1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
|
package magnolia.examples
import magnolia._
import scala.language.experimental.macros
/** shows one type as another, often as a string
*
* Note that this is a more general form of `Show` than is usual, as it permits the return type to
* be something other than a string. */
trait Show[Out, T] { def show(value: T): Out }
trait GenericShow[Out] {
/** the type constructor for new [[Show]] instances
*
* The first parameter is fixed as `String`, and the second parameter varies generically. */
type Typeclass[T] = Show[Out, T]
def join(typeName: String, strings: Seq[String]): Out
/** creates a new [[Show]] instance by labelling and joining (with `mkString`) the result of
* showing each parameter, and prefixing it with the class name */
def combine[T](ctx: CaseClass[Typeclass, T]): Show[Out, T] = new Show[Out, T] {
def show(value: T) = {
val paramStrings = ctx.parameters.map { param =>
s"${param.label}=${param.typeclass.show(param.dereference(value))}"
}
join(ctx.typeName.split("\\.").last, paramStrings)
}
}
/** choose which typeclass to use based on the subtype of the sealed trait */
def dispatch[T](ctx: SealedTrait[Typeclass, T]): Show[Out, T] = new Show[Out, T] {
def show(value: T): Out = ctx.dispatch(value) { sub =>
sub.typeclass.show(sub.cast(value))
}
}
/** bind the Magnolia macro to this derivation object */
implicit def gen[T]: Show[Out, T] = macro Magnolia.gen[T]
}
/** companion object to [[Show]] */
object Show extends GenericShow[String] {
/** show typeclass for strings */
implicit val string: Show[String, String] = new Show[String, String] {
def show(s: String): String = s
}
def join(typeName: String, params: Seq[String]): String =
params.mkString(s"$typeName(", ",", ")")
/** show typeclass for integers */
implicit val int: Show[String, Int] = new Show[String, Int] {
def show(s: Int): String = s.toString
}
}
|