Abstract: These course notes are addressed to a wide audience of people interested in modern programming languages in general, ML-like languages in particular, or simply in OCaml, whether they are programmers or language designers, beginners or knowledgeable readers —little prerequiresite is actually assumed.

They provide a formal description of the operational semantics (evaluation) and statics semantics (type checking) of core ML and of several extensions starting from small variations on the core language to end up with the OCaml language —one of the most popular incarnation of ML— including its object-oriented layer.

The tight connection between theory and practice is a constant goal: formal definitions are often accompanied by OCaml programs: an interpreter for the operational semantics and an algorithm for type reconstruction are included. Conversely, some practical programming situations taken from modular or object-oriented programming patterns are considered, compared with one another, and explained in terms of type-checking problems.

Many exercises with different level of difficulties are proposed all along the way, so that the reader can continuously checks his understanding and trains his skills manipulating the new concepts; soon, he will feel invited to select more advanced exercises and pursue the exploration deeper so as to reach a stage where he can be left on his own.