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aiprompts/v_advanced/generics.md

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+
+```v
+
+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.
+
+```v
+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
+```
+