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scalalanguage-design

Scala implicit class restriction

Scala documentation states that implicit class "must be defined inside of another trait/class/object"

What is the reason for this constraint?

Solution

  • The referenced SIP describes implicit classes as being syntactic sugar for a class and a def.

    Annotations on implicit classes default to attaching to the generated class and the method. For example,

    @bar
implicit class Foo(n: Int)

    will desugar into:

    @bar implicit def Foo(n: Int): Foo = new Foo(n)
@bar class Foo(n:Int)

    Since a def must be within a trait/class/object, an implicit class, being partially "composed of" a def, must also obey this condition.