Unable to append variable-length parameters after optional parameter

[davidkpiano]

A function that is called with an optional parameter followed by a variable-length parameter throws the error "Optional parameters may not be combined with variable-length parameters" despite being valid in Ruby Sass.

@function foo($value, $default: 13, $args...) {
  $res: $value + $default;
  @if length($args) != 0 {
    $res: $res + nth($args, 1);
  }
  @return $res;
}

.test {
  value: foo(3); // expected: 16
  value: foo(3, 4); // expected: 7
  value: foo(3, 4, 5, 6); // expected: 12
}

// RUBY SASS
.test {
  value: 16;
  value: 7;
  value: 12;
}

// LIBSASS
// Throws "optional parameters may not be combined with variable-length parameters"

[mgreter]

I guess this has changed in ruby sass without us noticing. I just had to remove the existing check and everything seems to work correctly (#981). I took the liberty to adjust the test case from @davidkpiano above and I hope someone would volunteer to create a PR on sass-specs for this, thanks!

[xzyfer]

We need to be careful here. Optional and variable-length parameters can be mixed, but only in specific orders i.e. variable-parameters can follow optional parameters but not the other way around.

[ncoden]

variable-parameters can follow optional parameters but not the other way around.

@xzyfer Why not ? I understand that when passed inline we cannot split optional parameters from variable-parameters, but it would work well with named parameters, and would be really useful to provide additional "second-hand" options to a function.

For example:

@function please-call-this-for-me($func, $args..., $warn: true) {
    @if type-of($func) == 'function' and function-exists($func) {
        @return call($func, $args...);
    }
    @else if $warn == true {
        @warn 'Function #{$func} does not exists';
    }
}

// Call "foo" with (bar, baz, warn)
$_: please-call-this-for-me(foo, bar, baz, warn);

// Call "foo" with (bar, baz)
$_: please-call-this-for-me(foo, bar, baz, $warn: false);

That's a stupid example I know, but we can see that the "main job" of please-call-this-for-me is to call a function with arguments (this is why using variable-parameters seems legit to me, instead of a list) and that we need beside this "main job" to customize it with optional arguments that will not be used often but that we still need.

What do you think ?