Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.14)
// Compass (v1.0.3)
// ----

$layers: (
  forefront: 9001,
  modal: (
    base:    9000,
    header:  801,
    main:    101,
    footer:  201
  ),
  header:    801,
  main:      1,
  footer:    101,
  back:      -1
);

@function map-deep-get($map, $keys...) {
  $value: $map;
  @each $key in $keys {
    $value: map-get($value, $key);
  }

  @return $value;
}

@function layer-get($keys...) {
  @if not variable-exists(layers) {
    @error "`$layers` is not defined.";
  }

  @if not function-exists(map-deep-get) {
    @error "`map-deep-get` function is not available.";
  }
  
  @return map-deep-get($layers, $keys...);
}

.modal {
  position: relative;
  z-index: layer-get("modal", "base");
}

SassMeister-output.css

.modal {
  position: relative;
  z-index: 9000;
}