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Comment by chunkyslink | Hacker News Reader

Could someone please explain what this could be used for ?

Instead of using if or switch statements to compute a value, you can use a prettier interface.

So this:

  function getStatus(status) {
    if (status === 200){
      return "Success";
    } else if (status === 401){
      return "Fail!";
    }
  }

  var myString = getStatus(response.status);

Now becomes this:

  var myString = match (status) {
    {200} => "Success",
    {401} => "Fail!"
  }

More like:

  var myString = match (status) {
    200 => "Success",
    401 => "Fail!"
  }

Smooshes the evaluation of an object that can be many different shapes into an easy to understand operation.

I think the HTTP response is a great example. It can have many status codes which can determine how you want to proceed with that response. If you've worked with handling HTTP responses, you've most likely implemented some version of a 'match' operation before.

Is not `switch` enough for this?

It's more concise than switch, as well as more powerful (can destructure objects). You can't set things equal to the result of a switch. I find the pattern very convenient.

  let day;
  switch (date) {
      case 1:
          day = "mon";
          break;
      case 2:
          day = "tues";
          break;
      default:
          day = "wed";
          break;

  let day = match(date) {
      1 => 'mon',
      2 => 'tues'
  }

In fairness I think that would look a bit more like this:

    const getDate = (date) => {
      switch (date) {
        case 1:
          return 'mon'

        case 2:
          return 'tues'

        default:
          return 'wed'
      }
    }

    let date = getDate(1)

Which is, of course, still less terse. What I normally use when I have cases like this is an object-as-a-map. IE:

    const dates = {
      1: 'mon',
      2: 'tues'
    }

    let day = dates[3] || 'wed';

By the way, I don't think it is a good idea to write a function as a constant. Please compare this

    function getDate(date) { .. }

to this:

    const getDate = (date) => { ... }

The first version is more readable. We instantly see that it is a function and in a second case it looks like a constant at first. Also without the equal and arrow sign it looks simpler.

Life is more fun with fat arrows sometimes

    new Promise(function (resolve, reject) {
        methodOne(data, function (error, response) {
            if (erorr) {
                reject(error);
            } else {
                resolve(response);
            }
        })
    })

    new Promise((resolve, reject) => methodOne(data, (error, response) => error ? reject(error) : resolve(response)));

That one liner has a lot going on. The first one is easier to read.

Please also look at my idea how to rewrite the code for readability using `deferred` pattern: https://news.ycombinator.com/item?id=16933983

Yes, there are cases when arrow functions are useful: when small functions are used inline, like this:

    var numbers = [1, 2, 3, 4].map(x => x * x);
    var bestUsers = users.filter(u => u.getRating() > 100);

But for a case when you have a large non-anonymous function, `function` keyword suits better. You don't need to use `const` keyword just becase it is something trendy now.

In your example, the code with arrow functions is smaller, but it is not more readable. Because there is no indentation, it is difficult to understand how code is nested. I cannot read that.

It can be rewritten using `deferred` pattern:

    var deferred = new Deferred;

    methodOne(data, function (error, response) {
        if (erorr) {
            deferred.reject(error);
        } else {
            deferred.resolve(response);
        }
    });

    return deferred.getPromise();

This way we can get rid of a callback in the Promise constructor. Please note that our code now looks sequential and we clearly see what happens after what. Asynchronous code is difficult to write and read; therefore we must put an extra effort to make it easier.

In my opinion it is generally bad idea to nest more that 1-2 levels of functions inside each other.

> A Deferred object is returned by the obsolete Promise.defer() method to provide a new promise along with methods to change its state.

> Starting from Gecko 30, this object is obsolete and should not be used anymore. Use the new Promise() constructor instead (or use the above backwards/forwards compatible Deferred function given below). For example, the equivalent of

Seems like it's obsolete. [0]

If you want to write async code, use async / await.

    const run = async () => {
        const response = await new Promise((resolve, reject) => methodOne(data, (e, res) => e ? reject(e) : resolve(res)));
    };

If you spend enough time with fat arrow, it's as easy to read as `function` is. On top of that, IMO it looks cleaner. It also allows you to do scope binding in a different manor which in React is much better.

This:

    <a onClick={this.onClickEvent.bind(this)}>Click Me</a>

becomes this:

    <a onClick={(event) => this.onClickEvent(event)}>Click Me</a>

[0] https://developer.mozilla.org/en-US/docs/Mozilla/JavaScript_...

> Seems like it's obsolete.

Then you can write your own Deferred implementation. I didn't even know it was implemented in browsers.

     const run = async () => { ... }

How to read this? run is a constant that equals the result of calling function async() thas maps to something in curly brackets? This is confusing.

> If you want to write async code, use async / await.

That is a good idea, but it has nothing to do with arrow functions.

> It also allows you to do scope binding in a different manor

`this` binding in JS in classic functions is broken by design, so yes, that is the advantage of arrow functions.

But function getDate(date) is hoisted. The const version isn't.

I don't think it is a good idea to rely on such subtle differences that are not well known among developers. We want to write code that is easy to maintain, not compete in knowing ECMA specs, right?

From looking at the proposal, there are substantial differences. The biggest is that switch is an equality test on a single value, while pattern matching as proposed allows matching on multiple properties of an object, as well as conditional clauses.

I need to look at it more closely, as some languages have matching be an expression, rather than a statement or block like the conventional switch.

It doesn't add something that you can't already do- tcomb and other libraries offer functions that mimic it- but it changes the way you can express certain ideas without needing them, in a way that is already familiar from other languages.

EDIT: yes, it does look like an expression, which means it can be used in many more places than a standard switch.

> Smooshes the evaluation

I see what you did there...

Pattern matching is very common in ML languages and is a core part of Rust.

As another commenter said, it's like conditionals on steroids.

Pattern matching can be quite powerful, e.g. in C# (which I wish was more indepth / more featured, it still feels a bit immature :( )

```

    switch(foo)
    {
       case TextBox t:
          Console.Writeline(t.Text);
          break;
       case TextBox when t.Text = "Bob":
          Console.WriteLine("Hello Bob");
          break;
       case Combobox c:
          Console.WriteLine($"{c.SelectedItem}");
          break;
       case null:
          Console.WriteLine("Ooops, null!");
          break;
       case int i when i == 5:
          Console.WriteLine("got an int, and it was 5!");
          break;
       case default:
           // handle the default case.
           break;
     }

```

IN languages like F#, pattern matching can compile time check you have covered all bases as well.

e.g, this won't build

````

    type VariableResult =
      | E of string
      | V of string

    let result = V "variable"

    match result with
      | E e -> printf "was error"

```

As i haven't told it how to handle the V case for that discriminated union. So you get nice compile time checking.

Ugh, how do you format code?

> Pattern matching can be quite powerful, e.g. in C# (which I wish was more indepth / more featured, it still feels a bit immature :( )

C# doesn't really deserve the name of pattern matching, just a type-based switch (with guards), you can't match on values let alone destructure them.

> Ugh, how do you format code?

4 spaces indent.

Basically simplifying and shortening a ton of common patterns of code, very similarly to how the new-ish destructuring syntax did a few years ago.

So, there are example use cases in the proposal [1]. I think it's meant to standardize how we handle conditional inputs or shapes of inputs (some people use if statements, some switch, and others store it all in an object and use keys).

1. https://github.com/tc39/proposal-pattern-matching#motivating...

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