Enforce type difference

by

I have a simpler solution, which also leverages ambiguity,

trait =!=[A, B]

implicit def neq[A, B] : A =!= B = null

// This pair excludes the A =:= B case
implicit def neqAmbig1[A] : A =!= A = null
implicit def neqAmbig2[A] : A =!= A = null

The original use case,

case class Foo[A,B](a : A, b : B)(implicit ev: A =!= B)
new Foo(1, "1")
new Foo("foo", Some("foo"))

// These don't compile
// new Foo(1, 1)
// new Foo("foo", "foo")
// new Foo(Some("foo"), Some("foo"))

Update

We can relate this to my “magical typesystem tricks” (thanks @jpp 😉 as follows,

type ¬[T] = T => Nothing
implicit def neg[T, U](t : T)(implicit ev : T =!= U) : ¬[U] = null

def notString[T <% ¬[String]](t : T) = t

Sample REPL session,

scala> val ns1 = notString(1)
ns1: Int = 1

scala> val ns2 = notString(1.0)
ns2: Double = 1.0

scala> val ns3 = notString(Some("foo"))
ns3: Some[java.lang.String] = Some(foo)

scala> val ns4 = notString("foo")
<console>:14: error: No implicit view available from 
  java.lang.String => (String) => Nothing.
       val ns4 = notString2("foo")
                            ^

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