Simulate Sass 3.3 Maps data type when compiling with libsass (v2.0.0), still compiles successfully with Sass 3.3.

List Map

// Parameters:
// ```scss
// $map : Map converted to List
// $property : CSS property
// $loop : Loop through list. Predefined as true
// $map-key: Key for desired value in key/value pair. Predefined as null
// $namespace : Prefix for placeholders. Predefined as null
// ```
// 
// Usage:
// ```scss
// @include loop-map($type, font-weight, type);
//
// .example2 {
//   font-weight: list-map($type, subheading);
// }
// ```
//
// _**Note** : Key/value pairs in Maps are parenthesized and separated by comma to convert them to lists, e.g. $type: ((heading, $bold),(subheading, $normal),(body, $normal),(bold, $bold),);.

$extra-light: 100;
$light: 200;
$demi: 300;
$normal: 400;
$medium: 500;
$semi-bold: 600;
$bold: 700;
$black: 800;
$extra-black: 900;

$type: (
  (heading, $bold),
  (subheading, $normal),
  (body, $normal),
  (bold, $bold),
);

@function list-map($map, $map-key) {
  @each $val in $map {
    $key: nth($val, 1);
    $value: nth($val, 2);
    @if $map-key == $key {
      @return $value;
    }
  }
}

@mixin loop-map($map, $property, $namespace) {
  @each $val in $map {
    $key: nth($val, 1);
    $value: nth($val, 2);
    %#{$namespace}-#{$key} {
      #{$property}: $value;
    }
  }
}

@include loop-map($type, font-weight, type);

.heading {
  @extend %type-heading;
}
.subheading {
  @extend %type-subheading;
}
.body {
  @extend %type-body;
}
.bold {
  @extend %type-bold;
}

.extend-heading {
  @extend %type-heading;
}

.media-query-subheading {
  font-weight: list-map($type, subheading);
}