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scalaimplicitsabstract-type

Find implicit value by abstract type member

With a type like trait A[T], finding an implicit in scope is simply implicitly[A[SomeType]]

Can this be done and, if so, how is this done where the type-parameter is replaced with an abstract type member, like in trait A { type T }?

Solution

  • You can do

    implicitly[A { type T = Int }

    but you risk to lose precision:

    scala> trait Foo { type T ; val t: T }
defined trait Foo

scala> implicit val intFoo: Foo { type T = Int } = new Foo { type T = Int ; val t = 23 }
intFoo: Foo{type T = Int} = $anon$1@6067b682

scala> implicitly[Foo].t // implicitly loses precision
res0: Foo#T = 23

    To solve this problem, you can use the newly introduced the method, from the shapeless library (from which I've take the example above)

    scala> the[Foo].t // the retains it
res1: Int = 23

scala> the[Foo].t+13
res2: Int = 36