@mixin _assert-ascending($map, $map-name) {
  $prev-key: null;
  $prev-num: null;
  @each $key, $num in $map {
    @if $prev-num != null and $prev-num >= $num {
      @warn "Invalid value for #{$map-name}: This map must be in ascending order," +
        "but key '#{$key}' has value #{$num} which isn't greater than #{$prev-num}," +
        "the value of the previous key '#{$prev-key}' !";
    }
    $prev-key: $key;
    $prev-num: $num;
  }
}

// Grid breakpoints
$grid-breakpoints: (
  xs: 0,
  sm: 544px,
  md: 768px,
  lg: 992px,
  xl: 1200px,
) !default;
@include _assert-ascending($grid-breakpoints, '$grid-breakpoints');

// Grid containers
$container-max-widths: (
  sm: 576px,
  md: 720px,
  lg: 940px,
  xl: 1140px,
) !default;
@include _assert-ascending($container-max-widths, '$container-max-widths');

// Grid columns
$grid-columns: 12 !default;
$grid-gutter-width: 30px !default;
