What precisely is a scala evidence parameter

Question

I\'ve been trying to find an authoritative definition of what is an evidence parameter, to no avail, for solving a case of \"could not find implicit value for evidence parameter

Answer 1

The language specification uses the term "evidence" in §7.4 Context Bounds and View Bounds:

A type parameter A of a method or non-trait class may also have one or more context bounds A : T. In this case the type parameter may be instantiated to any type S for which evidence exists at the instantiation point that S satisfies the bound T. Such evidence consists of an implicit value with type T[S].

With this syntactic sugar you get synthetic parameters which the spec calls "evidence parameters". (Note that this also covers view-bounds <% which are now deprecated).

Since often explicitly written implicit parameters are also named evidence, I think it's valid to call any implicit parameter an "evidence" if it witnesses a specific property of a type. Take for example <:< [A, B] that evidences that A is a sub-type of B:

trait Foo

trait Bar[A] {
  def baz(implicit evidence: A <:< Foo): Unit
}

Then if you try this:

trait Test {
  def bar: Bar[Any]

  bar.baz
}

This fails with a compilation error:

:58: error: Cannot prove that Any <:< Foo.
         bar.baz
             ^

The exact wording can be specified with the implicitNotFound annotation. Without a specific code example, it's unclear what generates the "could not find implicit value for evidence parameter of type....".

Here's an example of a custom message:

@annotation.implicitNotFound(msg = "Oh noes! No type class for ${A}")
trait MyTypeClass[A]

trait Bar[A] {
  def baz(implicit evidence: MyTypeClass[A]): Unit
}

Then:

trait Test {
  def bar: Bar[Any]

  bar.baz
}

Fails with the custom message:

:58: error: Oh noes! No type class for Any
         bar.baz
             ^