$breakpoints: (
  m: 768px,
  l: 1240px,
  lg: 1800px
);

@mixin mq($breakpoint) {
  @if map-has-key($breakpoints, $breakpoint) {
    $breakpoint-value: map-get($breakpoints, $breakpoint);

    @media (max-width: ($breakpoint-value - 1)) {
      @content;
    }
  }
}