Computer History
Tracing the History of the Computer - CLU Programming Language

CLU is a programming language created at MIT by Barbara Liskov and her students between 1974 and 1975. It was notable for its use of constructors for abstract data types that included the code that operated on them, a key step in the direction of object oriented programming (OOP). However many of the other features of OOP are missing or incomplete, notably inheritance, and the language is also hindered by a sometimes frustrating syntax. CLU and Alphard both seem to get as close as possible to being a full OO language without actually being one.

Clusters

The syntax of CLU was based on ALGOL, then the starting point for most new language design. The key addition was the concept of a cluster, CLU's type extension system and the root of the language's name (CLUster). Clusters correspond generally to the concept of an "object" in an OO language, and have roughly the same syntax. For instance, here is the CLU syntax for a cluster that implements complex numbers:

    complex_number = cluster is add, subtract, multiply, ....
         rep = record [ real_part: real, imag_part: real ]
         add = proc ... end add;
         subtract = proc ... end subtract;
         multiply = proc ... end multiply;
         ...
    end complex_number;

While clusters offered a then-advanced system for structuring programs, CLU did not offer any sort of structure for the clusters themselves. Cluster names are global, and no namespace mechanism was provided to group clusters or allow them to be created "locally" inside other clusters. This problem is not unique to CLU, but it is surprising that so many languages have lacked this feature - given the centralness in ALGOL of giving scope to variables, it seems that giving scope to cluster/object names would be an obvious extension.

CLU does not perform 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, the latter being "base types" such as integers.