$positions: (
    t: ('top'),
    r: ('right'),
    b: ('bottom'),
    l: ('left'),
    x: ('left', 'right'),
    y: ('top', 'bottom')
);

@mixin sizeValue($prop, $i) {
    #{$prop}: ($i * .5em);
};

@for $i from 0 to 6 {
    @each $prop, $propValue in (m: 'margin', p: 'padding') {
        .#{$prop}-#{$i} {
            @include sizeValue((#{$propValue}), $i);
        }

        @each $var, $value in $positions {
            .#{$prop}#{$var}-#{$i} {
                @each $pos in $value {
                    @include sizeValue((#{$propValue}-#{$pos}), $i);
                }
            }

            @if ($i == 0 and $prop == 'm') {
                .m#{$var}-auto {
                    @each $pos in $value {
                        margin-#{$pos}: auto;
                    }
                }
            }
        }
    }
}