Scala Implicits and Type Classes 2

Question 1

What is the primary use of an implicit class in Scala 2?

Accepted Answer

To add extension methods to an existing type

Answer

An implicit class wraps an existing type so the compiler can automatically apply its methods when called on values of the wrapped type, effectively adding extension methods.

Question 2

In Scala 3, which keyword marks a parameter as one that should be filled by a `given` instance?

Accepted Answer

using

Answer

Scala 3 uses `using` in the parameter list to declare that an argument should be supplied by a `given` instance from the scope.

Question 3

Which syntax correctly defines a `Show[Int]` type class instance in Scala 2?

Accepted Answer

implicit val intShow: Show[Int] = new Show[Int] { def show(n: Int): String = n.toString }

Answer

In Scala 2, type class instances are provided as `implicit val` (or `implicit object`) definitions that the compiler can inject automatically.

Question 4

What does the `@implicitNotFound` annotation do in Scala?

Accepted Answer

Provides a custom, user-friendly error message when an implicit for that type cannot be found

Answer

`@implicitNotFound("...")` placed on a type class trait lets library authors show a descriptive error message instead of the default compiler message when an instance is missing.

Question 5

In Scala 2, in what priority order does the compiler search for implicit values?

Accepted Answer

Local scope first, then imports, then companion objects of involved types

Answer

Scala 2 searches for implicits in local scope (highest priority), then imported scope, then the companion objects of the types involved (the 'implicit scope').

Scala Practice Test

Scala Practice Test

Scala Implicits and Type Classes 2