The Free On-line Dictionary of Computing (30 December 2018):
(CLUster) An object-oriented programming language
developed at MIT by Liskov et al in 1974-1975.
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.
CLU was one of Kamin's interpreters. clu2c compiled CLU to
C. Concurrent CLU was an extension designed to support
["CLU Reference Manual", Barbara Liskov et al, LNCS 114,
E-mail: Paul R. Johnson .
Versions for Sun and VAX/VMS
(ftp://pion.lcs.mit.edu/pub/clu/). Portable version