Scala 3: getting rid of implicit. Typeclasses

My previous article was about implicit conversions and extension methods. In this article, we'll discuss the new way to declare typeclasses in Scala 3.

Having learned how to add external methods to arbitrary classes, we want to go even deeper, namely to learn how to convert arbitrary classes to "external" interfaces, that is, without directly inheriting from them. This task is solved by typeclasses.

But first, let's figure out what a typeclass is. Like the concept itself, the term " type class" originated in Haskell. The word "class" is used here not in the narrow sense that is accepted in OOP, but in a broader sense - as a designation of a set of entities that have something in common. (I understand that most people who will read this article have an OOP background, and for them the term "typeclass" sounds something like "oil of oils", although it means "category of oils". To avoid confusion with conventional OOP -classes, instead of "type class" I will use simply transliteration "typeclass" - approx. transl.)

The syntax of the examples is up to date Scala 3.0.0-M3



.

, , , . Scala 3:

// Adapted from this Dotty documentation:
// https://dotty.epfl.ch/docs/reference/contextual/type-classes.html

trait Semigroup[T]:
  extension (t: T)
    def combine(other: T): T
    def <+>(other: T): T = t.combine(other)

trait Monoid[T] extends Semigroup[T]:
  def unit: T
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

, <+>



. — , , 0 — . , Semigroup



Monoid



.

Semigroup



T



extension- combine



<+>



, combine



. unit



Monoid



, extension-. , unit



T



, , , T



, .

:

given StringMonoid: Monoid[String] with
  def unit: String = ""
  extension (s: String) def combine(other: String): String = s + other

given IntMonoid: Monoid[Int] with
  def unit: Int = 0
  extension (i: Int) def combine(other: Int): Int = i + other
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

. , given foo: Bar



— implicit-. Scala3 REPL, , : StringMonoid



IntMonoid



.

- :

"2" <+> ("3" <+> "4")             // "234"
("2" <+> "3") <+> "4"             // "234"
StringMonoid.unit <+> "2"         // "2"
"2" <+> StringMonoid.unit         // "2"

2 <+> (3 <+> 4)                   // 9
(2 <+> 3) <+> 4                   // 9
IntMonoid.unit <+> 2              // 2
2 <+> IntMonoid.unit              // 2
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

StringMonoid



IntMonoid



unit



. <+>



extension-, String



Int



. <+>



, .

: given Monoid[String] with ...



. unit



summon[Monoid[String]]



. summon



— implicitly



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). .

, . , , IntMonoid



Numeric[T]



:

given NumericMonoid[T](using num: Numeric[T]): Monoid[T] with
  def unit: T = num.zero
  extension (t: T) def combine(other: T): T = num.plus(t, other)

2.2 <+> (3.3 <+> 4.4)             // 9.9
(2.2 <+> 3.3) <+> 4.4             // 9.9

BigDecimal(3.14) <+> NumericMonoid.unit
NumericMonoid[BigDecimal].unit  <+> BigDecimal(3.14)
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

using



, Scala 2 implicit



. .

. NumericMonoid



— , Monoid[T]



— . T



, . NumericMonoid[BigDecimal]



, NumericMonoid



BigDecimal



. num



— NumericMonoid



, using



.

, unit



. -, <+>



. Scala obj1.method(obj2)



.

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using



.