Decoder.scala

package foo

trait Decoder[E, D] {
  def decode(e: E): D
}

package.scala

package object foo {
  type StringDecoder[A] = Decoder[String, A]
}

REPL-session.txt

scala> import foo._
import foo._

scala> implicit val test: StringDecoder[Float] = StringDecoder(_.toFloat)
test: foo.StringDecoder[Float] = foo.StringDecoder$$anon$1@40a4337a

scala> StringDecoder.foobar
<console>:15: error: value foobar is not a member of object foo.StringDecoder
       StringDecoder.foobar

StringDecoder.scala

package foo

object StringDecoder {
  def apply[A](f: String => A): StringDecoder[A] = new StringDecoder[A] { override def decode(s: String) = f(s) }

  def apply[A](implicit da: StringDecoder[A]): StringDecoder[A] = da

  def foobar: Unit = println("foobar")
}