Coercion

Support for coercing basic data types.

Provides:

• coercion protocols which define asInt, asString and so on
• implementations of these protocols for foundation data types
• a Converter protocol which can take an Any object and turn it into a known type using the coercion protocols
• subscript operators for Dictionary which look up a key by type: asInt:, asString: etc, performing coercion where possible