Callable objects (Reference)

std.traits.callable declares three traits, for values that are called like functions with zero, one or two arguments. A bound names a trait imported from std.traits.callable. Function types, the Callable[Params, R] sugar and mut parameters are in Functions and calls.

Trait	Hook	Call form
Callable0[R]	def __call__(self) -> R	value()
Callable1[A, R]	def __call__(self, arg: A) -> R	value(x)
Callable2[A, B, R]	def __call__(self, a: A, b: B) -> R	value(x, y)

CallableN bounds

A type parameter bounded by CallableN[...] accepts:

• a named function or a closure with exactly N parameters, whose parameter types match the bound's leading type arguments and whose return type matches its last one; a closure's parameters take their types from the bound;
• a value of a model or class that adopts the same CallableN[...] with with and defines __call__.

Inside the function, the call form calls the value: a function or closure runs, and an adopting type runs its __call__. A worked program: Accept a function, a closure or a callable object.

from std.traits.callable import Callable1

def apply[M with Callable1[int, str]](mapper: M, value: int) -> str:
    return mapper(value)

Call	Result
apply((v) => f"item:{v}", 3)	"item:3"
apply(Prefixer(prefix="model"), 4), Prefixer adopting Callable1[int, str]	its __call__(4)
apply(shout, 5), shout of type (str) -> str	refused: INCAN-T0001

Fn, FnMut and FnOnce markers

A type parameter of a function or method bounded by Fn[...], FnMut[...] or FnOnce[...], imported from std.rust, with at most two type arguments, accepts a named function whose parameters match the marker's type arguments in number and type. The marker names no return type: the function's return type is the one it declares.

Refusals

Refused	Code
A function or closure whose parameter count, parameter types or return type differ from the bound	INCAN-T0001
A value of a type that does not adopt the bound's CallableN[...]	INCAN-T0001
A function whose parameter count or parameter types differ from an Fn, FnMut or FnOnce marker	INCAN-T0001
An Fn, FnMut or FnOnce marker from std.rust naming more than two parameters, or bounding a type parameter of a type alias, model, class, trait, enum or newtype	INCAN-T0106

See also

• Function types and callable traits
• Accept a function, a closure or a callable object