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. 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 numbers:
complex_number = cluster is add, subtract, multiply,... rep = record add = proc... end add; subtract = proc... end subtract; multiply = proc... end multiply; ... end complex_number;
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, but its representation type 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. 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 typeany, 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.
Other features
Another key feature of the CLU type system are iterators, which return objects from a collection serially, one after another. Iterators offer an identical application programming interfaceno matter what data they are being used with. Thus the iterator for a collection of complex_numbers can be used interchangeably with that for an array of integers. A distinctive feature of CLU iterators is that they are implemented as coroutines, with each value being provided to the caller via a yield statement. Iterators like those in CLU are now a common feature of many modern languages, such as C#, Ruby, and Python, though recently they are often referred to as generators. CLU also includes exception handling, based on various attempts in other languages; exceptions are raised using signal and handled with except. Unlike most other languages with exception handling, exceptions are not implicitly resignaled up the calling chain. Also unlike most other languages that provide exception handling, exceptions in CLU are considered part of ordinary execution flow and are considered a "normal" and efficient typesafe way to break out of loops or return from functions; this allows for direct assignment of return values "except when" other conditions apply. Exceptions that are neither caught nor resignaled explicitly are immediately converted into a special failure exception that typically terminates the program. CLU is often credited as being the first language with type-safe variant types, called oneofs, before the languageML had them. A final distinctive feature in CLU is parallel assignment, where more than one variable can appear on the left hand side of an assignment operator. For instance, 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. Parallel assignment predates CLU, appearing in CPL, named simultaneous assignment, but CLU popularized it and is often credited as the direct influence leading to parallel assignment in later languages. All objects in a CLU program live in the heap, and memory management is automatic. CLU supports type parameterized user-defined data abstractions. It was the first language to offer type-safe bounded parameterized types, using structure where clauses to express constraints on actual type arguments.
CLU has influenced many other languages in many ways. In approximate chronological order, these include: CLU and Ada were major inspirations for C++ templates. CLU's exception handling mechanisms influenced later languages like C++ and Java. Sather, Python, and C# include iterators, which first appeared in CLU. Perl and Lua took multiple assignment and multiple returns from function calls from CLU. Python and Ruby borrowed several concepts from CLU, such as call by sharing, the yield statement, and multiple assignment
