Defining functions

Simple example

Use the fn keyword to create a function. Use -> to specify its return type. Use return to return a value from a function. All the code paths must return a value of the specified return type.

fn add_two_i32s(a: i32, b: i32) -> i32:
    return a + b

If a function does not return anything (i.e its return type is void), its return type can be omitted:

fn print_i32(x: i32):
    println(x)

Recursive and mutually recursive functions

In Boa, functions can be recursive and mutually recursive:

// A recursive function

fn factorial(i: u32) -> u32:
    return 1 if i == 0 else i * factorial(i - 1)

// Two mutually recursive functions

fn is_even(i: u32) -> bool:
    return true if i == 0 else is_odd(i - 1)

fn is_odd(i: u32) -> bool:
    return false if i == 0 else is_even(i - 1)

Tail-call optimization (TCO)

Functions that can be optimized with TCO will be optimized to a simple loop. Here is an example of such a function:

fn factorial2(i: i32, accumulator: u32) -> u32:
    return accumulator if i == 0 else factorial2(i - 1, i * accumulator)

If you do not understand what this whole subsection is about, it's okay. Just continue to the next section!

First-class functions

In Boa, functions are said to be "first-class", meaning that they can be stored inside variables and passed around.

Higher-order functions