doobie-composite.scala

case class Foo(a: Int, b: String, c: Bar)
case class Bar(x: Int, y: Baz)
case class Baz(z: String)

// Each case class can be generalized in to a product, where that product is just the parts of the case class

// Foo === Int :: String :: Bar :: HNil
// Bar === Int :: Baz :: HNil
// Baz === String :: HNil

// doobie then flattens the structure, so 
// `Bar` is substituted with Int :: Baz :: HNil, 
// and `Baz` is substituted with `String :: HNil`.

// So, a Foo is really `Int :: String :: Int :: String :: HNil`.  
// Doobie will derive a composite instance from this and use the position for the mappings.