Allow dynamic keys for object (hash) definition

[aurium]

This feature is available in Javascript, but not in Coffee.

opts = { s('param'): 'some value' }
# unexpected (

replacements =
  "#{prefix}keyname1": 'some value 1'
  "#{prefix}keyname2": 'some value 2'
# unexpected string interpolation

[davidchambers]

This feature is available in Javascript, but not in Coffee.

Not true. { s('param'): 'some value' } isn't valid in either language, and opts = {}; opts[s('param')] = 'some value' works in both languages.

[michaelficarra]

Though ES6 will have computed keys. We should be aware of CoffeeScript's coming obsolescence if it doesn't adapt.

[lydell]

ES6 computed keys will be available in Firefox 34: https://developer.mozilla.org/en-US/Firefox/Releases/34#JavaScript.

[akre54]

Yeah it'd be nice to have this. Either with the ES6-style brackets { [key()]: val } or a more CS-style interpolation delimiters { #{key()}: val }

[aurium]

I don't like @Argorate's sugar, because it looks like prototype sugar, and may cause mistakes. I believe @akre54 comes with a perfect coffeeish syntax #{...}, despite the conflict with the coffee comment.

On other rand @Argorate compatible compilation for "old" javascript is the same as i was thinking. Someone see any problem on the translation above?

myObj =
  x: 1
  #{name+i}: 'some value'   # as we can see the comment tokenization
  #{name+j}: 'other value'  # must be updated with this proposal.
  somekey: someValue + myObj.x

compiles to:

myObj = {
  x: 1
};
myObj[name+i] = 'some value';
myObj[name+j] = 'other value';
myObj[somekey] = someValue + myObj.x;

[akre54]

somekey: someValue + myObj.x => myObj[somekey] = someValue + myObj.x;

We can't just break the unquoted syntax unfortunately. That's the reason for the delimiters in the first place.

I'd rather see that input compile to:

myObj = {
  x: 1,
  somekey: someValue + myObj.x // (runtime TypeError)
};
myObj[name + i] = 'some value';
myObj[name + j] = 'other value';

For the record this is Traceur's output.

[carlsmith]

If we're bike-shedding, can I throw in hash comprehensions again? It'd be nice if we could naturally extend the key expression syntax to comprehensions, one day.

[akre54]

That seems like a real slippery slope with some ugly unintended consequences.

Dynamic keys are a much more modest and useful addition.

[carlsmith]

Agreed. I just meant that it'd be worth leaving 'space' in the syntax for comprehensions, for the future, but it shouldn't get in the way of key expressions.

[MatrixFr]

@aurium : yes but in this use case, we will never call "prototype", so I think it's not realy in conflict, but the sugar is not realy important for me, I just hope CS will add one ;)

[lydell]

I’d vote for "a#{b}c": value to compile to {["a" + b + "c"]: value}. Just like yield, this would be a feature that works only in supporting environments; it’s up to the developer not to use it if support for an older environment is needed. No need to a any new syntax.

[michaelficarra]

@lydell: I like that.

[epidemian]

@lydell, but, unlike yield, this is a feature that can be easily implemented with ES3.

I'd vote for using the same syntax and a similar implementation than LiveScript (see "Dynamic Keys"), where:

myObj =
  x: 1
  "#{name}#{i}": 'some value' # Using string interpolation.
  (name + j): 'other value' # Using a parenthesised expression.
  somekey: someValue + myObj.x

Compiles to:

// Generated by LiveScript 1.3.1
var myObj, ref$;
myObj = (ref$ = {
  x: 1
}, ref$[name + "" + i] = 'some value', ref$[name + j] = 'other value', ref$.somekey = someValue + myObj.x, ref$);

(the output probably needs better whitespace handling)

I like the LS syntax because dynamic string literals look like strings, which is the benefit of string interpolation in the first place; and because it also allows using any value as a key (using the parentheses syntax), which in turn would be useful if we want to support Symbol properties at some point:

countdown = 
  (Symbol.iterator): *->
    yield 3
    yield 2
    yield 1

# Assuming this compiles to JS's `for ... of`
for n of countdown
  console.log n

[lydell]

We must not forget #786 and #1731, though.

[lydell]

Good point about Symbol keys. That means that we need to support an alternate syntax in addition to string interpolation. Both old CoffeeScript and @epidemian used (). An argument for this syntax is ease of implementation, as said in https://github.com/jashkenas/coffeescript/issues/786#issuecomment-484874:

"#{key}": val

Lexer converts interpolated strings into parenthesized concatenations, so that's actually the same thing for parser.

Another option is to use ES6-style [] notation. An argument for this is: Less syntax to learn.

Which is better?

[jashkenas]

We don't need to worry about Symbol keys — they're probably going to end up being considered a "nasty" part of JavaScript that CoffeeScript can enthusiastically avoid.

"#{key}": val

Is probably the best syntax. In an alternate universe, something like:

key: val

... might be preferable, but that ship has sailed.

[pepkin88]

I vote for [] syntax. For majority of my use cases it's shorter by 3 characters comparing to "#{}" syntax. Also is compatible with the ES6 syntax, which means it will gain more familiarity. Moreover, I think it is more intuitive, as it's analogous to the property access syntax:

foo = { bar: 123 }
foo.bar

foo = { [bar]: 123 }
foo[bar]

foo = { "#{bar}": 123 }
foo"#{bar}" # not quite, looks like tagged template string, also doesn't work

foo = { ["#{bar}"]: 123 }
foo["#{bar}"]

If string interpolation is needed, it can still be used like this ["#{}"].

Optionally the "#{}" syntax could be implemented in addition, because they don't collide with each other.

[lydell]

Yes, if we're adding new syntax in addition to interpolation it should definitely be []: since:

• No need to deviate from JS syntax.
• The symmetry argument from #3828 can be applied in reverse: Since 'a': b works, c.'a' should work. Since c[a] works, [a]: b should work.
• Because of #3840 the "ease of implementation" argument no longer applies.

[pepkin88]

@lydell Oh I'm sorry, I didn't notice, that the "#{}" syntax is already implemented, I thought that it is still an open debate. But still, I think the [] syntax for computed properties would be a nice addition, for reasons you stated :)

[jashkenas]

Let's not add new syntax for dynamic keys at the moment. If we end up needing it, we can add it later.