I would like to add that optimizations may also be performed in addition to type-checking (type-checking, by the way, depends on safety optimization level). What's also handy is that you can specify the optimization levels (space, safety, speed, debug) per function or per file (or in a `locally' block).

Function return types may also be specified, with serapeum's ->, it may be as simple as (-> f (fixnum fixnum) boolean) for instance, right above the function.

Classes and structs let you specify types for slots.

Expressions may be typed with `the' special operator aka (the fixnum (first list)).

PS CL has gradual typing.