Data types

FQL supports both primitive data types consisting of exactly one value and compound data types comprised of multiple values. The following types are available:

Data type Description
none An empty value, also: the absence of a value
boolean Boolean truth value with possible values false and true
integer Signed whole number
float Signed floating point number
string UTF-8 encoded text value
array Sequence of values, referred to by their positions
object Sequence of values, referred to by their names
binary Sequence of binary values
custom User defined types

Primitive types

None value

A none value can be used to represent an empty or absent value. It is different from a numerical value of zero (null != 0) and other falsy values (false, zero-length string, empty array or object). It is also known as nil or null in other languages.

Boolean data type

The Boolean data type has two possible values, true and false. They represent the two truth values in logic and mathematics.

Numeric literals

Numeric literals can be integers or floating-point numbers. They can optionally be signed with the + or - symbols. A decimal point . is used as separator for the optional fractional part.

1 +1 42 -1 -42 0.5 1.23 -99.99

String literals

String literals must be enclosed in single or double quotes. If the used quote character is to be used itself within the string literal, it must be escaped using the backslash symbol. A literal backslash also needs to be escaped with a backslash.

"yikes!" "don't know" "this is a \"quoted\" word" "this is a longer string." "the path separator on Windows is \\" 'yikes!' 'don\'t know' 'this is a "quoted" word' 'this is a longer string.' 'the path separator on Windows is \\'

Compound types

FQL supports two compound types:

  • array: A composition of unnamed values, each accessible by their positions.
  • object: A composition of named values, each accessible by their names.
  • binary: An array of binary values.


The first supported compound type is the array type. Arrays are effectively sequences of (unnamed / anonymous) values. Individual array elements can be accessed by their positions. The order of elements in an array is important.

An array declaration starts with a left square bracket [ and ends with a right square bracket ]. The declaration contains zero, one or more expressions, separated from each other with the comma , symbol. Whitespace around elements is ignored in the declaration, thus line breaks, tab stops and blanks can be used for formatting.

In the easiest case, an array is empty and thus looks like:

[ ]

Array elements can be any legal expression values. Nesting of arrays is supported.

[ true ] [ 1, 2, 3 ] [ -99, "yikes!", [ false, ["no"], [] ], 1 ] [ [ "fox", "marshal" ] ]

Individual array values can later be accessed by their positions using the [] accessor. The position of the accessed element must be a numeric value. Positions start at 0.

// access 1st array element (elements start at index 0) u.friends[0] // access 3rd array element u.friends[2]


The other supported compound type is the object type. Objects are a composition of zero to many attributes. Each attribute is a name/value pair. Object attributes can be accessed individually by their names. This data type is also known as dictionary, map, associative array and other names.

Object declarations start with a left curly bracket { and end with a right curly bracket }. An object contains zero to many attribute declarations, separated from each other with the , symbol. Whitespace around elements is ignored in the declaration, thus line breaks, tab stops and blanks can be used for formatting.

In the simplest case, an object is empty. Its declaration would then be:

{ }

Each attribute in an object is a name/value pair. Name and value of an attribute are separated using the colon : symbol. The name is always a string, whereas the value can be of any type including sub-objects.

The attribute name is mandatory - there can’t be anonymous values in an object. It can be specified as a quoted or unquoted string:

{ name: … } // unquoted { 'name': … } // quoted (apostrophe / "single quote mark") { "name": … } // quoted (quotation mark / "double quote mark")

It must be quoted if it contains whitespace, escape sequences or characters other than ASCII letters (a-z, A-Z), digits (0-9), underscores (_) and dollar signs ($). The first character has to be a letter, underscore or dollar sign.

If a keyword is used as an attribute name then the attribute name must be quoted or escaped by backticks:

{ return: … } // error, return is a keyword! { 'return': … } // quoted { "return": … } // quoted { `return`: … } // escaped (backticks) { ´return´: … } // escaped (ticks)

Attribute names can be computed using dynamic expressions, too. To disambiguate regular attribute names from attribute name expressions, computed attribute names must be enclosed in square brackets [ … ]:

{ [ CONCAT("test/", "bar") ] : "someValue" }

There is also shorthand notation for attributes which is handy for returning existing variables easily:

LET name = "Peter" LET age = 42 RETURN { name, age }

The above is the shorthand equivalent for the generic form:

LET name = "Peter" LET age = 42 RETURN { name: name, age: age }

Any valid expression can be used as an attribute value. That also means nested objects can be used as attribute values:

{ name : "Peter" } { "name" : "Vanessa", "age" : 15 } { "name" : "John", likes : [ "Swimming", "Skiing" ], "address" : { "street" : "Cucumber lane", "zip" : "94242" } }

Individual object attributes can later be accessed by their names using the dot . accessor: u.friends[0].name.first

Attributes can also be accessed using the square bracket [] accessor:

u["address"]["city"]["name"] u["friends"][0]["name"]["first"]

In contrast to the dot accessor, the square brackets allow for expressions:

LET attr1 = "friends" LET attr2 = "name" u[attr1][0][attr2][ CONCAT("fir", "st") ]

Binary value

Binary value is an array of bytes that represent binary data like files, images, audio or video.


Custom types

Custom types are the values defined by a user or author of a 3rd party library that extends FQL functionality. The values can represent either specific primitives or compound types. For example, tt could be http cookies, database records, images and etc.

The feature is a runtime specific. Currently, it's the Go runtime.
At this moment, the feature is supported in embedded mode only and not available via CLI.

In order to define a custom value, it needs to have 2 Go types. One should implement core.Type and another core.Value.

type Type interface { ID() int64 String() string Equals(other Type) bool }
type Value interface { json.Marshaler Type() Type String() string Compare(other Value) int64 Unwrap() interface{} Hash() uint64 Copy() Value }

For simplicity, there is a helper function in core package, that allows to create a type:

var MyValueType = core.NewType("MyValueType")

core.Value interface provides a basic set of functionality which allows the FQL runtime to use a custom value.

LET myType = SOME_FUNC() RETURN [myType]

Reading data

By default, implementing core.Value gives the FQL runtime just a basic set of functionality to work with values. This means that a type which implemented only core.Value interfaces is treated as a primitive data type.

If the underlying data type is compound and there is a need for providing access to underlying data, the type should implement core.Getter interface:

type Getter interface { GetIn(ctx context.Context, path []Value) (Value, error) }

After that, it will be possible to use . or [] accessors:

LET myType = SOME_FUNC() RETURN myType.someProperty


In order to use a custom type as source for FOR IN statement, it must implement core.Iterable interface:

type Iterable interface { Iterate(ctx context.Context) (Iterator, error) }
type Iterator interface { Next(ctx context.Context) (value Value, key Value, err error) }

Clean up

If a custom type needs to do some clean up after a query execution, it needs to implement Go’s io.Closer interface.