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 frustratingly terrible syntax. CLU and Alphard both seem to get as close as possible to being a full OO language without actually being one. Ruby borrowed many concepts from CLU and has similar syntax.

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 complex number cluster:

    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 were global, and no namspace mechanism was provided to group clusters or allow them to be created "locally" inside other clusters. This problem is hardly unique to CLU, but it is surprising that so many languages have lacked this feature given that the "whole idea" of ALGOL was to offer scope to variables, it seems that cluster/object names should be an obvious extension of this concept.

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, the later being "base types" such as integers.

Another key feature of the CLU type system is iterators, which return objects from a collection one after the other. Iterators were "black boxes" that offered an identical API no matter what data they were being used with, the iterator for a collection of complex_numbers would be identical to that for an array of integers. Iterators are now a common feature of most modern languages.

Exceptions were another basic feature of CLU, largely copied from various attempts to add this feature in other languages. Exceptions are raised using signal and handled with except.

One of features of CLU was multiple assignment: e.g. writing x,y=y,x would exchange values of x and y.  In the same way, functions could return several values, like x,y,z=f(t). Oddly, given the focus on type design, CLU did not offer enumerated types, nor any obvious way to make them.

External links

• CLU Home Page
• A History of CLU
• clu2c: a program to compile CLU code to C