Small talk that will describe what is meta programming on example of Scala Shapeless library.

1. FLAVOUR OF M-
PROGRAMMING
WITH SHAPELESS
LETS COMPILER WRITE
CODE FOR US

2. ...lets imagine that your program its just a data.
Meta programming is about:
META
FLAVOUROFM-PROGRAMMINGWITHSHAPELESS
flatMap
Exposing part of compiler outside
Code generation
Moving to compile time evaluation

3. SHAPELESS
FLAVOUROFM-PROGRAMMINGWITHSHAPELESS
Library that implements heterogeneous list and bunch of macroses for it
How we can store different types
of data in one data structure?

4. SHAPELESS
FLAVOUROFM-PROGRAMMINGWITHSHAPELESS
Library that implements heterogeneous list and bunch of macroses for it
List[Any]
List[AbstractClass]
HList

5. SHAPELESS
FLAVOUROFM-PROGRAMMINGWITHSHAPELESS
Library that implements heterogeneous list and bunch of macroses for it
HList - its infinite static typed list based on
abstract data types.
Its like tuple: (String, Int, Long, ...), but
without limit on amount of elements inside.

6. SHAPELESS
FLAVOUROFM-PROGRAMMINGWITHSHAPELESS
Library that implements heterogeneous list and bunch of macroses for it
HList example:
val list: String :: Int :: HNil =
"Word" :: 99 :: HNil

7. SHAPELESS
FLAVOUROFM-PROGRAMMINGWITHSHAPELESS
Library that implements heterogeneous list and bunch of macroses for it
What we can do with
String :: Int :: HNil type
Everything!
def transform(hList: HList.Repr): Seq[String]

8. SHAPELESS
FLAVOUROFM-PROGRAMMINGWITHSHAPELESS
Library that implements heterogeneous list and bunch of macroses for it
String :: Int :: HNil
HList[String, HList[Int, HNil]]
and we want achieve Seq[String]
Basic types transformers: Int => Seq[String]
Shapeless type transformer: HList[A, B] => Seq[String]

9. SHAPELESS
FLAVOUROFM-PROGRAMMINGWITHSHAPELESS
Library that implements heterogeneous list and bunch of macroses for it
trait Transformer[A] {
def transform(a: A): Seq[String]
}
implicit val intT = new Transformer[Int] {
def transform(a: Int): Seq[String] = Seq(a.toString)
}

10. SHAPELESS
FLAVOUROFM-PROGRAMMINGWITHSHAPELESS
Library that implements heterogeneous list and bunch of macroses for it
// HList[String, HList[Int, HNil]]
implicit def hListT[H, T <: HList]
(implicit headT: Transformer[H],
tailT: Transformer[T]): Transformer[H :: T] = ???
implicit val hNilT = new Transformer[HNil] {
override def transform(a: HNil): Seq[String] = Nil
}

11. SHAPELESS
FLAVOUROFM-PROGRAMMINGWITHSHAPELESS
Library that implements heterogeneous list and bunch of macroses for it
implicitly[Transformer[String :: Int :: Nil]].transform(ourHList)
Search for implicit to convert HList[A, B]
Build HList[String, HList[B, C]]
Search for implicit to convert String
Search for implicit to convert Int and Nil
Provide converter from HList[Int, Nil] to Seq[String]
Provide converter from HList[String, HList[Int, Nil]] to Seq!

12. USE CASE
FLAVOUROFM-PROGRAMMINGWITHSHAPELESS
case class User (
name: String,
age: Int
)
User("MyName", 99)
def serialize(user: User): String = ???
// "MyName, 99"

13. USE CASE
FLAVOUROFM-PROGRAMMINGWITHSHAPELESS
implicit val customerGen = Generic[Customer]
def createEncoder[A, R]
(implicit gen: Generic.Aux[A, R],
encoder: Transformer[R]): Transformer[A] =
new Transformer[A] {
override def transform(a: A) =
encoder.encode(gen.to(a))
}

14. THANKS!
TIME TO ARGUE