JSON.parse(f))
  .fold(
    x => 3000,
    x => x.port);

console.log(getPort('config.json'))

We wrote the function 'tryCatch', the idea is put the code need to be checked in try, when success call 'Right', error, call 'Left()'.

tryCatch(() => fs.readFileSync('config.json'))

Read the file, is success, will return file content. If not, then goes to set default 3000.

  .fold(
    x => 3000,
    x => x.port);

It works, but we still miss one things, in the old code, the 'JSON.parse' are also wrapped into try catch.

const getPort = () =>
  tryCatch(() => fs.readFileSync('config.json')) //Right('{port:8888}')
  .map(f => tryCatach(() => JSON.parse(f))) //Right(Right({port:8888}))

But once we also wrap parseing code into tryCatch function, the return value is 2d-Right. So we want to flatten it.

const Right = x => ({
    map: f => Right(f(x)),
    flatMap: f => f(x),
    fold: (f, g) => g(x),
    toString: () => `Right(${x})`
  });

const Left = x => ({
  map: f => Left(x),
  flatMap: f => f(x),
  fold: (f, g) => f(x),
  toString: () => `Left(${x})`
});

We add 'flatMap', so instead of putting the value into Right() or Left(), we just return the value. Because we know the value passed in is already a Right or Left.

const getPort = () =>
  tryCatch(() => fs.readFileSync('config.json')) //Right('{port:8888}')
  .flatMap(f => tryCatch(() => JSON.parse(f))) //Right({port:8888})
  .fold(
    x => 3000,
    x => x.port);

---------

// SETUP: fake fs
//==========
const fs = {
  readFileSync: name => {
    if(name === 'config.json') {
      return JSON.stringify({port: 8888})
    } else {
      throw('missing file!')
    }
  }
}

//=================

const Right = x => ({
    map: f => Right(f(x)),
    flatMap: f => f(x),
    fold: (f, g) => g(x),
    toString: () => `Right(${x})`
  });

const Left = x => ({
  map: f => Left(x),
  flatMap: f => f(x),
  fold: (f, g) => f(x),
  toString: () => `Left(${x})`
});

const fromNullable = x => 
  x != null ? Right(x): Left(null);

const tryCatch = f => {
  try {
    return Right(f());
  } catch(e) {
    return Left(e);
  }
}

//=========================
const getPort = () =>
  tryCatch(() => fs.readFileSync('config.json')) //Right({port:8888})
  .flatMap(f => tryCatch(() => JSON.parse(f))) //Right(Right({port:8888}))
  .fold(
    x => 3000,
    x => x.port);

console.log(getPort('config.json'))

-----

You can also rename 'flatMap' to 'chain'. Sometime 'flatMap' or 'chain' looks similar to 'fold' implementation. But the meaning is different, here 'chain / flatMap' says It says, "If we're going to return another Either, we are going to use chain instead of map." 'fold' says just get the value out of the box, it's done!