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permalink: docs/bifs.html
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# Built-in Functions

## Color Functions

### red(color[, value])

Return the red component of the given `color`, or set the red component to the optional second `value` argument.

    red(#c00)
    // => 204

    red(#000, 255)
    // => #f00

### green(color[, value])

Return the green component of the given `color`, or set the green component to the optional second `value` argument.

    green(#0c0)
    // => 204

    green(#000, 255)
    // => #0f0

### blue(color[, value])

Return the blue component of the given `color`, or set the blue component to the optional second `value` argument.

    blue(#00c)
    // => 204

    blue(#000, 255)
    // => #00f

### alpha(color[, value])

Return the alpha component of the given `color`, or set the alpha component to the optional second `value` argument.

    alpha(#fff)
    // => 1

    alpha(rgba(0,0,0,0.3))
    // => 0.3

    alpha(#fff, 0.5)
    // => rgba(255,255,255,0.5)

### dark(color)

Check if `color` is dark:

    dark(black)
    // => true

    dark(#005716)
    // => true

    dark(white)
    // => false


### light(color)

Check if `color` is light:

    light(black)
    // => false

    light(white)
    // => true

    light(#00FF40)
    // => true

### hue(color[, value])

Return the hue of the given `color`, or set the hue component to the optional second `value` argument.

    hue(hsl(50deg, 100%, 80%))
    // => 50deg

    hue(#00c, 90deg)
    // => #6c0

### saturation(color[, value])

Return the saturation of the given `color`, or set the saturation component to the optional second `value` argument.

    saturation(hsl(50deg, 100%, 80%))
    // => 100%

    saturation(#00c, 50%)
    // => #339

### lightness(color[, value])

Return the lightness of the given `color`, or set the lightness component to the optional second `value` argument.

    lightness(hsl(50deg, 100%, 80%))
    // => 80%

    lightness(#00c, 80%)
    // => #99f

### hsla(color | h,s,l,a)

Convert the given `color` to an `HSLA` node, or h,s,l,a component values.

    hsla(10deg, 50%, 30%, 0.5)
    // => HSLA

    hsla(#ffcc00)
    // => HSLA

### hsl(color | h,s,l)

Convert the given `color` to an `HSLA` node, or h,s,l,a component values.

    hsl(10, 50, 30)
    // => HSLA

    hsl(#ffcc00)
    // => HSLA

### rgba(color | r,g,b,a)

Return `RGBA` from the r,g,b,a channels, or provide a `color` to tweak the alpha.

    rgba(255,0,0,0.5)
    // => rgba(255,0,0,0.5)

    rgba(255,0,0,1)
    // => #ff0000

    rgba(#ffcc00, 0.5)
    // rgba(255,204,0,0.5)

Alternatively, Stylus supports the `#rgba` and `#rrggbbaa` notations as well:

    #fc08
    // => rgba(255,204,0,0.5)

    #ffcc00ee
    // => rgba(255,204,0,0.9)

### rgb(color | r,g,b)

Return a `RGBA` from the r,g,b channels or cast to an `RGBA` node.

    rgb(255,204,0)
    // => #ffcc00

    rgb(#fff)
    // => #fff

### blend(top[, bottom])

Blends the given `top` color over the `bottom` one using the normal blending. The `bottom` argument is optional, and defaults to `#fff`.

    blend(rgba(#FFF, 0.5), #000)
    // => #808080

    blend(rgba(#FFDE00,.42), #19C261)
    // => #7ace38

    blend(rgba(lime, 0.5), rgba(red, 0.25))
    // => rgba(128,128,0,0.625)

### lighten(color, amount)

Lighten the given `color` by `amount`. 

This function is unit-sensitive; it supports percentages, for example, as shown here:

    lighten(#2c2c2c, 30)
    // => #787878

    lighten(#2c2c2c, 30%)
    // => #393939

### darken(color, amount)

Darken the given `color` by `amount`.

This function is unit-sensitive; it supports percentages, for example, as shown here:

    darken(#D62828, 30)
    // => #551010

    darken(#D62828, 30%)
    // => #961c1c

### desaturate(color, amount)

Desaturate the given `color` by `amount`.

    desaturate(#f00, 40%)
    // => #c33

### saturate(color, amount)

Saturate the given `color` by `amount`.

    saturate(#c33, 40%)
    // => #f00

### complement(color)

Gives the complementary color. Equivalent to spinning hue by `180deg`.

    complement(#fd0cc7)
    // => #0cfd42

### invert(color)

Inverts the color. The red, green, and blue values are inverted. Opacity is left alone.

    invert(#d62828)
    // => #29d7d7

### spin(color, amount)

Spins hue of the given `color` by `amount`.

    spin(#ff0000, 90deg)
    // => #80ff00

### grayscale(color)

Gives the grayscale equivalent of the given color. Equivalent to `desaturate(color, 100%)`.

    grayscale(#fd0cc7)
    // => #0cfd42

### mix(color1, color2[, amount])

Mix two colors by a given amount. The `amount` is optional and is defaulted to `50%`.

    mix(#000, #fff, 30%)
    // => #b2b2b2

### tint(color, amount)

Mix the given color with white.

    tint(#fd0cc7,66%)
    // => #feaceb

### shade(color, amount)

Mix the given color with black.

    shade(#fd0cc7,66%)
    // => #560443

### luminosity(color)

Returns the [relative luminance](http://www.w3.org/TR/WCAG20/#relativeluminancedef) of the given `color`.

    luminosity(white)
    // => 1

    luminosity(#000)
    // => 0

    luminosity(red)
    // => 0.2126

### contrast(top[, bottom])

Returns the [contrast ratio](http://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef) object between `top` and `bottom` colors, based on [script](https://github.com/LeaVerou/contrast-ratio/blob/gh-pages/color.js#L108) underlying “[contrast ratio](http://leaverou.github.io/contrast-ratio/)” tool by Lea Verou.

The second argument is optional and is defaulted to `#fff`.

The main key in the returned object is `ratio`, it also have `min` and `max` values that are different from the `ratio` only when the `bottom` color is transparent. In that case the `error` also contains an error margin.

    contrast(#000, #fff).ratio
    => 21
    contrast(#000, rgba(#FFF, 0.5))
    => { "ratio": "13.15;", "error": "-7.85", "min": "5.3", "max": "21" }

### transparentify(top[, bottom, alpha])

Returns the transparent version of the given `top` color, as if it was blend over the given `bottom` color (or the closest to it, if it is possible).

The second argument is optional and is defaulted to `#fff`.

The third argument is optional and overrides the autodetected alpha.

    transparentify(#808080)
    => rgba(0,0,0,0.5)

    transparentify(#414141, #000)
    => rgba(255,255,255,0.25)

    transparentify(#91974C, #F34949, 0.5)
    => rgba(47,229,79,0.5)


## Path Functions

### basename(path[, ext])

Returns the basename of `path`, (optionally) with `ext` extension removed.

    basename('images/foo.png')
    // => "foo.png"

    basename('images/foo.png', '.png')
    // => "foo"

### dirname(path)

Returns the dirname of `path`.

    dirname('images/foo.png')
    // => "images"

### extname(path)

Returns the filename extension of `path` including the dot.

    extname('images/foo.png')
    // => ".png"


### pathjoin(...)

Peform a path join.

    pathjoin('images', 'foo.png')
    // => "images/foo.png"

    path = 'images/foo.png'
    ext = extname(path)
    pathjoin(dirname(path), basename(path, ext) + _thumb + ext)
    // => 'images/foo_thumb.png'


## List / Hash Functions

### push(expr, args...)

Push the given `args` to `expr`.

    nums = 1 2
    push(nums, 3, 4, 5)

    nums
    // => 1 2 3 4 5

Aliased as `append()`.

### pop(expr)

Pop a value from `expr`.

    nums = 4 5 3 2 1
    num = pop(nums)

    nums
    // => 4 5 3 2
    num
    // => 1

### shift(expr)

Shift an element from `expr`.

    nums = 4 5 3 2 1
    num = shift(nums)

    nums
    // => 5 3 2 1
    num
    // => 4

### unshift(expr, args...)

Unshift the given `args` to `expr`.

    nums = 4 5
    unshift(nums, 3, 2, 1)

    nums
    // => 1 2 3 4 5

Aliased as `prepend()`.

### index(list, value)

Returns the (zero-based) index of `value` within a `list`.

    list = 1 2 3

    index(list, 2)
    // => 1

    index(1px solid red, red)
    // => 2

### keys(pairs)

Return keys in the given `pairs`:

    pairs = (one 1) (two 2) (three 3)
    keys(pairs)
    // => one two three

### values(pairs)

Return values in the given `pairs`:

    pairs = (one 1) (two 2) (three 3)
    values(pairs)
    // => 1 2 3

### list-separator(list)

Return the separator of the given `list`.

    list1 = a b c
    list-separator(list1)
    // => ' '

    list2 = a, b, c
    list-separator(list2)
    // => ','

### length([expr])

Parenthesized expressions may act as tuples, the `length()` function returns the length of such expressions.

    length((1 2 3 4))
    // => 4

    length((1 2))
    // => 2

    length((1))
    // => 1

    length(())
    // => 0

    length(1 2 3)
    // => 3

    length(1)
    // => 1

    length()
    // => 0

### last(expr)

Return the _last_ value in the given `expr`:

    nums = 1 2 3
    last(nums)
    last(1 2 3)
    // => 3

    list = (one 1) (two 2) (three 3)
    last(list)
    // => (three 3)



## Unit Functions

### typeof(node)

Return type of `node` as a string.

    type(12)
    // => 'unit'

    typeof(12)
    // => 'unit'

    typeof(#fff)
    // => 'rgba'

    type-of(#fff)
    // => 'rgba'

Aliased as `type-of` and `type`.

### unit(val[, type])

Return a string indicating the type of `val` or an empty string,
or assign the given `type` without unit conversion.

    unit(10)
    // => ''

    unit(15in)
    // => 'in'

    unit(15%, 'px')
    // => 15px

    unit(15%, px)
    // => 15px

### percentage(num)

Convert a `num` to a percentage.

    percentage(.5)
    // => 50%

    percentage(4 / 100)
    // => 4%


## Math Functions

### abs(unit)

    abs(-5px)
    // => 5px

    abs(5px)
    // => 5px

### ceil(unit)

    ceil(5.5in)
    // => 6in

### floor(unit)

    floor(5.6px)
    // => 5px

### round(unit)

    round(5.5px)
    // => 6px

    round(5.4px)
    // => 5px

**Note:** All rounding functions can accept optional `precision` argument — you can pass the number of digits you want to save after the period:

    ceil(5.52px,1)
    // => 5.6px

    floor(5.57px,1)
    // => 5.5px

    round(5.52px,1)
    // => 5.5px

### sin(angle)

Returns the value of sine for the given `angle`. If the angle is given as a degree unit, like `45deg`, it is treated as a degree, otherwise it is treated as radians.

    sin(30deg)
    // => 0.5

    sin(3*PI/4)
    // => 0.707106781

### cos(angle)

Returns the value of cosine for the given `angle`. If the angle is given as a degree unit, like `45deg`, it is treated as a degree, otherwise it is treated as radians.

    cos(180deg)
    // => -1

### tan(angle)

Returns the value of tangent for the given `angle`. If the angle is given as a degree unit, like `45deg`, it is treated as a degree, otherwise it is treated as radians.

    tan(45deg)
    // => 1

    tan(90deg)
    // => Infinity

### min(a, b)

    min(1, 5)
    // => 1

### max(a, b)

      max(1, 5)
      // => 5

### even(unit)

    even(6px)
    // => true

### odd(unit)

    odd(5mm)
    // => true

### sum(nums)

    sum(1 2 3)
    // => 6

### avg(nums)

    avg(1 2 3)
    // => 2

### range(start, stop[, step])

Returns a list of units from `start` to `stop` (included) by given `step`. 

The `step` argument defaults to `1` if omitted.  It cannot be `0`.

    range(1, 6)
    // equals to `1..6`
    // 1 2 3 4 5 6

    range(1, 6, 2)
    // 1 3 5

    range(-6, -1, 2)
    // -6 -4 -2

    range(1px, 3px, 0.5px)
    // 1px 1.5px 2px 2.5px 3px

It is most often used in `for` loops:

    for i in range(10px, 50px, 10)
      .col-{i}
        width: i

Yields:

    .col-10 {
      width: 10px;
    }
    .col-20 {
      width: 20px;
    }
    .col-30 {
      width: 30px;
    }
    .col-40 {
      width: 40px;
    }
    .col-50 {
      width: 50px;
    }


### base-convert(num, base, width)

Returns a `Literal` `num` converted to the provided `base`, padded with `width` zeroes. 

Width defaults to `2`.

    base-convert(1, 10, 3)
    // => 001

    base-convert(14, 16, 1)
    // => e

    base-convert(42, 2)
    // => 101010


## String Functions

### match(pattern, string[, flags])

Returns any matches of `pattern` (regular expression) in `string`.

    match('^(height|width)?([<>=]{1,})(.*)', 'height>=1024px')
    // => 'height>=1024px' 'height' '>=' '1024px'

    match('^foo(?:bar)?', 'foo')
    // => 'foo'

    match('^foo(?:bar)?', 'foobar')
    // => 'foobar'

    match('^foo(?:bar)?', 'bar')
    // => null

    match('ain', 'The rain in SPAIN stays mainly in the plain')
    // => 'ain'

    match('ain', 'The rain in SPAIN stays mainly in the plain', g)
    // => 'ain' 'ain' 'ain'

    match('ain', 'The rain in SPAIN stays mainly in the plain', 'gi')
    // => 'ain' 'AIN' 'ain' 'ain'


### replace(pattern, replacement, val)

Return the string `val`, after replacing with all `pattern` matches with `replacement`.

    replace(i, e, 'griin')
    // => 'green'

    replace(i, e, griin)
    // => #008000


### join(delim, vals...)

Join the given `vals` with `delim`.

    join(' ', 1 2 3)
    // => "1 2 3"

    join(',', 1 2 3)
    // => "1,2,3"

    join(', ', foo bar baz)
    // => "foo, bar, baz"

    join(', ', foo, bar, baz)
    // => "foo, bar, baz"

    join(', ', 1 2, 3 4, 5 6)
    // => "1 2, 3 4, 5 6"


### split(delim, val)

The `split()` method splits a string or ident into a list of strings by separating the string into substrings.

    split(_, bar1_bar2_bar3)
    // => bar1 bar2 bar3

    split(_, 'bar1_bar2_bar3')
    // => 'bar1' 'bar2' 'bar3'


### substr(val, start, length)

The `substr()` method returns the characters in a string beginning at the specified location through the specified number of characters.

    substr(ident, 1, 2)
    // => de

    substr('string', 1, 2)
    // => 'tr'

    val = dredd
    substr(substr(val, 1), 0, 3)
    // => #f00


### slice(val, start[, end])

The `slice()` method extracts a section of a string/list and returns a new string/list.

    slice('lorem' 'ipsum' 'dolor', 1, 2)
    slice('lorem' 'ipsum' 'dolor', 1, -1)
    // => 'ipsum'
    
    slice('lorem ipsum', 1, 5)
    // => 'orem'
    slice(rredd, 1, -1)
    // => #f00
    
    slice(1px solid black, 1)
    // => solid #000


### unquote(str | ident)

Unquote the given `str`, and return it as a `Literal` node.

    unquote("sans-serif")
    // => sans-serif

    unquote(sans-serif)
    // => sans-serif

    unquote('1px / 2px')
    // => 1px / 2px


### convert(str)

Like `unquote()` but tries to convert the given `str` to a Stylus node.

    unit = convert('40px')
    typeof(unit)
    // => 'unit'

    color = convert('#fff')
    typeof(color)
    // => 'rgba'

    foo = convert('foo')
    typeof(foo)
    // => 'ident'


### s(fmt, ...)

The `s()` function is similar to `unquote()`, in that it returns a `Literal` node. However, it accepts a format string similar to C's `sprintf()`. 

Currently, the only specifier is `%s`.

    s('bar()');
    // => bar()

    s('bar(%s)', 'baz');
    // => bar("baz")

    s('bar(%s)', baz);
    // => bar(baz)

    s('bar(%s)', 15px);
    // => bar(15px)

    s('rgba(%s, %s, %s, 0.5)', 255, 100, 50);
    // => rgba(255, 100, 50, 0.5)

    s('bar(%Z)', 15px);
    // => bar(%Z)

    s('bar(%s, %s)', 15px);
    // => bar(15px, null)

Check out the `%` string operator for equivalent behaviour.


## Utility Functions

### `called-from` property

`called-from` property contains the list of the functions the current function was called from in reverse order (the first item is the deepest function).

    foo()
      bar()

    bar()
      baz()

    baz()
      return called-from

    foo()
    // => bar foo

### current-media()

`current-media()` function returns the string of the current block's `@media` rule (or `''` if there is no `@media` above the block).

    @media only screen and (min-width: 1024px)
      current-media()
    // => '@media (only screen and (min-width: (1024px)))'


### +cache(keys...)

`+cache` is a really powerful built-in function that allows you to create your own “cachable” mixins.

A “cachable mixin” is one that would apply its contents to the given selector on the first call, but would `@extend` the first call's selector at the second call with the same params.

    size($width, $height = $width)
      +cache('w' + $width)
        width: $width
      +cache('h' + $height)
        height: $height

    .a
      size: 10px 20px
    .b
      size: 10px 2em
    .c
      size: 1px 2em

Would yield to

    .a,
    .b {
      width: 10px;
    }
    .a {
      height: 20px;
    }
    .b,
    .c {
      height: 2em;
    }
    .c {
      width: 1px;
    }

See how the selectors are grouped together by the used property.

### +prefix-classes(prefix)

Stylus comes with a block mixin `prefix-classes` that can be used for prefixing the classes inside any given Stylus block. For example:

    +prefix-classes('foo-')
      .bar
        width: 10px

Yields:

    .foo-bar {
      width: 10px;
    }

### lookup(name)

Allows looking up the value of a variable called `name`, passed as a string.
Returns `null` if the variable is undefined.

Useful when you need to get a value of a variable with dynamically
generated name:

    font-size-1 = 10px
    font-size-2 = 20px
    font-size-3 = 30px

    for i in 1..3
      .text-{i}
        font-size: lookup('font-size-' + i)

Yields:

    .text-1 {
      font-size: 10px;
    }
    .text-2 {
      font-size: 20px;
    }
    .text-3 {
      font-size: 30px;
    }

### define(name, expr[, global])

Allows to create or overwrite a variable with a given `name`, passed as a string, onto current scope (or global scope if `global` is true).

This <abbr title="Built-In Function">BIF</abbr> can be useful in those cases where you need interpolation in variable names:

    prefix = 'border'
    border = { color: #000, length: 1px, style: solid }

    for prop, val in border
      define(prefix + '-' + prop, val)

    body
      border: border-length border-style border-color

yields:

    body {
      border: 1px solid #000;
    }

### operate(op, left, right)

Perform the given `op` on the `left` and `right` operands:

    op = '+'
    operate(op, 15, 5)
    // => 20

### selector()

Returns the compiled current selector or `&` if called at root level.

    .foo
      selector()
    // => '.foo'

    .foo
      &:hover
        selector()
    // '.foo:hover'

### selector-exists(selector)

Returns true if `selector` exists..

    .foo
      color red

      a
        font-size 12px

    selector-exists('.foo') // true
    selector-exists('.foo a') // true
    selector-exists('.foo li') // false
    selector-exists('.bar') // false

This method does not take into account the current context meaning:

    .foo
      color red

      a
        font-size 12px

      selector-exists('a') // false
      selector-exists(selector() + ' a') // true


### opposite-position(positions)

Return the opposite of the given `positions`.

    opposite-position(right)
    // => left

    opposite-position(top left)
    // => bottom right

    opposite-position('top' 'left')
    // => bottom right

### image-size(path)

Returns the `width` and `height` of the image found at `path`. Lookups are performed in the same manner as `@import`, altered by the `paths` setting.

    width(img)
      return image-size(img)[0]

    height(img)
      return image-size(img)[1]

    image-size('tux.png')
    // => 405px 250px

    image-size('tux.png')[0] == width('tux.png')
    // => true

### embedurl(path[, encoding])

Returns an inline image as a `url()` literal, encoded with `encoding`. 

(Available encodings: `base64` (default), and `utf8`).

    background: embedurl('logo.png')
    // => background: url("data:image/png;base64,…")

    background: embedurl('logo.svg', 'utf8')
    // => background: url("data:image/svg+xml;charset=utf-8,…")

### add-property(name, expr)

Adds property `name`, with the given `expr` to the closest block.

For example:

    something()
      add-property('bar', 1 2 3)
      s('bar')

    body
      foo: something()

yields:

    body {
      bar: 1 2 3;
      foo: bar;
    }

Next, the "magic" `current-property` local variable comes into play. This variable is automatically available to function bodies, and contains an expression with the current property's name, and value.

For example, if we were to inspect this local variable using `p()`, we get the following:

    p(current-property)
    // => "foo" (foo __CALL__ bar baz)

    p(current-property[0])
    // => "foo"

    p(current-property[1])
    // => foo __CALL__ bar baz

Using `current-property` we can take our example a bit further, and duplicate the property with new values, and a conditional to ensure the function is only used within a property value.

    something(n)
      if current-property
        add-property(current-property[0], s('-webkit-something(%s)', n))
        add-property(current-property[0], s('-moz-something(%s)', n))
        s('something(%s)', n)
      else
        error('something() must be used within a property')

    body {
      foo: something(15px) bar;
    }

yields:

    body {
      foo: -webkit-something(15px);
      foo: -moz-something(15px);
      foo: something(15px) bar;
    }

If you noticed in the example above, `bar` is only present for the initial call, since we returned `something(15px)`, it remained in-place within the expression, however the others do not take the rest of the expression into account.

Our more robust solution below defines a function named `replace()` which clones the expression to prevent mutation, replaces the string value of an expression with another, and returns the cloned expression. We then move on to replace `__CALL__` within the expressions, which represents the cyclic call to `something()`.

    replace(expr, str, val)
      expr = clone(expr)
      for e, i in expr
        if str == e
          expr[i] = val
      expr

    something(n)
      if current-property
        val = current-property[1]
        webkit = replace(val, '__CALL__', s('-webkit-something(%s)', n))
        moz = replace(val, '__CALL__', s('-moz-something(%s)', n))
        add-property(current-property[0], webkit)
        add-property(current-property[0], moz)
        s('something(%s)', n)
      else
        error('something() must be used within a property')

    body
      foo: something(5px) bar baz

yields:

    body {
      foo: -webkit-something(5px) bar baz;
      foo: -moz-something(5px) bar baz;
      foo: something(5px) bar baz;
    }

Our implementation is now fully transparent both in regards to the property it is called within, and the position of the call. 

This powerful concept aids in transparent vendor support for function calls, such as gradients.


## Console Functions
 
### warn(msg)

Warn with the given error `msg`. Does not exit.

    warn("oh noes!")

### error(msg)

Exit Stylus with the given error `msg`.

    add(a, b)
      unless a is a 'unit' and b is a 'unit'
        error('add() expects units')
      a + b

### p(expr)

Inspect the given `expr`:

    fonts = Arial, sans-serif
    p('test')
    p(123)
    p((1 2 3))
    p(fonts)
    p(#fff)
    p(rgba(0,0,0,0.2))

    add(a, b)
      a + b

    p(add)

stdout:

    inspect: "test"
    inspect: 123
    inspect: 1 2 3
    inspect: Arial, sans-serif
    inspect: #fff
    inspect: rgba(0,0,0,0.2)
    inspect: add(a, b)


## External File Functions

### json(path[, options])

Convert the JSON file at `path` into Stylus variables or an object. Nested variable object keys are joined with a dash (`-`).

For example, the following sample `media-queries.json` file:

    {
        "small": "screen and (max-width:400px)",
        "tablet": {
            "landscape": "screen and (min-width:600px) and (orientation:landscape)",
            "portrait": "screen and (min-width:600px) and (orientation:portrait)"
        }
    }

May be used in the following ways:

    json('media-queries.json')

    @media small
    // => @media screen and (max-width:400px)

    @media tablet-landscape
    // => @media screen and (min-width:600px) and (orientation:landscape)

    vars = json('vars.json', { hash: true })
    body
      width: vars.width

    vars = json('vars.json', { hash: true, leave-strings: true })
    typeof(vars.icon)
    // => 'string'

    // don't throw an error if the JSON file doesn't exist
    optional = json('optional.json', { hash: true, optional: true })
    typeof(optional)
    // => 'null'


### use(path)

You can use any JS-plugin at given `path` with `use()` function right inside your `.styl` files, like this:

    use("plugins/add.js")

    width add(10, 100)
    // => width: 110

And the `add.js` plugin in this case looks this way:

    var plugin = function(){
      return function(style){
        style.define('add', function(a, b) {
          return a.operate('+', b);
        });
      };
    };
    module.exports = plugin;

If you'd like to return any Stylus objects like `RGBA`, `Ident`, or `Unit`, you could use the provided Stylus nodes like this:

    var plugin = function(){
      return function(style){
        var nodes = this.nodes;
        style.define('something', function() {
          return new nodes.Ident('foobar');
        });
      };
    };
    module.exports = plugin;

You can pass any options as an optional second argument, using a [hash object](hashes.html):

    use("plugins/add.js", { foo: bar })


### Undefined Functions

Undefined functions are output as literals. So, for example,
we may call `rgba-stop(50%, #fff)` within our css, and it will
output as you would expect. We can use this within helpers as well.

In the example below we simply define the function `stop()` which
returns the literal `rgba-stop()` call.

    stop(pos, rgba)
      rgba-stop(pos, rgba)

    stop(50%, orange)
    // => rgba-stop(50%, #ffa500)
