Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.1)
// Compass (v1.0.1)
// ----

@mixin child($pos) {
    @if $pos == "first" {
      &:#{$pos}-child {
        float: left;
      }
    } 
    @else if $pos == "last" {
      &:#{$pos}-child {
        float: right;
      }
    }
    @else {
      @error "`#{$pos}` is not a valid pseudo-class.";
    }
}

.foo {
  @include child(last);
}

SassMeister-output.css

.foo:last-child {
  float: right;
}