DefaultsTo

Neat helper to obtain a default type should one not be given eg:

def find[T]()(implicit e: T DefaultsTo Document) { ... }

The signature of the find method ensures that it can only be called if the caller can supply an object of type DefaultsTo[T, Document]. Of course, the DefaultsTo.default and DefaultsTo.overrideDefault methods make it easy to create such an object for any type T. Since these methods are implicit, the compiler automatically handles the business of calling one of them and passing the result into find.

But how does the compiler know which method to call? It uses its type inference and implicit resolution rules to determine the appropriate method. There are three cases to consider:

1. find is called with no type parameter. In this case, type T must be inferred. Searching for an implicit method that can provide an object of type DefaultsTo[T, Document], the compiler finds default and overrideDefault. default is chosen since it has priority (because it's defined in a proper subclass of the trait that defines overrideDefault). As a result, T must be bound to Document.

2. find is called with a non-Document type parameter (e.g., find[BsonDocument]()). In this case, an object of type DefaultsTo[BsonDocument, Document] must be supplied. Only the overrideDefault method can supply it, so the compiler inserts the appropriate call.

3. find is called with Document as the type parameter. Again, either method is applicable, but default wins due to its higher priority.