This method enables the following syntax:

     Array(1, 2) should (contain (2) and contain (1))
                                 ^
     

This method enables the following syntax:

     map should (contain key ("fifty five") or contain key ("twenty two"))
                         ^
     

The map's value type parameter cannot be inferred because only a key type is provided in an expression like (contain key ("fifty five")). The matcher returned by this method matches scala.collection.Maps with the inferred key type and value type Any. Given Map is covariant in its value type, and Matcher is contravariant in its type parameter, a Matcher[Map[Int, Any]], for example, is a subtype of Matcher[Map[Int, String]]. This will enable the matcher returned by this method to be used against any Map that has the inferred key type.

This method enables the following syntax:

     Map("one" -> 1, "two" -> 2) should (not contain value (5) and not contain value (3))
                                                     ^
     

The map's key type parameter cannot be inferred because only a value type is provided in an expression like (contain value (5)). The matcher returned by this method matches scala.collection.Maps with the inferred value type and the existential key type [K] forSome { type K }. Even though Matcher is contravariant in its type parameter, because Map is nonvariant in its key type, a Matcher[Map[Any, Int]], for example, is not a subtype of Matcher[Map[String, Int]], so the key type parameter of the Map returned by this method cannot be Any. By making it an existential type, the Scala compiler will not infer it to anything more specific. This will enable the matcher returned by this method to be used against any Map that has the inferred value type.