Handoff sets of Sass variables to mixin's content block

_handoff.scss

// handoff mixin variables for use inside content
$batons: () !default;

/// get the last set of variables handed-off
/// @param $baton=() map of variables
/// @example calling from inside a handoff-enabled mixin with content block
/// @mixin foo($foo, $free) {
///   
///   // send handoff right before `@content`
///   @include handoff((
///     foo: bar,
///     free: beer
///   ));
///   @content
/// }
/// 
/// @include foo() {
///   // get handoff first thing
///   $handoff: handoff();
///   
///   foo: map-get($handoff, foo);
///   free: map-get($handoff, beer);
/// }
@function handoff($baton) {
  // pop the last set of variables and return and return
  @if($baton == null) {
    $batons-except-last: ();

    @for $i from 1 through length($batons) {
      @if $i == length($handoff) {
        $last: nth($handoff, length($handoff));
      }
      @else {
        $batons-except-last: append($batons-except-last, nth($batons, $i));
      }
    }

    $batons: $batons-except-last !global;

    @return $last;
  }

  // add to relay list
  @if type-of($baton) == map {
    $batons: append($batons, $baton) !global; 
  }
  @else {
    @error 'handoff parameter must be a map';
  }
}

@mixin handoff($baton) {
  @if type-of($baton) != map {
    @error 'handoff parameter must be a map';
  }

  $_: handoff($baton);
}