[macro] Splicing a list of parameters with default values

[Woody Robert]

Hi,

We have a macro that generates a value having a structural type, so we generate each of its members. One of them is a method with default parameter values.

E.g., we want the following invokation:

case class User(name: String, age: Int)
val user = User("Peter", 42)

val result = myMacro(user)

To generate a value with a method copy like the following:

new {
  def copy(name: String = get(user, "name"), age: Int = get(user, "age")): User = ???
}

So we can then write result.copy or result.copy(age = 0) and so on.

Here is a relevant part of our code:

val paramsCopy = for(param <- params) yield {
  val paramName = param.name.toTermName
  val paramNameString = param.name.toString
  val paramType = param.typeSignature
  q"val $paramName: $paramType = get($obj , $paramNameString)"
}

val defCopy = q"""def copy(..${paramsCopy.reverse}) = ???"""

We have two questions:

1. We defined each parameter with a quasiquote containing a val def. If we replace it with just q"$paramName: $paramType = get($obj , $paramNameString)", we get a syntax error when the macro is expanded (';' expected but '=' found). With methods having parameters with no default value, if we remove the leading val keyword we get an AST type error (found Typed, required: ValDef). What is the right way to build a parameter list?
2. The default parameters don't work. I can call result.copy("John", 0), though. Are default parameters filled before macro invocations?

Thanks for your help!

[Eugene Burmako]

Could you please elaborate on defaults not working? What error messages are you getting?

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[Woody Robert]

We have this result :

result.copy(name = "John", age = 0) // Ok

result.copy(name = "John")                   // Error: not enough arguments for method copy; Unspecified value parameter age

result.copy                                                 // Error: not enough arguments for method copy; Unspecified value parameters name, age

Thanks for helping.

[Lukas Rytz]

Maybe it's due to some flags (DEFAULTPARAM) that are not set by the quasiquote parser?

  q"val $paramName: $paramType = get($obj , $paramNameString)"