Example of generics in V. Implements the V documentation skeleton about Generics

generics.v

// Example of generics in V. 
// Its taken from the quasi-complete code in the documentation 
// https://github.com/vlang/v/blob/master/doc/docs.md#generics
// credit:  lazalong@gmail.com
module main

interface IData {}

struct User {
	id int
	name string
}

struct Post {
	id int
	user_id int
	title string
}

[heap]
struct DB {
pub mut:
	data map[string]map[int]IData // nested maps
}

pub fn new_db() DB {
	mut db := DB{}
	db.data = map[string]map[int]IData{}
	return db
}

pub fn (mut db DB[T]) query_one<T>(t string, id int) T {
//	if id in db.data[T.name] {
	if x := db.data[T.name][id] {
		if x is T {
			return *x
		}
	}
// }
	return T{}
}

/* alternative but then the none must be handled
pub fn (mut db DB[T]) query_one<T>(t string, id int) ?T {
    if x := db.data[T.name][id] {
        if x is T {
            println('found ${id} | x: $x')
            return *x
        }
    }
    return none
}
*/ 

pub fn (mut db DB) add<T>(data T) {
	db.data[T.name][data.id] = IData(data)
}

struct Repo<T> {
mut:
	db &DB
}

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 (mut 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)
}

pub fn (mut r Repo<T>) add<T>(data T) {
	r.db.add(data)
}

fn main() {
	db := new_db()
   mut users_repo := new_repo<User>(&db) // returns Repo<User>
	mut posts_repo := new_repo<Post>(&db) // returns Repo<Post>

	users_repo.add(User{1, 'test'})
	users_repo.add(User{2, 'test2'})
	posts_repo.add(Post{3, 4, 'bob' })
	users_repo.add(User{4, 'test3'})

	user := users_repo.find_by_id(1)? // find_by_id<User>
	post := posts_repo.find_by_id(6)? // find_by_id<Post>

	println ('user 1: ${user}')
	println ('post 3: ${post}')

	println('User all values ${db.data['User']}')
	println('Post all values ${db.data['Post']}')
}