Components of the Interpreter Design Pattern

1. AbstractExpression

This is an abstract class or interface that declares an abstract interpret() method. It represents the common interface for all concrete expressions in the language.

2. TerminalExpression

These are the concrete classes that implement the AbstractExpression interface. Terminal expressions represent the terminal symbols or leaves in the grammar. These are the basic building blocks that the interpreter uses to interpret the language.

• For example, in an arithmetic expression interpreter, terminal expressions could include literals such as numbers or variables representing numeric values.
• These terminal expressions would evaluate to their respective values directly without further decomposition.

3. NonterminalExpression

These are the also concrete classes that implement the AbstractExpression interface. Non-terminal expression classes are responsible for handling composite expressions, which consist of multiple sub-expressions. These classes are tasked to provide the interpretation logic for such composite expressions.

• Another aspect of non-terminal expressions is their responsibility to coordinate the interpretation process by coordinating the interpretation of sub-expressions.
• This involves coordinating the interpretation calls on sub-expressions, aggregating their results, and applying any necessary modifications or operations to achieve the final interpretation of the entire expression
• Non-terminal expressions facilitate the traversal of expression trees during the interpretation process.
• As part of this traversal, they recursively interpret their sub-expressions, ensuring that each part of the expression contributes to the overall interpretation.

4. Context

This class contains information that is global to the interpreter and is maintained and modified during the interpretation process. The context may include variables, data structures, or other state information that the interpreter needs to access or modify while interpreting expressions.

5. Client

The client is responsible for creating the abstract syntax tree (AST) and invoking the interpret() method on the root of the tree. The AST is typically created by parsing the input language and constructing a hierarchical representation of the expressions.

6. Interpreter

The interpreter is responsible for coordinating the interpretation process. It manages the context, creates expression objects representing the input expression, and interprets the expression by traversing and evaluating the expression tree. The interpreter typically encapsulates the logic for parsing, building the expression tree, and interpreting the expressions according to the defined grammar.

The Interpreter design pattern is a behavioral design pattern that facilitates the interpretation and evaluation of expressions or language grammars.