breakpoint.sass

@mixin breakpoint( $start, $stop: null ) {
	$breakpoints: (
		'small':     480px,
		'medium':    720px,
		'large':     960px,
		'huge':      1200px,
		'container': $container,
	);

	$start: validate-point( $start, $breakpoints );

	@if $stop {
		$stop: validate-point( $stop, $breakpoints );

		@media ( min-width: $start ) and ( max-width: $stop ) {
			@content;
		}
	} @else {
		@media ( min-width: $start ) {
			@content;
		}
	}
}

validate-point.sass

@function validate-point( $point, $breakpoints ) {
	@if map-get( $breakpoints, $point ) {
		$point: map-get( $breakpoints, $point );
	} @elseif 'number' == type-of( $point ) {
		@if unitless( $point ) {
			@error "Using custom breakpoints requires units (i.e. px), was #{$point}.";
		}
	} @else {
		@error "Neither a set or custom breakpoint was provided, was #{$point}.";
	}

	@return $point;
}