Closure Pattern

The most fundamental of the general programming patterns derived from ARS is the closure pattern. A closure is an encapsulation of a lambda abstraction with its total environment. This environment consists of all the names that this lambda abstraction has access to. Access to names in a lambda abstraction is controlled by the so called `lexical scope'. Lexical scope can also be described as the context of a lambda abstraction in the program text. A detailed explanation is given in the book Programmierung pur.

A closure is a first class object, which means that it can be treated like any data item:

• it can be stored in memory,
• it can be passed as an argument to a function and
• it may be returned as a value from a function.

The slight difference between a closure and an object in OOP is the following:

• A closure is essentially a function that can be called, but may have all kinds of data and procedures encapsulated in it.
• An object is essentially a data item with all kinds of data and procedures encapsulated in it. One of its procedures (normally called methods) may be `apply', allowing to apply the object to a set of arguments, which essentially is the same as calling a function.

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