Add nil literal

[petebrowne]

Here's a common use use case for a nil literal:

@mixin block-colors($background-color, $border-color: nil) {
  $border-color: darken($background-color, 10%) !default;

  background-color: $background-color;
  border-color: $border-color;
}

This is semantically different than using false in conjunction with an @if directive. Plus, guarded assignment can be used to set a dynamic default value.

[nex3]

I think this is a good thing to add, but I think "null" is a better name for this. It's used by a wider variety of languages, including in particular Javascript.

I'll add more comments on the patch.

[petebrowne]

I agree with the name change - I'll work on incorporating the changes you're suggesting. Thanks

[Anahkiasen]

I agree with the idea that null == 0 (but null !== 0), seems simpler and more logical to me.

[petebrowne]

What do you think @nex3 @chriseppstein? Should 1 + null return 1 or raise a SyntaxError?

[chriseppstein]

Syntax error. We can relax it later if we feel it's wrong, but changing to syntax error later is very hard.

[nex3]

I'm not at all fond of Javascript's aggressive type coercion. 1 + null should be an error, and null != 0.

[petebrowne]

I've opened a separate pull request, #374, with the suggested changes.