Consider the following code snippet:
trait Foo {
type T
def value: T
}
object Foo {
def apply[A](v: A): Foo = new Foo {
override type T = A
override def value = v
}
}
trait Decode[A] {
def apply(x: A): String
}
object Decode {
def apply[A](f: A => String): Decode[A] = new Decode[A] {
override def apply(x: A) = f(x)
}
implicit val decodeStr: Decode[String] = Decode(identity)
}
class Sandbox {
def decodeFoo(foo: Foo)(implicit decoder: Decode[foo.T]): String =
decoder(foo.value)
val foo = Foo("hello")
println(decodeFoo(foo))
}
The above code should work fine and print hello
, but instead it will not compile:
could not find implicit value for parameter decoder: Decode[Sandbox.this.foo.T]
[error] println(decodeFoo(foo))
Even when I explicitly pass an implicit parameter:
println(decodeFoo(foo = foo)(decoder = Decode.decodeStr))
I still get this error:
type mismatch;
[error] found : Decode[String]
[error] required: Decode[Sandbox.this.foo.T]
[error] println(decodeFoo(foo = foo)(decoder = Decode.decodeStr))
[error] ^
Of course, I can do Foo
a Foo[T]
and define the decoders for it, but this is not the point of this question - I want to understand why the code above is not compiled.
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