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SassMeister-input.scss

// ----
// Sass (v3.3.5)
// Compass (v1.0.0.alpha.18)
// ----

// Returns opposite direction
// Quite similar to the one from Compass
// Rely on Sass 3.3 but could easily be adapted to Sass 3.2
// ---
// @param [list] $directions: list of directions
// ---
// @return opposite directions

@function opposite-direction($directions) {
  $opposite-directions: ();
  $direction-map: (
    'top':    'bottom',
    'right':  'left',
    'bottom': 'top',
    'left':   'right',
    'ltr':    'rtl',
    'rtl':    'ltr'
  );
  
  // Loop through each value in $directions
  // 1. Get opposite from map
  // 2. If direction exists, append it to result list
  // 3. Or warn the user
  @each $direction in $directions {
    $opposite-direction: map-get($direction-map, $direction); // 1
    @if $opposite-direction != null { 
      $opposite-directions: append($opposite-directions, #{$opposite-direction}); // 2
    }
    @else {
      @warn "No opposite direction can be found for `#{$direction}`."; // 3
    }
  }
  
  @return if($opposite-directions != (), $opposite-directions, false);
}

// Test
test {
  $directions: 
    left, right, top, bottom, 
    top left, top right, bottom left, bottom right,
    left top, right top, left bottom, right bottom,
    ltr, rtl,
    error;
    
  @each $direction in $directions {
    #{$direction}: opposite-direction($direction);
  }
}

SassMeister-output.css

test {
  left: right;
  right: left;
  top: bottom;
  bottom: top;
  top left: bottom right;
  top right: bottom left;
  bottom left: top right;
  bottom right: top left;
  left top: right bottom;
  right top: left bottom;
  left bottom: right top;
  right bottom: left top;
  ltr: rtl;
  rtl: ltr;
  error: false;
}