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Starlark Types

The Starlark 'types' extension is highly experimental and likely to be modified in the future.

Types can be added to function arguments, or function return types.

For example:

def fib(i: int) -> int:
    ...

There are moments where types can be checked:

  1. At runtime, as a function is executed, when a value of the appropriate type is available.
  2. Statically, without executing anything.
  3. At compile time, when the definitions of all symbols imported using load are available.

Currently runtime is the normal way of checking, but other systems built on Starlark (e.g. Buck2) may also perform additional types of checking. In all cases the meaning of the types is the same.

The rest of this document lays out what types mean and what type-supporting values are available (records and enums).

What does a type mean?

A type is a Starlark expression that has a meaning as a type:

  • When fib(3) is called, the value 3 is passed to fib as parameter i.
  • When the execution of fib is started, the expression int is evaluated to the value of the int function.
  • A check is then made that the value 3 matches the type represented by int.

If the value doesn't match, it is a runtime error. Similarly, on return statements, or the end of the function, a check is made that result type matches int.

As some examples of types:

  • The type typing.Any matches any value, with no restrictions.
  • The types int, bool, str all represent the values produced by the respective functions.
  • The type None represents the value None.
  • The type list[int] represents a list of int types, e.g. list[typing.Any] represents a list containing any types.
  • The type dict[int, bool] represents a dictionary with int keys and bool values.
  • The type tuple[int, bool, str] represents a tuple of arity 3 with components being int, bool and str.
  • The type tuple[int, ...] represents a tuple of unknown arity where all the components are of type int.
  • The type int | bool represents a value that is either an int or a bool.
  • The type typing.Callable represents something that can be called as a function.
  • The type typing.Iterable represents something that can be iterated on.
  • The type typing.Never represents a type with no valid values - e.g. the result of fail is typing.Never as the return value of fail can never be observed, given the program terminates.

The goals of this type system are:

  • Reuse the existing machinery of Starlark as much as possible, avoiding inventing a special class of type values. As a consequence, any optimisations for values like string/list are reused.
  • Provide a pleasing syntax.
  • Some degree of compatibility with Python, which allows types as expressions in the same places Buck2 allows them (but with different meaning and different checking).
  • And finally, a non-goal is to provide a complete type system capable of representing every type invariant: it's intended to be a lossy approximation.

In addition to these built-in types, records and enumerations are provided as special concepts.

Record types

A record type represents a set of named values, each with their own type.

For example:

MyRecord = record(host=str, port=int)

This above statement defines a record MyRecord with 2 fields, the first named host that must be of type str, and the second named port that must be of type int.

Now MyRecord is defined, it's possible to do the following:

  • Create values of this type with MyRecord(host="localhost", port=80). It is a runtime error if any arguments are missed, of the wrong type, or if any unexpected arguments are given.
  • Get the type of the record suitable for a type annotation with MyRecord.
  • Get the fields of the record. For example, v = MyRecord(host="localhost", port=80) will provide v.host == "localhost" and v.port == 80. Similarly, dir(v) == ["host", "port"].

It is also possible to specify default values for parameters using the field function.

For example:

MyRecord = record(host=str, port=field(int, 80))

Now the port field can be omitted, defaulting to 80 is not present (for example, MyRecord(host="localhost").port == 80).

Records are stored deduplicating their field names, making them more memory efficient than dictionaries.

Enum types

The enum type represents one value picked from a set of values.

For example:

MyEnum = enum("option1", "option2", "option3")

This statement defines an enumeration MyEnum that consists of the three values "option1", "option2" and "option3".

Now MyEnum is defined, it's possible to do the following:

  • Create values of this type with MyEnum("option2"). It is a runtime error if the argument is not one of the predeclared values of the enumeration.
  • Get the type of the enum suitable for a type annotation with MyEnum.
  • Given a value of the enum (for example, v = MyEnum("option2")), get the underlying value v.value == "option2" or the index in the enumeration v.index == 1.
  • Get a list of the values that make up the array with MyEnum.values() == ["option1", "option2", "option3"].
  • Treat MyEnum a bit like an array, with len(MyEnum) == 3, MyEnum[1] == MyEnum("option2") and iteration over enums [x.value for x in MyEnum] == ["option1", "option2", "option3"].

Enumeration types store each value once, which are then efficiently referenced by enumeration values.