Generated by SassMeister.com.

SassMeister-input.scss

@mixin side__panel($b: side, $e: panel, $m: alert) {
	$selector: $b;
	@if $e != false {
		$selector: #{$selector}__#{$e};
	}
	@if $m != false {
		$selector: #{$selector}--#{$m};
	}

	.#{$selector} {
		width: 100%;
		@content;
	}
}

@include side__panel(side, panel, alert);


@include side__panel(side, panel, test) {
  border: 1px solid red;
};

@include side__panel(side, false, test) {
  border: 1px solid red;
};

@include side__panel(side, bar, false) {
  border: 1px solid red;
};

SassMeister-output.css

.side__panel--alert {
  width: 100%;
}

.side__panel--test {
  width: 100%;
  border: 1px solid red;
}

.side--test {
  width: 100%;
  border: 1px solid red;
}

.side__bar {
  width: 100%;
  border: 1px solid red;
}