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

@for $i from 0 to 5 {
    @each $var, $value in $positions {
        @each $prop, $propValue in (m: 'margin', p: 'padding') {
            .#{$prop}#{$var}-#{$i} {
                @each $pos in $value {
                    #{$propValue}-#{$pos}: ($i * .5) + 'em';
                }
            }
        }
    }
}