three-column-result.css

body.standard #footer,
body.standard #header,
body.standard #sidebar,
body.standard #content,
body.standard #ads {
  float: left;
  margin-left: 10px;
  margin-right: 10px;
}

body.standard #footer,
body.standard #header,
body.standard #sidebar {
  margin-left: 0;
}

body.standard #footer,
body.standard #header,
body.standard #ads {
  margin-right: 0;
}

body.standard #footer,
body.standard #header {
  width: 960px;
}

body.standard #sidebar {
  width: 190px
}

body.standard #content {
  width: 470px
}

body.standard #ads {
  width: 270px
}

three-column.sass

$gutter-width: 10px
$column-width: 40px

=column-base
  float: left
  margin:
    left: $gutter-width
    right: $gutter-width

=last-column
  margin-right: 0

=first-column
  margin-right: 0

=column($columns, $first: false, $last: false)
  @extend column-base
  @if $first
    @extend first-column
  @if $last
    @extend last-column
  width: $column-width * $columns - $gutter-width

=three-column-layout($left-cols, $middle-cols, $right-cols)
  =full-width
    +column($left-cols + $middle-cols + $right-cols, $first: true, $last: true)
  =header
    @extend full-width
  =footer
    @extend full-width
  =left-column
    +column($left-cols, $first: true)
  =middle-column
    +column($middle-cols)
  =right-column
    +column($right-cols, $last: true)
  @yield

body.standard
  +three-column-layout(5,12,7)
    #footer
      +footer
    #header
      +header
    #sidebar
      +left-column
    #content
      +middle-column
    #ads
      +right-column

I like it. The only thing I don’t like about extending mixins is the name “mixin”. Maybe “block” would be better. For the yield thing I prefer @content.

Maybe we could auto-@yield if there’s no @yield but a closure.

I think this could be possible (even this example won’t make useful code):

=columns($count)
  @for $i from 1 through $count
    .column-#{$i}
      width: $i * 10
      @yield

.table
  +columns(7)
    height: 10px

What about scopes?

=color-steps($color-start, $color-end, $steps)
  @for $step from 1 through $steps
    $color = mix-colors($color-end, $color-start, 100% / $step * $steps)
    @yield
    // or: @yield $step, $color

+color-steps(red, green, 10)
  .column-#{$step}
    background: lighten($color, 50%)
    border: 1px $color solid

Some interesting ideas and observations there, @hagenburger. Yes, if you called @yield (or whatever it is called) more than once you'd end up placing the contents more than once -- and while I can't think of a good use for that at this instant, I'm sure I will ;)

I'm not sure about auto-yielding -- at first I really liked it, but it would preclude the ability to create a mixin that performs conditional inclusion.

The idea of letting the mixin pass variables to the child block, is interesting too -- I think I prefer the explicit approach.

It had occurred to me that using mixins as a scoping/iteration mechanism like you suggest is a very interesting way of creating localized behaviors within a stylesheet. I've wanted such a capability for compass on many occasions.

Nathan and I discussed this last night... I don't think we'll see these features anytime soon.