Minimalistic core. Supports mixins: +m(<list of modifiers>) and +e(<list of elements>) with nest-in-state support.

npm i -S bem-sass-mixins

Webpack

@import '~bem-sass-mixins/bem-sass-mixins' (Sass 4.0 will give us improved resolving)

Gulp

See gulp-sass options. You'll need includePaths=['path/to/node_modules/bem-sass-mixins'] option to resolve @import 'bem-sass-mixins' (from node_modules/bem-sass-mixins) or includePaths=['path/to/node_modules'] to resolve @import 'bem-sass-mixins/bem-sass-mixins' (from node_modules/). Looks simple enough to configure.

Define preferrable separators in your variables file like $bemSeparators: (element: '-', modifier: '--') (these are mine for .MyBlock-myElement--myModifier style) or rely on defaults __ and --.

I came to conclusion that it was ALWAYS better to write blocks as pure classes. Problems happen when composing mixins (like +b(BlockA BlockB)) which represent blocks. We cannot have two block mixins each under another, so we may think about list of blocks passed to mixin. Ok, but what if we also want to specify namespaces, which may also be different per block or even absent for some blocks?

Nevermind... Looks like unnecessary work at all. Just keep in mind, that block mixins are not implemented here.

Look at the example of usage:

.Block
  color: red
  +e(element1 element2)
    color: green
    +m(elementModifier1 elementModifier2)
      color: blue
  +m(blockModifier1 blockModifier2)
    color: red
    +e(element)
      color: green
      +m(elementModifier)
        @extend %TestPlaceholder // Stylus can't do this without ugly prefix :)
        color: blue

Will produce:

.Block {
  color: red;
}
.Block-element1, .Block-element2 {
  color: green;
}
.Block-element1--elementModifier1, .Block-element1--elementModifier2, .Block-element2--elementModifier1, .Block-element2--elementModifier2 {
  color: blue;
}
.Block--blockModifier1, .Block--blockModifier2 {
  color: red;
}
.Block--blockModifier1 .Block-element, .Block--blockModifier2 .Block-element {
  color: green;
}
.Block--blockModifier1 .Block-element--elementModifier, .Block--blockModifier2 .Block-element--elementModifier {
  color: blue;
}

.Block--blockModifier1 .Block-element--elementModifier, .Block--blockModifier2 .Block-element--elementModifier {
  font-weight: bold;
}