Some of the highlights in release 3.12 are:
* Polymorphic recursion is supported, using explicit type
declarations on the recursively-defined identifiers.
* First-class modules: module expressions can be embedded as
values of the core language, then manipulated like any other
first-class value, then projected back to the module level.
* New operator to modify a signature a posteriori: S with type
t := tau denotes signature S where the t type component is
removed and substituted by the type tau elsewhere.
* New notations for record expressions and record patterns:
{ lbl } as shorthand for { lbl = lbl }, and { ...; _ } marks
record patterns where some labels were intentionally omitted.
* Local open let open ... in ... now supported by popular demand.
* Type variables can be bound as type parameters to functions;
such types are treated like abstract types within the
function body, and like type variables (possibly generalized)
outside.
* The module type of construct enables to recover the module
type of a given module.
* Explicit method override using the method! keyword, with
associated warnings and errors.
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