A type is a kind of data, such as String or Int. Every type has an associated class, but more than one type may map to the same class: for instance, String and String? are two distinct types, but are the same class (frost.core.String).

The full syntax for a class type is:


For example, the type Array<Int64?>? refers to a nullable array of nullable Int64s.

<class> is the name of any Frost class, such as Int32 or String. Class names may or may not be fully qualified.

The <parameters> are an optional comma-separated list of types, surrounded by angle brackets (<type1, type2>). Parameters are used by generic types.

The optional question mark at the end of a type name identifies the type as being nullable. Without a trailing question mark, the type is non-nullable and thus null is not a legal value for the type.

Every expression in Frost has a type. Frost uses type inference to automatically determine the types of fields and variables.

The run-time type of a value is not necessarily the same as its compile-time type. For instance, in the code:

def str:Object := getString()

presuming that getString() does as its name suggests and returns a String, then the variable str will contain a String and thus have a run-time type of String. But as its type was explicitly declared as Object, it has a compile-time type of Object. You therefore could not write:


because you would be passing an Object to a method expecting a String. A typecast instructs the compiler to treat an object as a different compile-time type; in this case you could write:


in order to make this call. Of course, in this particular case it would have been better to simply declare str as the right type to begin with!

Method Types

In addition to the class types described above, methods have types. The type of a function encapsulates its parameter types and return type in the form (<parameterTypes>)=>(<returnType>), for example:

(Real, Real)=>(Real)

This type represents a function taking two Real parameters and returning a Real result, as in:

function add(x:Real, y:Real):Real {
    return x + y

Methods which are not functions (i.e. they potentially have side effects) have a slightly different type signature:

(Real, Real)=&>(Real)

The ampersand in the middle of the arrow signifies "and other effects", as a method may perform unspecified other actions. Methods may also be declared to not return a value, in which case the return type is left blank. The type signature of the System.exit(Int) method, for example, is:


signifying that it takes a single Int as a parameter, has side effects, and does not return a value.

Function types such as (Int)=>(Int) may be used in contexts where the equivalent method type (in this case (Int)=&>(Int)) is expected, but the reverse is not true.

Because method references, inline methods, and lambdas can capture values, and those values may be mutable, the resulting method reference can itself be mutable. The types described above permit both mutable and immutable method references to be assigned to them. To restrict a method type to immutable method references only, add an asterisk (*) after the arrow. This gives us four different method types for any given combination of parameters and return type:

()=>(Int)   -- mutable function
()=&>(Int)  -- mutable method
()=>*(Int)  -- immutable function
()=&>*(Int) -- immutable method

Functions may be used wherever methods are allowed, and immutable method references may be used wherever mutable method references are allowed.

Tuple Types

The type of a tuple is described by a parenthesized list of type names:

def tuple:(Object, Int) := ("Tuple", 12)

Tuples must contain at least two types. Tuples containing only immutable types will themselves be considered immutable.