16.1: Reflection

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Pharo is a reflective programming language. In a nutshell, this means that programs are able to reflect on their own execution and structure. More technically, this means that the metaobjects of the runtime system can be reified as ordinary objects, which can be queried and inspected. The metaobjects in Pharo are classes, metaclasses, method dictionaries, compiled methods, but also the run-time stack, processes, and so on. This form of reflection is also called introspection, and is supported by many modern programming languages.

Conversely, it is possible in Pharo to modify reified metaobjects and reflect these changes back to the runtime system (see Figure \(\PageIndex{1}\)). This is also called intercession, and is supported mainly by dynamic programming languages, and only to a very limited degree by static languages. So pay attention when people say that Java is a reflective language, it is an introspective one not a reflective one.

Figure \(\PageIndex{1}\): *Reification and reflection.*

A program that manipulates other programs (or even itself) is a metaprogram. For a programming language to be reflective, it should support both introspection and intercession. Introspection is the ability to examine the data structures that define the language, such as objects, classes, methods and the execution stack. Intercession is the ability to modify these structures, in other words to change the language semantics and the behavior of a program from within the program itself. Structural reflection is about examining and modifying the structures of the run-time system, and behavioural reflection is about modifying the interpretation of these structures.

In this chapter we will focus mainly on structural reflection. We will explore many practical examples illustrating how Pharo supports introspection and metaprogramming.