Scala, generic tuple - scala

Question

I have a generic method that can accept any tuple of any size, the only constraint is that the first element of this tuple should be of type MyClass.
Something like this:
trait MyTrait[T <: (MyClass, _*)] {
getMyClass(x: T): MyClass = x._1
}
I've tried this
trait MyTrait[T <: (MyClass, _) with (MyClass, _, _) with (MyClass, _, _) with ...] {
getMyClass(x: T): MyClass = x._1
}
but I get the error unboud wildcard type

Answer 1

If you want to do this without either boilerplate or runtime reflection, Shapeless is your best bet. You can use the IsComposite type class to put type-level constraints on the first element of a tuple:
import shapeless.ops.tuple.IsComposite
trait MustBeFirst
class MyClass[P <: Product](p: P)(implicit ev: IsComposite[P] { type H = MustBeFirst }) {
def getMustBeFirst(x: P): MustBeFirst = ev.head(p)
}
And then:
scala> val good2 = (new MustBeFirst {}, "")
good2: (MustBeFirst, String) = ($anon$1#7294acee,"")
scala> val good3 = (new MustBeFirst {}, "", 123)
good3: (MustBeFirst, String, Int) = ($anon$1#6eff9288,"",123)
scala> val good4 = (new MustBeFirst {}, "", 'xyz, 123)
good4: (MustBeFirst, String, Symbol, Int) = ($anon$1#108cdf99,"",'xyz,123)
scala> val bad2 = ("abc", 123)
bad2: (String, Int) = (abc,123)
scala> new MyClass(good2)
res0: MyClass[(MustBeFirst, String)] = MyClass#5297aa76
scala> new MyClass(good3)
res1: MyClass[(MustBeFirst, String, Int)] = MyClass#3f501844
scala> new MyClass(good4)
res2: MyClass[(MustBeFirst, String, Symbol, Int)] = MyClass#24e15478
scala> new MyClass(bad2)
<console>:15: error: could not find implicit value for parameter ev: shapeless.ops.tuple.IsComposite[(String, Int)]{type H = MustBeFirst}
new MyClass(bad2)
^
If you need to use a trait, you can put the ev (for "evidence") requirement inside the definition instead of in the constructor:
trait MyTrait[P <: Product] {
implicit def ev: IsComposite[P] { type H = MustBeFirst }
}
Now any class instantiating MyTrait will have to provide evidence that P is a tuple with MustBeFirst as its first element.

Answer 2

It's a little bit unsafe but you can use Structural type in this case:
trait MyTrait {
def getMyClass(x: {def _1: MyClass}): MyClass = x._1
}

Answer 3

Scala can't use generic tuple with unknown size because Products don's inherit themeselfs. You can try to use Shapeless or Products from play json lib.

Answer 4

This is now possible in Scala 3 and very straightforward:
class MyClass
trait MyTrait[T <: MyClass *: _] {
def getMyClass(x: T): MyClass = x.head
}
The *: infix operator is the type level equivalent of the +: sequence prepend (or :: for lists). So we essentially require type T to be a tuple such that its first member is of type MyClass. Note that the members of generic tuples cannot be retrieved using the usual _1, _2, ... attributes, but should be accessed using list-like methods (head, tail, apply, etc.). More surprising, these methods are type-safe since they carry precise type information.
Demo:
object Test1 extends MyTrait[(MyClass, Int, String)] // Compiles
//object Test2 extends MyTrait[(Int, String)] // Does not compile!
//object Test3 extends MyTrait[EmptyTuple] // Neither does this
val myClass = Test1.getMyClass((new MyClass, 1, "abc"))
summon[myClass.type <:< MyClass] // Compiles
Read more about match types, type inference.

Answer 5

You need to inherit your trait from Product, through which you can have productIterator, productArity and, productElement to handle the returned value. Here is an example
case class MyClass()
trait MyTrait[T <: Product] {
def getMyClass(x: T): Option[MyClass] =
if(
x.productIterator.hasNext
&&
x.productIterator.next().isInstanceOf[MyClass]
){
Some(x.productIterator.next().asInstanceOf[MyClass])
} else {
None
}
}
case class Test() extends MyTrait[Product]
And you can invoke like this
Test().getMyClass((MyClass(), 1,3,4,5))
//res1: Option[MyClass] = Some(MyClass())
Test().getMyClass((1,3,4,5))
//res2: Option[MyClass] = None
Hope this helps you.

Answer 6

If you are looking for compile time guarantee then this is one of the use cases for
Shapeless,
You need to add Shapeless in your build.sbt,
libraryDependencies ++= Seq("
com.chuusai" %% "shapeless" % "2.3.3"
)
Now, you can use Shapeless to define a typesafe getter which comes with compile time guarantees,
scala> import shapeless._
// import shapeless._
scala> import ops.tuple.IsComposite
// import ops.tuple.IsComposite
scala> import syntax.std.tuple._
// import syntax.std.tuple._
scala> case class Omg(omg: String)
// defined class Omg
scala> val myStringTuple = ("All is well", 42, "hope")
// myStringTuple: (String, Int, String) = (All is well,42,hope)
scala> val myOmgTuple = (Omg("All is well"), 42, "hope")
// myOmgTuple: (Omg, Int, String) = (Omg(All is well),42,hope)
Now if you want to enrich your tuples with a "first" getter with a specific type then,
scala> implicit class GetterForProduct[B <: Product](b: B) {
| def getFirst[A](implicit comp: IsComposite[B] { type H = A }): A = b.head
| }
// defined class GetterForProduct
scala> val myString = myStringTuple.getFirst[String]
// myString: String = All is well
scala> val myOmgError = myOmgTuple.getFirst[String]
// <console>:24: error: could not find implicit value for parameter comp: shapeless.ops.tuple.IsComposite[(Omg, Int, String)]{type H = String}
// val myOmgError = myOmgTuple.getFirst[String]
// ^
scala> val myOmg = myOmgTuple.getFirst[Omg]
// myOmg: Omg = Omg(All is well
If you don't need the implicit enrichment and are just looking for a way to "lock" the type in a getter and use it for corresponding types,
scala> trait FirstGetterInProduct[A] {
| def getFirst[B <: Product](b: B)(implicit comp: IsComposite[B] { type H = A }): A = b.head
| }
// defined trait FirstGetterInProduct
scala> object firstGetterInProductForString extends FirstGetterInProduct[String]
// defined object firstGetterInProductForString
scala> object firstGetterInProductForOmg extends FirstGetterInProduct[Omg]
// defined object firstGetterInProductForOmg
// Right tuple with right getter,
scala> val myString = firstGetterInProductForString.getFirst(myStringTuple)
// myString: String = All is well
// will fail at compile time for tuple with different type for first
scala> val myOmgError = firstGetterInProductForString.getFirst(myOmgTuple)
// <console>:23: error: could not find implicit value for parameter comp: shapeless.ops.tuple.IsComposite[(Omg, Int, String)]{type H = String}
// val myOmgError = firstGetterInProductForString.getFirst(myOmgTuple)
// ^
scala> val myOmg = firstGetterInProductForOmg.getFirst(myOmgTuple)
// myOmg: Omg = Omg(All is well)