Type mismatch on implicit type class conversion

Question

I have the following piece of code

 trait HTMLWritable { def toHTML: String } case class User(name: String, age: Int, email: String) extends HTMLWritable { override def toHTML: String = s"<div> $name ($age) <a href=$email/> </div>" } trait HTMLSerializer[T] { def serialize(value: T): String } implicit object UserSerializer extends HTMLSerializer[User] { override def serialize(user: User): String = s"<div> ${user.name} (${user.age}) <a href=${user.email}/> </div>" } implicit class HTMLEnrichment[T](value: T) { def toHTML(serializer: HTMLSerializer[T]): String = serializer.serialize(value) } val john = User("John", 32, "[email protected]") println(john.toHTML(UserSerializer))

which produces the following error

Error:(30, 23) type mismatch; found : advanced.StackOverflowQuestion.UserSerializer.type required: Int println(john.toHTML(UserSerializer))

the error will disappear if I rename HTMLWritable's toHTML method to toHtml or in general to something else and keeping the print statement intanct like so

 trait HTMLWritable { def tohtml: String } case class User(name: String, age: Int, email: String) extends HTMLWritable { override def tohtml: String = s"<div> $name ($age) <a href=$email/> </div>" }

and the question: Why is the compiler confused here?

I would expect that the class method would be preferred and only defer to the implicit if the method was not found but this type mismatch is a completely different beast.

Dmytro Mitin · Accepted Answer · 2020-04-15 15:38:41Z

2

Compiler thinks that .toHTML in john.toHTML(UserSerializer) is method HTMLWritable#toHTML instead of extension method HTMLEnrichment#toHTML.

Then according to HTMLWritable#toHTML signature, john.toHTML is a String and john.toHTML(UserSerializer) is john.toHTML.apply(UserSerializer) i.e. String#apply expecting Int.

answered Apr 15, 2020 at 15:38

Dmytro Mitin

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2 Comments

Luis Miguel Mejía Suárez Over a year ago

It would be good to mention that the Scala compiler will never search for an extension method if a method with the same name already exists in that class.

Niko Over a year ago

aye aye aye... of course man. Now I see... It is the apply method of the String that produces the type mismatch. Thanks a ton.

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