Using detached rulesets as lambda delegates without scoping issues; in reply to: https://gist.github.com/seven-phases-max/5280eacdf3d591f35163

foreach.less

// @mixin
// Loops over all items in the specified list and for each item executes
// a special `.\\` mixin declared by the specified ruleset. The ruleset
// and mixin combination is used to emulate a lambda function delegate.
// @param {List} list
// The list over which to loop.
// @param {Ruleset} lambda
// A ruleset that may define the `.\\` mixin that is used to process the
// individual list items. The mixin should correspond to the
// following signature:
//   `.\\(@item, @index, @list) { }`
// where `@item` is the current list item; `@index` is the current
// list item's index and `@list` is the original list.
.foreach( @list, @lambda ) {
  // Enclose all nested variables and mixins and do not allow them to
  // leak back up to parent scope. This prevents problems with nested
  // calls to the mixin.
  & {
    @n : length(@list);

    // A catch-all declaration which ensures that the `.\\` mixin will
    // always have a compatible signature available to be called, also
    // when the `@lambda` ruleset didn't emit one.
    .\\(@item, @index, @list ){}

    .iterate(0);
    .iterate(@index) when (@index < @n) {
      @item  : extract(@list, @index + 1);

      // Comment items are skipped over.
      // See the `.is-comment` declaration below.
      .is-comment(@item);

      // Need an isolated scope again for each individual item.
      // A sequence of mixin calls would leak scope into another
      // again.
      & when not (@is-comment) {
        @lambda();

        // Call the special `.\\` mixin. The previously defined
        // catch-all declaration ensures that this always succeeds.
        .\\(@item, @index, @list );
      }

      .iterate(@index + 1);
    }

    // Works around a problem where comments are erroneously seen as
    // members of list variables in older versions of the LESS
    // compiler.
    .is-comment(@item) {
      @n : length(@item);

      ._();
      ._() when (@n > 1), (isstring(@item)) { @is-comment : false; }
      ._() when (default()) {
        @single : replace( ~"@{item}", "\/\/.*", "");
        @multi  : replace( ~"@{item}", "\/\*.*", "");

        .__();
        .__() when (@single = ~""), (@multi = ~"") { @is-comment : true; }
        .__() when (default()) { @is-comment : false; }
      }
    }
  }
}

usage.less

@import "foreach";

.mixin(@args...) {
  .foreach(@args, { .\\(@arg, @index, @list) {
    box-shadow+: @arg black;
  }});
}