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Interpreter

The Interpreter design pattern is a behavioral pattern used to define a grammar for interpreting sentences in a language. It provides a way to evaluate sentences or expressions based on a given set of rules. This pattern is useful when working with languages, expressions, or simple interpreters.

Simple Math Expression Interpreter

We'll create a simple interpreter that evaluates arithmetic expressions consisting of addition (+) and numbers.

PlantUML model
plantuml code

Explanation:

  1. Expression Interface - Defines an abstract interpret() method.
  2. Number (Terminal Expression) - Represents numbers and directly returns their value.
  3. Addition (Non-Terminal Expression) - Evaluates the sum of two expressions.
  4. Main Function - Creates an expression (5 + 10) and evaluates it.

This approach can be extended to support more operators and a full expression tree interpreter.

Designing a Robot Command Interpreter

You're developing a robot control system where users can send simple text-based commands like:

MOVE FORWARD 10
TURN LEFT 90
MOVE BACKWARD 5

Instead of hardcoding every possible command, you decide to implement an Interpreter Pattern to parse and execute these commands dynamically.

PlantUML model
plantuml code

What's Happening in Our Code?

We are designing a mini command language for our robot, where each command is an expression that gets interpreted and executed. The system will:

  1. Read commands as strings.
  2. Convert them into objects (expressions).
  3. Evaluate and execute them dynamically.

Instead of manually handling each command with if-else statements, we let the Interpreter Pattern handle parsing and execution flexibly.

⚙️ How It Works

  1. Define an abstract Command interface – Every robot command follows this interface.
  2. Create concrete command classes – Each command (MOVE, TURN) is implemented as a separate class.
  3. Use a parser – Converts strings into Command objects.
  4. Interpret and execute – Each command executes its logic when interpreted.

📌 When to Use the Interpreter Pattern

Use the Interpreter pattern when:

  1. You have a simple grammar to interpret – If your problem involves parsing and evaluating structured sentences or expressions (e.g., math expressions, custom scripting languages, or rule engines), the Interpreter pattern is a good fit.
  2. The grammar is stable and small – This pattern works best when the number of rules in the grammar is relatively limited and does not change often.
  3. You need a customizable evaluation process – If users or developers need to define new rules or expressions dynamically, this pattern provides flexibility.
  4. You want an object-oriented approach to parsing – It allows defining expressions as objects and evaluating them using a tree structure.

❌ When NOT to Use the Interpreter Pattern

Avoid the Interpreter pattern when:

  1. The grammar is too complex – If you have a large set of rules or a complex language (e.g., a full programming language), this pattern becomes inefficient and hard to maintain.
    • 🔹 Alternative: Use a parser generator like ANTLR or regular expressions for efficient parsing.
  2. Performance is critical – Since this pattern represents expressions as objects and builds trees, it consumes more memory and has execution overhead.
    • 🔹 Alternative: Convert expressions into bytecode or an Abstract Syntax Tree (AST) and evaluate them efficiently.
  3. The number of expression types keeps increasing – If you frequently add new grammar rules, the codebase becomes hard to maintain.
    • 🔹 Alternative: Use the Visitor Pattern to separate interpretation logic from the structure.

🔗 Related Design Patterns

The Interpreter pattern is often used alongside or as an alternative to:

  1. Composite Pattern – The tree structure used in the Interpreter pattern is similar to the Composite pattern, which treats individual objects and compositions uniformly.
  2. Visitor Pattern – When interpretation logic becomes too complex, the Visitor pattern helps separate the grammar structure from the logic.
  3. Flyweight Pattern – To optimize memory usage in case of repeated expressions (e.g., large ASTs), Flyweight can be used to share common subexpressions.
  4. Builder Pattern – If parsing an input language is required before interpreting, the Builder pattern can help construct expression trees.

Source code examples: Robot Command Interpreter, Math Expression Interpreter