Evidence that types are not equal in Scala

Question

Is there any way to constraint a method so that it only makes sense if two types are proved not to be equal?

trait Something[A, B] {
  // I can only be called if type A is the same as type B
  def ifEqual(implicit ev: A =:= B)

  // Now I cannot be called if type A is proven to be the same as type B
  def ifNotEqual(implicit ev: A ??? B)
}

score 14 · Accepted Answer · answered Jul 01 '14 at 21:35

14

Yes. From shapeless,

// Type inequalities
trait =:!=[A, B] 

implicit def neq[A, B] : A =:!= B = new =:!=[A, B] {}
implicit def neqAmbig1[A] : A =:!= A = ???
implicit def neqAmbig2[A] : A =:!= A = ???

answered Jul 01 '14 at 21:35

Miles Sabin

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As always, amazing type-fu! – Hugo Sereno Ferreira Jul 01 '14 at 21:51
3

This must be a duplicate, except it's so cool Miles stepped in to answer it. It's like that scene from Annie Hall when Marshall McLuhan shows up in the movie line. "If life were only like this!" – som-snytt Jul 02 '14 at 03:12
Defintely a duplicate. See http://stackoverflow.com/questions/6909053/enforce-type-difference. – Régis Jean-Gilles Jul 02 '14 at 08:11

Evidence that types are not equal in Scala

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