From ML to C via Modula-3 an approach to teaching programming

types: Types can be defined in terms of permitted operations while keeping implementation details private. Static scoping: All identifiers are resolved at compile time. However, procedure execution can be controlled by pattern matching of arguments at run time. Type-safe exceptions: Exceptions allow procedures to return out-of-band results (often arising from abnormal conditions) to be communicated in a type-safe way.