Passing a singleton as argument in Scala

Question

I have this:

final case class MyClass(....) extends G with M[MyClass]

object MyObject extends GMC[MyClass] {....}

trait GMC[A <: G with M[A]] {....}

def f(gmc: GMC[G])

f(MyObject)

I get compiler error as:

Type mismatch: Required GMC[G], found MyObject.type

Is this wrong way of passing singleton?

Does X conforms to T[A] what is A? Which type did X used when inheriting T? Is T covariant, contravariant or invariant? — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented Mar 16, 2020 at 13:06
Please show us a complete, self-contained example, ie. the exact same code you use which reproduces the compiler error. — cbley
– cbley, Commented Mar 16, 2020 at 13:07

Tim · Accepted Answer · 2020-03-16 14:20:09Z

2

Answer to updated question

The problem lies with the definition of f. If you remove the call of f you get this error message:

type arguments [G] do not conform to trait GMC's type parameter bounds [A <: G with M[A]]

In other words, you can't use a bare G as a type parameter for GMC where the type constraint for GMC is G with M[A].

You need to constrain the type that f takes to match the definition of GMC:

def f[A <: G with M[A]](gmc: GMC[A]): Unit = ???

Previous answer

You need to specify the type parameter for T when you create the object:

object X extends T[A] {....}

If you make that change it will compile:

class A
trait T[A]
object X extends T[A] {}
def f(t: T[A]) = {}

f(X) // No error

edited Mar 16, 2020 at 14:20

answered Mar 16, 2020 at 12:57

Tim

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