With Statement

[danielribeiro]

It would be nice to have with statement on coffescript. Actually, instance eval would be nicer, but with stament is close enough for most dsls.

[hen-x]

The with statement is excluded from Coffeescript for reasons best expressed by Douglas Crockford, and reflected by most of the JS geekdom. It buys barely more clarity than you can already achieve with a single-letter temporary variable, yet the costs entail semantic ambiguity, potential global leaks (which are thankfully impossible in current Coffeescript), and the defeat of compiler optimizations.

If you really don't want to use a temporary variable, you can always cook up something like this instead:

_with = (object, block) -> block.call object

_with document, ->
    img = @createElement 'img'
    img.src = url
    @body.appendChild img

[danielribeiro]

We have several dsls in javascript, and some of them are incredibly more readable in with statement. For instance:

    with(dsl()) {
        div('class', 'menu_list',
            ul(li(alink('/user_config', 'User Configuration')),
                li(alink('/create_user', 'create user')),
                li(alink('/logout')),
                li(alink('/login'))))

        }

Instead of your old fashioned overly verbose, full of noise:

• User Configuration


• create user


• logout


• login

This example is from a port of Ruby's Erector to JS, which we are building, and plan to make open source shortly. We were even considering to switch to coffescript, but the lack of instance_eval and with statement in the language make it a deal breaker.

[satyr]

`with(dsl()){`
...
`}`

[danielribeiro]

It is an ok workaround. We'll keep coffesscript on our radar. We'll see if we can change the current nested function style to a Method chaining one.

It is important to note that nested function from inherited methods in javascript is quite horrible, as it is in python, because of the obligatory this/self keyword (respectively), which the with keyword removes in js. This is a point where javascript would be actually worse than java in dsls expressiveness, if weren't for 'with(this)'. And I am not even comparing with real pro-dsl languages, such as Scala and Ruby.

[weepy]

you might want to check out Kaffeine - it's got a similar goal to CoffeeScript, but it's closer to Javascript and doesn't disallow any syntax. Additionally it's got a using plugin that will bring in an objects to the local scope - something that might be useful for your DSLs'. http://github.com/weepy/kaffeine

[danielribeiro]

Very nice to hear. Definitely considering it.

[carlsmith]

Sorry to reopen such an old thread, but I wanted to propose something slightly different.

It would make sense in CoffeeScript specifically to support a with <expression> then <block> grammar that actually just wraps the block in a function and binds the expression to this:

with Math
  result = @floor @max x, y

Binding the value of the expression to this avoids the ambiguity contra that got the JavaScript version of with banned in Strict Mode.

This fix wouldn't make much sense for JavaScript (Math.floor becomes this.floor), but in CoffeeScript, it would work really well:

with canvas.getContext "2d"
  @fillStyle = "rgb(200, 0, 0)"
  @fillRect 10, 10, 50, 50
  @fillStyle = "rgba(200, 0, 200, 0.5)"
  @fillRect 30, 30, 50, 50

[GeoffreyBooth]

@carlsmith Can you add what the intended JavaScript output would be?

[carlsmith]

Hey, @GeoffreyBooth. Sure.

Assuming foo is assigned to {bar: true}, then this CoffeeScript...

with foo then console.log @bar

...would compile to something like...

!function() {
  console.log(this.bar);
}.bind(foo)();

... and would log true to the console.

[vendethiel]

Isn't do (o = foo) -> console.log o.bar good enough?

[mrmowgli]

I personally think it adds value, and it fulfills an underlying principle of CS, in that it improves a core idea of Ecmascript.

[jashkenas]

We shouldn't re-use with for this (fairly odd) pattern. Writing it this way is just fine:

(->
  @fillStyle = "rgb(200, 0, 0)"
  @fillRect 10, 10, 50, 50
  @fillStyle = "rgba(200, 0, 200, 0.5)"
  @fillRect 30, 30, 50, 50
).call canvas.getContext "2d"

[carlsmith]

Isn't do (o = foo) -> console.log o.bar good enough?

Not really. That code just assigns the longer name to a shorter name that is automatically cleaned up at the end of the block, which wouldn't be applicable to the original example, as you would just use a shorter name to begin with. It's examples like this that I think with would help with:

const context = canvas.getContext("2d");
context.fillStyle = "rgb(200, 0, 0)";
context.fillRect(10, 10, 50, 50);
context.fillStyle = "rgba(200, 0, 200, 0.5)";
context.fillRect(30, 30, 50, 50);

Most developers would use an abbreviation, so would probably do this:

const ctx = canvas.getContext("2d");
ctx.fillStyle = "rgb(200, 0, 0)";
ctx.fillRect(10, 10, 50, 50);
...

In these cases, there's no way to apply your suggestion, except to just use an even shorter abbreviation:

const c = canvas.getContext("2d");
c.fillStyle = "rgb(200, 0, 0)";
c.fillRect(10, 10, 50, 50);
...

It would be nice to get rid of the temporary variable altogether:

with canvas.getContext "2d"
  @fillStyle = "rgb(200, 0, 0)"
  @fillRect 10, 10, 50, 50
...

[carlsmith]

@jashkenas' example would work the same way, so yeah, I'd like to use with to sugar this (though it seems he doesn't):

(->
  @fillStyle = "rgb(200, 0, 0)"
  @fillRect 10, 10, 50, 50
  @fillStyle = "rgba(200, 0, 200, 0.5)"
  @fillRect 30, 30, 50, 50
).call canvas.getContext "2d"

I don't think the pattern is "odd". It's really just doing what with did in JavaScript, but using a feature unique to CoffeeScript to fix the problem with the original approach.