herolib/aiprompts/v_advanced/generics.md

struct Repo[T] {
    db DB
}

struct User {
    id   int
    name string
}

struct Post {
    id   int
    user_id int
    title string
    body string
}

fn new_repo[T](db DB) Repo[T] {
    return Repo[T]{db: db}
}

// This is a generic function. V will generate it for every type it's used with.
fn (r Repo[T]) find_by_id(id int) ?T {
    table_name := T.name // in this example getting the name of the type gives us the table name
    return r.db.query_one[T]('select * from ${table_name} where id = ?', id)
}

db := new_db()
users_repo := new_repo[User](db) // returns Repo[User]
posts_repo := new_repo[Post](db) // returns Repo[Post]
user := users_repo.find_by_id(1)? // find_by_id[User]
post := posts_repo.find_by_id(1)? // find_by_id[Post]

Currently generic function definitions must declare their type parameters, but in future V will infer generic type parameters from single-letter type names in runtime parameter types. This is why find_by_id can omit [T], because the receiver argument r uses a generic type T.

fn compare[T](a T, b T) int {
    if a < b {
        return -1
    }
    if a > b {
        return 1
    }
    return 0
}

// compare[int]
println(compare(1, 0)) // Outputs: 1
println(compare(1, 1)) //          0
println(compare(1, 2)) //         -1
// compare[string]
println(compare('1', '0')) // Outputs: 1
println(compare('1', '1')) //          0
println(compare('1', '2')) //         -1
// compare[f64]
println(compare(1.1, 1.0)) // Outputs: 1
println(compare(1.1, 1.1)) //          0
println(compare(1.1, 1.2)) //         -1