Useful sass functions

// ############################################################################
// core low level utility/helper functions
// ############################################################################

/// @function map-value
/// Returns a value from a nested map.
///
/// @access public
/// @param {Map | Number | String} $map-value - A map or value which is checked recursively.
/// @param {List} $map-value - The list of keys in sequential order.
/// @return {Number | String} - The value of the final key.
///
/// @example
/// map-value($map, (key1, key2));
@function map-value($map-value, $keys, $prev-key: null) {
	@if(type-of($map-value) != map or length($keys) == 0) {
		@if($map-value == null) {
			@error '"#{$prev-key}" not found in map-value keys.';
		}
		@return $map-value;
	}
	$curr-key: nth($keys, 1);
	@return map-value(map-get($map-value, $curr-key), slice($keys, 2), $curr-key);
}

/// @function slice
/// Returns a new list partial from a given list.
///
/// @access public
/// @param {List} $list - The target list from which to derive a partial list.
/// @param {Number} $start - The start index of the slice operation.
/// @return {Number} $end - The end index of the slice operation (Optional, defaults to list length).
///
/// @example
/// slice($list, 1, 2);
@function slice($list, $start: 1, $end: length($list)) {
	$result: ();
	@if $start > $end or $start < 1 or $end < 1 or $start > length($list) or $end > length($list)  {
		@return $result;
	}
	@else {
		@for $i from $start through $end {
			$result: append($result, nth($list, $i));
		}
	}
	@return $result;
}

/// jQuery-style extend function
/// About `map-merge()`:
/// * only takes 2 arguments
/// * is not recursive
/// @param {Map} $object - first map
/// @param {ArgList} $objects - other maps
/// @param {Bool} $deep - recursive mode
/// @return {Map}
@function extend($object, $objects.../*, $deep */) {
  $last: nth($objects, -1);
  $deep: $last == true;
  $max: if($deep, length($objects) - 1, length($objects));

  // Loop through all maps in $objects...
  @for $i from 1 through $max {
    // Store current map
    $current: nth($objects, $i);

    // If not in deep mode, simply merge current map with object
    @if not $deep {
      $object: map-merge($object, $current);
    }

    // If in deep mode
    @else {
      // Loop through all tuples in current map
      @each $key, $value in $current {
        // If value is a nested map and same key from object is a nested map as well
        @if type-of($value) == "map" and type-of(map-get($object, $key)) == "map" {
          // Recursive extend
          $value: extend(map-get($object, $key), $value, true);
        }
        // Merge current tuple with object
        $object: map-merge($object, ($key: $value));
      }
    }
  }
  @return $object;
}

/// @function get-color
/// Retreives colour values from the $s-colours map.
///
/// @access public
/// @param {String} $colour	- A String which represents a colour key in the $s-colours map.
/// @param {String} $tone - A String which represents a tone key of the given colour map.
/// @return {String} - A String representing the required colour.
///
/// @example
/// get-color(black);
@function get-color($color, $tone: base, $colors: $s-colors) {
  @if $color {
    @return map-value($colors, ($color, $tone));  
  }
}