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# Language Definition
**Expr** package uses a specific syntax. In this document, you can find all supported
syntaxes.
## Supported Literals
The package supports:
* **strings** - single and double quotes (e.g. `"hello"`, `'hello'`)
* **numbers** - e.g. `103`, `2.5`, `.5`
* **arrays** - e.g. `[1, 2, 3]`
* **maps** - e.g. `{foo: "bar"}`
* **booleans** - `true` and `false`
* **nil** - `nil`
## Digit separators
Integer literals may contain digit separators to allow digit grouping into more legible forms.
Example:
```
10_000_000_000
```
## Accessing Public Properties
Public properties on structs can be accessed by using the `.` syntax.
If you pass an array into an expression, use the `[]` syntax to access array keys.
```js
foo.Array[0].Value
```
## Functions and Methods
Functions may be called using `()` syntax. The `.` syntax can also be used to call methods on an struct.
```js
price.String()
```
## Supported Operators
The package comes with a lot of operators:
### Arithmetic Operators
* `+` (addition)
* `-` (subtraction)
* `*` (multiplication)
* `/` (division)
* `%` (modulus)
* `**` (pow)
Example:
```js
life + universe + everything
```
### Comparison Operators
* `==` (equal)
* `!=` (not equal)
* `<` (less than)
* `>` (greater than)
* `<=` (less than or equal to)
* `>=` (greater than or equal to)
### Logical Operators
* `not` or `!`
* `and` or `&&`
* `or` or `||`
Example:
```
life < universe || life < everything
```
### String Operators
* `+` (concatenation)
* `matches` (regex match)
* `contains` (string contains)
* `startsWith` (has prefix)
* `endsWith` (has suffix)
To test if a string does *not* match a regex, use the logical `not` operator in combination with the `matches` operator:
```js
not ("foo" matches "^b.+")
```
You must use parenthesis because the unary operator `not` has precedence over the binary operator `matches`.
Example:
```js
'Arthur' + ' ' + 'Dent'
```
Result will be set to `Arthur Dent`.
### Membership Operators
* `in` (contain)
* `not in` (does not contain)
Example:
```js
user.Group in ["human_resources", "marketing"]
```
```js
"foo" in {foo: 1, bar: 2}
```
### Numeric Operators
* `..` (range)
Example:
```js
user.Age in 18..45
```
The range is inclusive:
```js
1..3 == [1, 2, 3]
```
### Ternary Operators
* `foo ? 'yes' : 'no'`
Example:
```js
user.Age > 30 ? "mature" : "immature"
```
## Builtin functions
* `len` (length of array, map or string)
* `all` (will return `true` if all element satisfies the predicate)
* `none` (will return `true` if all element does NOT satisfies the predicate)
* `any` (will return `true` if any element satisfies the predicate)
* `one` (will return `true` if exactly ONE element satisfies the predicate)
* `filter` (filter array by the predicate)
* `map` (map all items with the closure)
* `count` (returns number of elements what satisfies the predicate)
Examples:
Ensure all tweets are less than 280 chars.
```js
all(Tweets, {.Size < 280})
```
Ensure there is exactly one winner.
```js
one(Participants, {.Winner})
```
## Closures
* `{...}` (closure)
Closures allowed only with builtin functions. To access current item use `#` symbol.
```js
map(0..9, {# / 2})
```
If the item of array is struct, it's possible to access fields of struct with omitted `#` symbol (`#.Value` becomes `.Value`).
```js
filter(Tweets, {len(.Value) > 280})
```
## Slices
* `array[:]` (slice)
Slices can work with arrays or strings.
Example:
Variable `array` is `[1,2,3,4,5]`.
```js
array[1:5] == [2,3,4]
array[3:] == [4,5]
array[:4] == [1,2,3]
array[:] == array
```
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