DefaultHelper
Related Doc:
package bson
Neat helper to obtain a default type should one not be given eg:
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.
Lower priority defaultsTo implicit helper
Companion object for DefaultsTo
A helper containing the sealed
DefaultsTotrait which is used to determine the default type for a given method.1.0