spacer.scss

@function str-replace($string, $search, $replace: '') {
  $index: str-index($string, $search);

  @if $index {
    @return str-slice($string, 1, $index - 1) + $replace + str-replace(str-slice($string, $index + str-length($search)), $search, $replace);
  }
  @return $string;
}


@mixin spacing($size, $unit) {
  $margin:  (type: 'm', prop: 'margin');
  $padding: (type: 'p', prop: 'padding');

  @each $item in $margin, $padding {
    $type: map-get($item, type);
    $prop: map-get($item, prop);

    $size_using_d_as_dot: str-replace("#{$size}", '.', 'd');

    .#{$type}r#{$size_using_d_as_dot}#{$unit} { #{$prop}-right:  #{$size}#{$unit} ;}
    .#{$type}b#{$size_using_d_as_dot}#{$unit} { #{$prop}-bottom: #{$size}#{$unit} ;}
    .#{$type}t#{$size_using_d_as_dot}#{$unit} { #{$prop}-top:    #{$size}#{$unit} ;}
    .#{$type}l#{$size_using_d_as_dot}#{$unit} { #{$prop}-left:   #{$size}#{$unit} ;}
  }
}


@for $i from 1 through 100 {
  $number: $i / 10;
  @include spacing($number, 'rem')
}

See the Pen oocss spacer by ononmktw (@ononmktw) on CodePen.

<script async src="//assets.codepen.io/assets/embed/ei.js"></script>

spacing部分は http://terkel.jp/archives/2011/03/oocss-spacing-classes-using-sass/
str-replacer部分はhttps://css-tricks.com/snippets/sass/str-replace-function/
をほぼそのまま使わせていただきました。ありがとうございました。