cairo by example

generics

Generic types allow us to create definitions that may be used with different data types.

In Cairo, you can use generics when defining functions, structs, enums, traits, implementations, and methods.

Example of a generic function:

// We can use this function to compare any integer (e.g. u8, u32, u256)
fn largest<T>(t1: T, t2: T) -> T {
    if t1 > t2 {
        t1
    } else {
        t2
    }
}

If you tried to run the previous function, you might had a compiler error saying Trait has no implementation in context ..., this happens because the compiler requires that some traits must be implemented for various reasons.

In our example we must implement 3 traits:

• PartialOrd: For the comparison to work
• Drop: Since we are dropping the value that is not returned
• Copy: To move the function inputs

Fix for the previous example:

fn largest<
    T,
    impl TOrd: PartialOrd<T>,
    impl TDrop: Drop<T>,
    impl TCopy: Copy<T>
    >(t1: T, t2: T) -> T {
    if t1 > t2 {
        t1
    } else {
        t2
    }
}

Here is a full example:

// We can use this function to compare any integer (e.g. u8, u32, u256)
fn largest<
    T,
    impl TOrd: PartialOrd<T>,
    impl TDrop: Drop<T>,
    impl TCopy: Copy<T>
    >(t1: T, t2: T) -> T {
    if t1 > t2 {
        t1
    } else {
        t2
    }
}

// You can check the result by running it
fn main() -> u8 {
    let a: u8 = 1;
    let b: u8 = 2;
    largest(a, b)
}

You can run the example by using cairo-run --single-file generics.cairo.