Sharing a great tool for generating media query based grid systems that I built in SASS. Feedback appreciated.

///////////////
// Variables //
///////////////
//map
$sizes__map : (
    small: 1px,
    medium: 768px,
    large: 960px,
    largest: 1080px,
    xlargest: 1240px
);

//desired ouput
$grid__includes : true; // true/false
$grid__classes : true; // true/false

//grid config
$grid__column-count : 12;
$grid__gutter-width : 10px;
$grid__column-width : 70px;


///////////////
// Functions //
///////////////
// Finds the next value in the give map, from the current position
@function nextKey($current-value, $mapped-list:$sizes__map) {
    // get current key from list
    $the-list: map-keys($mapped-list);

    //find index of current value and add 1
    $the-index: ( index( $the-list, $current-value) ) + 1;

    // get value from list with new index
    $new-value : nth($the-list, $the-index);

    @return $new-value;

}

// Generates context that can be converted to a percent, later on
@function gridContext($g-col-count, $g-col-w, $g-gutter-w) {
    $g-context: ( $g-col-count * $g-col-w ) + ( $g-col-count * ( $g-gutter-w - 1 ) );
    @return $g-context;
}


///////////////////////
// The Mammoth Mixin //
///////////////////////
@mixin media($breakpoint) {
    // turn the 
    $mapped__keys : map-keys($sizes__map);
    $index : index($mapped__keys, $breakpoint);
    @if $index {
        @if $index < length($sizes__map) {
            @media screen and (min-width : map-get($sizes__map, $breakpoint ) ) and (max-width: map-get($sizes__map, nextKey($breakpoint) )  - 1 ) {
                body {
                    height: $index;
                }
                @content;
            }
        }
        @elseif $index == length($sizes__map) {
            @media screen and (min-width : map-get($sizes__map, $breakpoint) ) {
                body {
                    width: $index;
                }
                @content;
            }
        }
    }
    @else {
        @error "Sorry but #{$breakpoint} is not a breakpoint in your setup";
    }
}

///////////////////
// The Grid Loop //
///////////////////

// Non-media based grid
@for $i from 1 through $grid__column-count {
    $context: gridContext($grid__column-count, $grid__column-width, $grid__gutter-width);
    $target: ( $i * $grid__column-width ) + ( $i * ( $grid__gutter-width - 1 ) );
    $set-the-width : percentage($target/$context);
    @if $grid__includes == true {
        %grid__col--#{$i} {
            width: $set-the-width;
        }
    }

    @if $grid__classes == true {
        .grid__col--#{$i} {
            width: percentage($target/$context);
        }
    }
}

// new loop for media grid system
@each $key, $value in $sizes__map {
    @include media($key) {
        @for $i from 1 through $grid__column-count {
            $context: gridContext($grid__column-count, $grid__column-width, $grid__gutter-width);
            $target: ( $i * $grid__column-width ) + ( $i * ( $grid__gutter-width - 1 ) );
            $set-the-width : percentage($target/$context);
            @if $grid__includes == true {
                %#{$key}__col--#{$i} {
                    width: $set-the-width;
                }
            }

            @if $grid__classes == true {
                .#{$key}__col--#{$i} {
                    width: percentage($target/$context);
                }
            }
        }
        .#{$key}__alpha {
            float: left;
            margin-left: 0;
        }

        .#{$key}__omega {
            float: right;
            margin-left: 0;
            margin-right: 0;
        }
    }
}