Scala is not inferring the appropriate method

Question

I have the following method ``` def show[E: Writes, T: Writes](block: RequestWithUser[AnyContent] => Either[E, T]): Action[AnyContent] = { withErr(block) } ``` which I use like this from a controller: ``` def show(id: Long) = CrudAuthAction.show { request => IdeaType.findByIdWithErr(id) } ``` And I want the request method to be optional, so I defined another signature for the same method: ``` def show[E: Writes, T: Writes](block: => Either[E, T]): Action[AnyContent] = { withErr(request => block) } ``` And it works fine, I can omit the request param But when I try to do the same with this other method ``` def list[T: Writes](block: RequestWithUser[AnyContent] => T): Action[AnyContent] = { fromRequest(block) } def list[T: Writes](block: => T): Action[AnyContent] = { fromRequest(request => block) } ``` When I want to use it like this: ``` def list = CrudAuthAction.list { request => IdeaType.find(request.queryString) } ``` it tells me that request is missing parameter type, and I have to…

Accepted Answer

The problem is that in the second example the compiler does not know which of the overloaded methods to pick, because T could well be a function type. Since `Either` is clearly not a function, it works in the first case.

To work around this issue, you could change the second method to

```
def list[T: Writes](block: => Foo[T]): Action[AnyContent] = {
  fromRequest(request => block.v)
}
```

With Foo defined like that:

```
case class Foo[T](val v:T)
```

Unfortunately, adding an implicit conversion to Foo breaks things again, unless you create an implicit conversion for each type in the `Writes` type class.

Scala is not inferring the appropriate method

Solution

Problem