CLU is an object-oriented language of the Pascal family designed to support data abstraction, similar to Alphard. It introduced the iterator: a coroutine yielding the elements of a data object, to be used as the sequence of values in a for loop.
A CLU program consists of separately compilable procedures, clusters and iterators, no nesting. A cluster is a module naming an abstract type and its operations, its internal representation and implementation. Clusters and iterators may be generic. Supplying actual constant values for the parameters instantiates the module.
There are no implicit type conversions. In a cluster, the explicit type conversions 'up' and 'down' change between the abstract type and the representation. There is a universal type 'any', and a procedure force to check that an object is a certain type. Objects may be mutable or immutable.
Exceptions are raised using 'signal' and handled with 'except'. Assignment is by sharing, similar to the sharing of data objects in Lisp. Arguments are passed by call-by-sharing, similar to call-by-value, except that the arguments are objects and can be changed only if they are mutable. CLU has own variables and multiple assignment.