Sass responsive grid implimentation

grid.scss

$grid-max: 12;
$grid-column: 80px;
$grid-gutter: 0px;
 
@function gridw ($span: $grid-max, $offset: 0px) { 
    @return ($grid-column * $span) + ($grid-gutter * ($span - 1)) + $offset; 
}
 
@mixin grid ($span: $grid-max, $offset: 0px) { 
    width: gridw($span, $offset); 
}
 
@mixin grid-respond ($min: -1, $max: -1, $media: "screen") {    
    $grid: null;
    @if $min != -1 and $max != -1 {
        $grid: "only " + $media + " and (min-width: " + gridw($min) + ") and (max-width: " + (gridw($max) - 1) + ")";
    } @else if $min == -1 and $max != -1 {
        $grid: "only " + $media + " and (max-width: " + (gridw($max) - 1) + ")";
    } @else if $min != -1 and $max == -1 {
        $grid: "only " + $media + " and (min-width: " + gridw($min) + ")";
    } @else {
        $grid: "only " + $media;
    }

    @media #{$grid} { 
        @content;
    };
}

usage:

@include grid-respond(0, 6) { 
    body { background: blue; }
}

@include grid-respond(6, 9) { 
    body { background: red; }
}

@include grid-respond(9, 12) {
    body { background: pink; }
}

@include grid-respond(12) {
    body { background: green; }
}