Constructs YAML values.
Each YAML scalar, sequence or mapping has a tag specifying its data type. Constructor uses user-specifyable functions to create a node of desired data type from a scalar, sequence or mapping.
Each of these functions is associated with a tag, and can process either a scalar, a sequence, or a mapping. The constructor passes each value to the function with corresponding tag, which then returns the resulting value that can be stored in a node.
If a tag is detected with no known constructor function, it is considered an error.