Make implicit names as unique as possible

When declaring an implicit value or method, try to give it a name that minimises the chances of conflicting with other libraries.

Reason

Implicit names shadow each other and confuse the compiler (and developers).

For example:

object foo {
  implicit val bar: Int = 1
}

object baz {
  implicit val bar: String = "foo"
}

Note how both foo and baz contain a value called bar, even though they do not share a type and only one is implicit.

This yields the following, confusing behaviour:

import foo._, baz._

implicitly[Int]
// error: could not find implicit value for parameter e: Int
// implicitly[Int]
// ^^^^^^^^^^^^^^^

There’s a single implicit value of type Int in scope - foo.bar. But the compiler can’t find it.

One way of demonstrating why the compiler cannot find it is:

import foo._, baz._

val a: String = bar
// error: reference to bar is ambiguous;
// it is imported twice in the same scope by
// import baz._
// and import foo._
// val a: String = bar
//                 ^^^

Even in the presence of sufficient type information (we know that a is a String, and only one of the two bars in scope is a String), the compiler considers two clashing names to be ambiguous and demands the ambiguity fixed.

The problem is easy to understand and fix in this last example - the error message is quite explicit. But in the context of implicit resolution, we’re just getting a generic could not find implicit value error message, which can be a bit of a nigthmare to debug.

In order to avoid this, and since you cannot control how other libraries name their implicits, you must:

Exceptions to the rule

There’s at least one scenario in which an implicit’s name isn’t used during resolution: when you declare an implicit of type T in the implicit scope of T.

For example:

trait Foo[A]
object Foo {
  implicit val foo: Foo[Int] = new Foo[Int] {}
}

trait Bar[A]
object Bar {
  implicit val foo: Bar[String] = new Bar[String] {}
}

Even though both implicit values share the name foo, implicit resolution works out:

// Brings Foo.foo and Bar.foo in scope
import Foo._, Bar._

// No conflict here
implicitly[Foo[Int]]
// res3: Foo[Int] = repl.Session$App2$Foo$$anon$1@6c5a331d

In this scenario, it’s safe to give terse names to your implicits.

Checked by

This rule is not checked by any linter that I know of.