Explicit types

It is possible to explicitly specify the types of variables or arguments in a function. The syntax is similar to type hints in Python.


# 'a: int' means the first argument `a` should be of type int
# 'b: str means the second argument `b` should be of type str
# The function returns nothing
def foo(a: int, b: str) -> None:
    print(a, b)
# A functions type signature can also be written on a separate line
bar : (int, str) -> None
def bar(a, b):
    print(a, b)

actor main(env):
    # i1 is explicitly specified as an integer while s1 is a str
    i1 : int = 1234
    s1 : str = "hello"
    foo(i1, s1)
    bar(i1, s1)

    # The literal value 1234 is an integer, so when we assign it to i2, the
    # compiler can easily infer that the type of i2 is int. Similarly for s2
    # since the literal value "hello" is clearly a string
    i2 = 1234
    s2 = "hello"
    foo(i2, s2)
    bar(i2, s2)

Compile and run:

actonc types.act


1234 hello
1234 hello
1234 hello
1234 hello

Try changing the type of i1 or s1 and you will find that the compiler complains that it cannot solve the type constraints of the program.