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Mixins

Both mixins and functions are defined in the same manner, but they are applied in different ways.

For example, we have a border-radius(n) function defined below, which is invoked as a mixin (i.e., invoked as a statement, rather than part of an expression).

When border-radius() is invoked within a selector, the properties are expanded and copied into the selector.

border-radius(n)
  -webkit-border-radius n
  -moz-border-radius n
  border-radius n

form input[type=button]
  border-radius(5px)

Compiles to:

form input[type=button] {
  -webkit-border-radius: 5px;
  -moz-border-radius: 5px;
  border-radius: 5px;
}

When using mixins you can omit the parentheses altogether, providing fantastic transparent vendor property support!

border-radius(n)
  -webkit-border-radius n
  -moz-border-radius n
  border-radius n

form input[type=button]
  border-radius 5px

Note that the border-radius within our mixin is treated as a property, and not a recursive function invocation.

To take this further, we can utilize the automatic arguments local variable, containing the expression passed, allowing multiple values to be passed:

border-radius()
  -webkit-border-radius arguments
  -moz-border-radius arguments
  border-radius arguments

Now we can pass values like border-radius 1px 2px / 3px 4px!

Another great use of this is the addition of transparent support for vendor-specifics—such as opacity support for IE:

    support-for-ie ?= true

    opacity(n)
      opacity n
      if support-for-ie
        filter unquote('progid:DXImageTransform.Microsoft.Alpha(Opacity=' + round(n * 100) + ')')

    #logo
      &:hover
        opacity 0.5

Rendering:

    #logo:hover {
      opacity: 0.5;
      filter: progid:DXImageTransform.Microsoft.Alpha(Opacity=50);
    }

Parent References

Mixins may utilize the parent reference character &, acting on the parent instead of further nesting.

For example, let's say we want to create a stripe(even, odd) mixin for striping table rows. We provide both even and odd with default color values, and assign the background-color property on the row. Nested within the tr we use & to reference the tr, providing the even color.

 stripe(even = #fff, odd = #eee)
   tr
     background-color odd
     &.even
     &:nth-child(even)
       background-color even

We may then utilize the mixin as shown below:

 table
   stripe()
   td
     padding 4px 10px

 table#users
   stripe(#303030, #494848)
   td
     color white

Alternatively, stripe() could be defined without parent references:

stripe(even = #fff, odd = #eee)
  tr
    background-color odd
  tr.even
  tr:nth-child(even)
    background-color even

If we wished, we could invoke stripe() as if it were a property:

stripe #fff #000

Mixing Mixins in Mixins

Mixins can (of course!) utilize other mixins, building upon their own selectors and properties.

For example, below we create comma-list() to inline (via inline-list()) and comma-separate an unordered list.

 inline-list()
   li
     display inline

 comma-list()
   inline-list()
   li
     &:after
       content ', '
     &:last-child:after
       content ''

 ul
   comma-list()

Rendering:

ul li:after {
  content: ", ";
}
ul li:last-child:after {
  content: "";
}
ul li {
  display: inline;
}
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