jade/support/stylus/docs/conditionals.md

Jade template engine

## Conditionals

 Conditionals provide control flow to a language which is otherwise static, providing conditional imports, mixins, functions, and more. The examples below are simply examples, and not recommended :)

### if / else if / else

 The `if` conditional works as you would expect, simply accepting an expression, evaluating the following block when `true`. Along with `if` are the typical `else if` and `else` tokens, acting as fallbacks.
 
 The example below would conditionally overload the `padding` property, swapping it for margin.

    overload-padding = true

    if overload-padding
      padding(y, x)
        margin y x

    body
      padding 5px 10px

Another example:

    box(x, y, margin = false)
      padding y x
      if margin
        margin y x

    body
      box(5px, 10px, true)

Another `box()` helper:

    box(x, y, margin-only = false)
      if margin-only
        margin y x
      else
        padding y x

### unless

 For users familiar with the ruby programming language, we have the `unless` conditional, which is essentially the opposite of `if`, essentially `if (!(expr))`.

In the example below, if `disable-padding-override` is undefined or `false` padding will be overridden, displaying `margin` instead. However when `true` padding will remain outputting `padding 5px 10px` as expected.

     disable-padding-override = true

     unless disable-padding-override is defined and disable-padding-override
       padding(x, y)
         margin y x

     body
       padding 5px 10px

### Postfix Conditionals

  Stylus supports postfix conditionals, meaning the `if` and `unless` act as operators, evaluating the left-hand operand, when the right-hand expression is truthy.
  
  
  For example let's define `negative()`, performing some basic type checking. Below we use block-style conditionals:
  
      negative(n)
        unless n is a 'unit'
          error('invalid number')
        if n < 0
          yes
        else
          no

  Next we utilize postfix conditionals to keep our function terse.

      negative(n)
        error('invalid number') unless n is a 'unit'
        return yes if n < 0
        no

  Of course we could take this further, and utilize `n < 0 ? yes : no`, or simply stick with booleans, and use only `n < 0`.

  Postfix conditionals may be applied to most single-line statements, for example `@import`, `@charset`, mixins, and of course properties as shown below:
  
  
      pad(types = margin padding, n = 5px)
        padding unit(n, px) if padding in types
        margin unit(n, px) if margin in types

      body
        pad()

      body
        pad(margin)

      body
        apply-mixins = true
        pad(padding, 10) if apply-mixins

yielding:

      body {
        padding: 5px;
        margin: 5px;
      }
      body {
        margin: 5px;
      }
      body {
        padding: 10px;
      }