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
• arrays - e.g. [1, 2, 3]
• maps - e.g. {foo: "bar"}
• booleans - true and false
• nil - nil

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.

foo.Array[0].Value

Calling Methods

The . syntax can also be used to call methods on an struct.

price.String()

Supported Operators

The package comes with a lot of operators:

Arithmetic Operators

• + (addition)
• - (subtraction)
• * (multiplication)
• / (division)
• % (modulus)
• ** (pow)

Example:

life + universe + everything

Digit separators

Integer literals may contain digit separators to allow digit grouping into more legible forms.

Example:

10_000_000_000

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:

not ("foo" matches "^b.+")

You must use parenthesis because the unary operator not has precedence over the binary operator matches.

Example:

'Arthur' + ' ' + 'Dent'

Result will be set to Arthur Dent.

Membership Operators

• in (contain)
• not in (does not contain)

Example:

user.Group in ["human_resources", "marketing"]

"foo" in {foo: 1, bar: 2}

Numeric Operators

• .. (range)

Example:

user.Age in 18..45

The range is inclusive:

1..3 == [1, 2, 3]

Ternary Operators

• foo ? 'yes' : 'no'

Example:

user.Age > 30 ? "mature" : "immature"

Builtin functions

• len (length of array 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)

Example:

// Ensure all tweets are less than 140 chars.
all(Tweets, {.Size < 140})

Closures

• {...} (closure)

Closures allowed only with builtin functions. To access current item use # symbol.

map(0..9, {# + 1})

If the item of array is struct, it's possible to access fields of struct with omitted # symbol (#.Value becomes .Value).

filter(Tweets, {.Size > 140})

Slices

• array[:] (slice)

Slices can work with arrays or strings.

Example:

// array is [1,2,3,4,5]
array[1:5] == [2,3,4] 
array[3:] == [4,5]
array[:4] == [1,2,3]
array[:] == array