Generated by SassMeister.com.

SassMeister-input.sass

// ----
// Sass (v3.4.21)
// Compass (v1.0.3)
// ----

$bemSeparators: (element: '__', modifier: '--', pseudo: ':') !default

// Return nest-in-state element if block has modifier class.
@function _nestInState($parent)
  $parent: str-slice(inspect($parent), 0) // To string.
  $modifierAt: str-index($parent, map-get($bemSeparators, 'modifier'))
  @if $modifierAt != null
    @return #{$parent + ' ' + str-slice($parent, 0, $modifierAt - 1)}
  @else
    $modifierAt: str-index($parent, map-get($bemSeparators, 'pseudo'))
    @if $modifierAt != null
      @return #{$parent + ' ' + str-slice($parent, 0, $modifierAt - 1)}
  @return $parent

// Generate prefixes for block(s) or element(s).
@function bemify($items, $type)
  $output: ()
  @each $parent in &
    @each $item in $items
      @if $type == 'element'
        $parent: _nestInState($parent)
      $output: append($output, #{$parent + map-get($bemSeparators, $type) + $item}, comma)
  @return $output

=e($elements)
  @at-root #{bemify($elements, 'element')}
    @content

=m($modifiers)
  @at-root #{bemify($modifiers, 'modifier')}
    @content
  
=pseudo($pseudos)
  @at-root #{bemify($pseudos, 'pseudo')}
    @content

.red
  background: red
  
.Block
  color: red
  +e(element1)
    color: green
    +m(elementModifier1)
      color: blue
  +m(blockModifier1)
    color: red
    +e(element)
      color: green
      +m(elementModifier)
        @extend .red // Stylus can't do this without ugly prefix :)
        color: blue
  +pseudo(hover)
    color: white
    +e(fancy)
      color: fancy
        

SassMeister-output.css

.red, .Block--blockModifier1 .Block__element--elementModifier {
  background: red;
}

.Block {
  color: red;
}
.Block__element1 {
  color: green;
}
.Block__element1--elementModifier1 {
  color: blue;
}
.Block--blockModifier1 {
  color: red;
}
.Block--blockModifier1 .Block__element {
  color: green;
}
.Block--blockModifier1 .Block__element--elementModifier {
  color: blue;
}
.Block:hover {
  color: white;
}
.Block:hover .Block__fancy {
  color: fancy;
}