Editing CLU (programming language) (section)

== Clusters ==
The [[syntax]] of CLU was based on [[ALGOL]], then the starting point for most new language designs. The key addition was the concept of a ''cluster'', CLU's type extension system and the root of the language's name (CLUster).<ref name=Liskov1977>{{Cite journal |last1= Liskov |first1= B. |author-link1= Barbara Liskov |last2= Snyder |first2= A. |last3= Atkinson |first3= R. |last4= Schaffert |first4= C. |title= Abstraction mechanisms in CLU |doi= 10.1145/359763.359789 |journal= [[Communications of the ACM]] |volume= 20 |issue= 8 |pages= 564–576 |date=August 1977 |citeseerx=10.1.1.112.656 |s2cid= 17343380 }}</ref> Clusters correspond generally to the concept of a "class" in an OO language. For instance, here is the CLU syntax for a cluster that implements [[complex number]]s:
<!-- Not actually Algol 68, but this attribute is required-->
<syntaxhighlight lang="text">
    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;
</syntaxhighlight>

A cluster is a module that encapsulates all of its components except for those explicitly named in the "is" clause. These correspond to the public components of a class in recent OO languages. A cluster also defines a type that can be named outside the cluster (in this case, "complex_number"), but its representation type (rep) is hidden from external clients.

Cluster names are global, and no namespace mechanism was provided to group clusters or allow them to be created "locally" inside other clusters.

In a cluster, the explicit type conversions ''up'' and ''down'' change between the abstract type and the representation; implicit conversions between these types are signified using the special type ''cvt''. CLU does not otherwise perform [[implicit type conversion]]s.  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, booleans, characters and strings.<ref name=Liskov1977/>