Interpreter

The interpreter pattern is specific implimentation of the composite pattern generally used for language parsing. In the interpreter pattern a class is produced for each symbol in the language. A statement can then be broken down into a syntax tree of these classes which can in turn be used to interpret the statement.

Use When

You should use Interpreter when there is a language that you need to interpret and when the statements of the language can be represented as abstract syntax trees. It works best when:

• the grammar of the language is simple. Otherwise, the class hierarchy for the grammar becomes large and complex.
• efficiency is not essential.

Structure

(from wikipedia)

Consequences

• Interpreter makes it easy to change and extend the grammar using inheritance.
• Implementing the grammar is easy because the nodes in the abstract syntax tree have similar interpretations are are usually easy to write. Often, writing these classes can be automated using a compiler or parser generator.
• It is hard to maintain complex grammar using Interpreter because it defines at least one class for every rule in the grammar. This can soon lead to a huge number of classes.
• It is easy to add new ways to interpret an expression.

Related Patterns

• Composite: The abstract syntax tree uses a Composite pattern.
• Flyweight: This pattern can be used to share the terminal symbols.
• Iterator: This pattern can be used to traverse the Interpreter structure.
• Visitor: This pattern can be used to collect the behavior for all abstract syntax tree nodes in one class.