# publicLearnBoost/stylus

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## Operator Precedence

Below is the operator precedence table, highest to lowest:

`````` []
! ~ + -
is defined
** * / %
+ -
... ..
<= >= < >
in
== is != is not isnt
is a
&& and || or
?:
= ?= += -= *= /= %=
not
if unless
``````

## Unary Operators

The following unary operators are available, `!`, `not`, `-`, `+`, and `~`.

``````!0
// => true

!!0
// => false

!1
// => false

!!5px
// => true

-5px
// => -5px

--5px
// => 5px

not true
// => false

not not true
// => true
``````

The logical `not` operator has low precedence, therefore the following example could be replaced with

``````a = 0
b = 1

!a and !b
// => false
// pased as: (!a) and (!b)
``````

with:

``````not a or b
// => false
// parsed as: not (a or b)
``````

## Binary Operators

### Subscript []

The subscript operator allows us to grab a value in an expression via index. Parenthesized expressions may act as tuples, so for example `(15px 5px)`, `(1 2 3)`.

Below is an example where we utilize tuples for error handling, showing the versatility of such a construct. As

`````` add(a, b)
if a is a 'unit' and b is a 'unit'
a + b
else
(error 'a and b must be units!')

body
// => padding: error "a and b must be units";

// => padding: "a and b must be units";
``````

A more complex example, invoking the `error()` built-in function with the error message returned, when the ident (the first value) equals `error`.

`````` if (val = add(1,'5'))[0] == error
error(val[1])
``````

## Range .. ...

Both the inclusive (`..`) and exclusive (`...`) range operators are provided, expanding to expressions:

`````` 1..5
// => 1 2 3 4 5

1...5
// => 1 2 3 4
``````

multiplicative and additive binary operators work as expected, and type conversion is applied within unit type classes, or default to the literal value. For example if we perform `5s - 2px` we will get `3s`.

``````15px - 5px
// => 10px

5 - 2
// => 3

5in - 50mm
// => 3.031in

5s - 1000ms
// => 4s

20mm + 4in
// => 121.6mm

"foo " + "bar"
// => "foo bar"

"num " + 15
// => "num 15"
``````

### Multiplicative: / * %

``````2000ms + (1s * 2)
// => 4ms

5s / 2
// => 2.5s

4 % 2
// => 0
``````

When using `/` within a property value you must wrap with parens. The following for example is taken literally, to support css line-height:

``````font: 14px/1.5;
``````

whereas the following is evaluated, dividing `14px` by `1.5`:

``````font: (14px/1.5);
``````

this exception is only required for the `/` operator.

### Exponent: **

The Exponent operator:

``````2 ** 8
// => 256
``````

### Equality: == != >= <= > <

Equality operators can be used to equate units, colors, strings, and even identifiers. This is a powerful concept, as even arbitrary identifiers such as as `wahoo` can be utilized as atoms, a function could return `yes` or `no` instead of `true` or `false` (although not advised).

``````5 == 5
// => true

10 < 5
// => true

#fff == #fff
// => true

true == false
// => false

wahoo == yay
// => false

wahoo == wahoo
// => true

"test" == "test"
// => true

true is true
// => true

'hey' is not 'bye'
// => true

'hey' isnt 'bye'
// => true
``````

Only exact values match, for example `0 == false`, and `null == false` are both `false`.

Aliases:

``````==    is
!=    is not
!=    isnt
``````

## Truthfulness

Nearly everything within Stylus resolves to `true`, including units with a suffix, for example even `0%`, `0px`, etc will resolve to `true`, since commonly in Stylus a mixin or function may accept such units as valid, however `0` itself is `false` in terms of arithmetic.

`true` examples:

``````  0%
0px
1px
-1
-1px
hey
'hey'
``````

`false` examples:

`````` 0
null
false
''
``````

### Logical Operators: && || and or

Logical operators `&&` and `||` are aliased `and` / `or` which apply the same precedence.

``````5 && 3
// => 3

0 || 5
// => 5

0 && 5
// => 0

#fff is a 'rgba' and 15 is a 'unit'
// => true
``````

### Existence Operator: in

Checks for the existence of the left-hand operand within the right-hand expression.

Simple examples:

``````  nums = 1 2 3
1 in nums
// => true

5 in nums
// => false
``````

Some undefined identifiers:

``````  words = foo bar baz
bar in words
// => true

HEY in words
// => false
``````

Works with tuples too:

``````  vals = (error 'one') (error 'two')
error in vals
// => false

(error 'one') in vals
// => true

(error 'two') in vals
// => true

(error 'something') in vals
// => false
``````

Example usage in mixin:

``````  pad(types = padding, n = 5px)
if margin in types
margin n

body

body

body
``````

yielding:

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

### Conditional Assignment: ?=

The conditional assignment operator `?=` lets us define variables without clobbering old values (when present). This operator expands to an `is defined` binary operation within a ternary, for example the following are equivalent:

``````color ?= white
color = color is defined ? color : white
``````

For example when using `=` we simply re-assign:

``````color = white
color = black

color
// => black
``````

However when using `?=` our second attempt fails since the variable is already defined:

``````color = white
color ?= black

color
// => white
``````

### Instance Check: is a

Stylus provides a binary operator named `is a` used to type check.

``````15 is a 'unit'
// => true

#fff is a 'rgba'
// => true

15 is a 'rgba'
// => false
``````

Alternatively we could use the `type()` BIF:

``````type(#fff) == 'rgba'
// => true
``````

'color' is the one special-case, evaluating to true when the left-hand operand is an `RGBA` or `HSLA` node.

### Variable Definition: is defined

This pseudo binary operator does not accept a right-hand operator, and does not evaluate the left. This allows us to check if a variable has a value assigned to it.

``````foo is defined
// => false

foo = 15px
foo is defined
// => true

#fff is defined
// => 'invalid "is defined" check on non-variable #fff'
``````

Alternatively one can use the `lookup(name)` built-in function to do this, or to perform dynamic lookups:

``````name = 'blue'
lookup('light-' + name)
// => null

light-blue = #80e2e9
lookup('light-' + name)
// => #80e2e9
``````

This operator is essential, as an undefined identifier is still a truthy value. For example:

``````body
if ohnoes
``````

will yield the following css when undefined:

``````body {
}
``````

however this will be safe:

``````body
if ohnoes is defined
``````

## Ternary

The ternary operator works as we would expect in most languages, being the only operator with three operands, the condition expression, the truth expression and the false expression.

``````num = 15
num ? unit(num, 'px') : 20px
// => 15px
``````

## Color Operations

Operations on colors provide a terse, expressive way to alter components. For example we can operate on each RGB:

``````#0e0 + #0e0
// => #0f0
``````

Another example is adjust the lightness value by adding or subtracting a percentage. To lighten a color we add, to darken we subtract.

``````#888 + 50%
// => #ccc

#888 - 50%
// => #444
``````

Adjust the hue is also possible by adding or subtracting with degrees, for example adding `50deg` to this red value, resulting in a yellow:

`````` #f00 + 50deg
// => #ffd500
``````

Values clamp appropriately, for example we can "spin" the hue 180 degrees, and if the current value is `320deg`, it will resolve to `140deg`.

We may also tweak several values at once, including the alpha by using `rgb()`, `rgba()`, `hsl()`, or `hsla()`:

``````  #f00 - rgba(100,0,0,0.5)
// => rgba(155,0,0,0.5)
``````

## Sprintf

The string sprintf-like operator `%` can be used to generate a literal value, internally passing arguments through the `s()` built-in:

``````   'X::Microsoft::Crap(%s)' % #fc0
// => X::Microsoft::Crap(#fc0)
``````

Multiple values should be parenthesized:

``````  '-webkit-gradient(%s, %s, %s)' % (linear (0 0) (0 100%))
// => -webkit-gradient(linear, 0 0, 0 100%)
``````
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