尝试使用隐式 class 扩展它时，代表扩展方法不可用？

Question

I'm trying to extend a Slick Rep with a new method =?= supplied as an extension method via Scala's implicit class :

import slick.ast.TypedType
import slick.lifted.LiteralColumn
import slick.lifted.Rep

object RepExtensions {
  implicit class RichRep[T](rep: Rep[T]) {
    def =?=(rhs: Option[T])(implicit tt: TypedType[Boolean]): Rep[Boolean] =
      rhs.map(rep === _).getOrElse(LiteralColumn[Boolean](true))
  }
}

However in this case, for reasons I'm unable to identify, the method === is not visible to the compiler:

[error] /Users/.../RepExtensions.scala:10:19: value === is not a member of slick.lifted.Rep[T]
[error]       rhs.map(rep === _).getOrElse(LiteralColumn[Boolean](true))
[error]

The === method is defined in Slick's ColumnExtensionMethods but I can't find any documentation explaining how to explicitly import that into scope - it certainly doesn't help to directly import that class itself. Indeed in the rest of my code where I'm dealing with Rep types, that === method seems to be freely available whenever I need it, without any special effort or explicit importing.

How can I get the === method into scope in this case?

Answer 1

The extension methods are implemented for types via the implicit defs in ExtensionMethodConversions (see the bottom of ExtensionMethods.scala ). That means, for === to be available, the implicit needs to apply to your type, T . As T is unconstrained, it doesn't match any of the implicit defs to bring === into scope.

One way to resolve that is to constrain T to match. For example:

import slick.ast.BaseTypedType
import slick.jdbc.H2Profile.api._ // or whichever database you use

object RepExtensions {
  implicit class RichRep[T : BaseTypedType](rep: Rep[T]) {
    def =?=(rhs: Option[T]): Rep[Boolean] =
      rhs.map(rep === _).getOrElse(LiteralColumn[Boolean](true))
  }
}

The next step up from that is to require a Shape , as described: https://stackoverflow.com/a/43014901/154248 -- but it doesn't look like you need that for this definition.