<===> input.scss
$config: (
        phone: (
                break-point-width:0px,
                break-point-name: xs
        ),
        tablet: (
                break-point-width:600px,
                break-point-name: sm
        ),
        laptop: (
                break-point-width:900px,
                break-point-name: md
        ),
        desktop: (
                break-point-width:1200px,
                break-point-name:lg
        ),
);

@each $key, $map in $config {
  $break-point-width: map_get($map, break-point-width);
  $break-point-name: map_get($map, break-point-name);
  $infix: if($break-point-width == 0px, null, -$break-point-name);
      .foo#{$infix} {
        content: '#{$break-point-name}';
      }
}

<===> output.css
.foo {
  content: "xs";
}

.foo-sm {
  content: "sm";
}

.foo-md {
  content: "md";
}

.foo-lg {
  content: "lg";
}