CODEWITHSUNDEEP
scalatype-systems
pathikrit
pathikrit

Reputation: 33469

How can I have a negation type in Scala?

I want to do something like this:

def iDontLikeStrings(arg: Not[String]) = {....}

Basically, this should compile:

iDontLikeStrings(23) 
iDontLikeStrings(true)

And this should NOT compile:

iDontLikeStrings("hello")

Upvotes: 4

Views: 766

Answers (2)

Martin Ring
Martin Ring

Reputation: 5426

In scala 3 this has become fairly simple:

import scala.util.NotGiven

def iDontLikeStrings[T: [T] =>> NotGiven[T =:= String]](t: T) = {....}

Upvotes: 2

pathikrit
pathikrit

Reputation: 33469

Here is my implementation (see gist):

Step 1: Encoding to capture A is not a subtype of B

trait NotSubTypeOf[A, B]

Note: We can use infix notation to write A NotSubTypeOf B instead of NotSubTypeOf[A, B]

Step 2: Evidence for any two arbitrary types A and B, A is not a subtype of B

implicit def isSub[A, B]: A NotSubTypeOf B = null

Step 3: Define ambigious implicits to trigger compile error in case A is a subtype of B (or A =:= B)

implicit def iSubAmbig1[A, B >: A]: A NotSubTypeOf B = null
implicit def iSubAmbig2[A, B >: A]: A NotSubTypeOf B = null

Step 4: Define a type-lambda for the negation type: 

type Not[T] = {
  type L[U] = U NotSubTypeOf T
}

With kind-projector, this can be made much more readable.

Step 5: Done!

def iDontLikeStrings[A: Not[String]#L](a: A) = {
  println(a)
}

iDontLikeStrings(23)   // compiles
iDontLikeStrings(true) // compiles
//iDontLikeStrings("hello") // does not compile

The compiler error message for the last line can be made better in Scala 2.12 which addresses SI-6806.

Also, all of this is built into Shapeless.

Upvotes: 7

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