# 1. Definition

The Interpreter Design Pattern provides a way to evaluate language grammar or expressions for particular languages. It primarily deals with interpreting a particular context based on the given grammar.

# 2. Problem Statement

Imagine you want to create a simple arithmetic calculator that evaluates expressions like "1 + 2" or "3 * 4". Parsing and evaluating such expressions directly within your main application logic can make it complex and hard to manage.

# 3. Solution

The Interpreter Pattern suggests modeling the domain with a set of classes representing possible operations and then interpreting expressions using these classes. In essence, for our calculator, we'd have classes for numbers, additions, multiplications, and so on.

# 4. Real-World Use Cases

1. Compilers and interpreters for programming languages.

2. Rule-based engines.

3. Text processing applications that need to process and analyze data based on certain patterns or grammar.

# 5. Implementation Steps

1. Define an abstract expression interface.

2. Implement concrete expression classes.

3. Use the expression classes to parse and evaluate expressions.

# 6. Implementation in Kotlin

``````// Step 1: Define the expression interface
interface Expression {
fun interpret(): Int
}
// Step 2: Implement concrete expression classes
class Number(private val number: Int) : Expression {
override fun interpret(): Int = number
}
class Add(private val leftExpression: Expression, private val rightExpression: Expression) : Expression {
override fun interpret(): Int = leftExpression.interpret() + rightExpression.interpret()
}
class Multiply(private val leftExpression: Expression, private val rightExpression: Expression) : Expression {
override fun interpret(): Int = leftExpression.interpret() * rightExpression.interpret()
}
// Step 3: Use the expressions to evaluate
fun main() {
val one = Number(1)
val two = Number(2)
val three = Number(3)
val multiplication = Multiply(two, three)
println("2 * 3 = \${multiplication.interpret()}")
}
``````

### Output:

```1 + 2 = 3
2 * 3 = 6
```

### Explanation:

1. The Expression interface is the root of our grammar. It has a single-method interpret, which returns the result of the expression.

2. We then have concrete implementations: Number, Add, and Multiply. Each of these classes knows how to evaluate itself. For example, the Add class adds the results of its two sub-expressions.

3. In the main function, we create simple expressions and evaluate them. The design allows us to easily expand the kinds of operations we support without changing existing classes.

# 7. When to use?

Use the Interpreter Pattern when:

1. You need to interpret a language with a well-defined grammar.

2. Grammar for the language is relatively simple.

3. Efficiency isn't a primary concern, as the pattern can be slower and more memory-intensive than other techniques.

Note: For complex grammars or performance-critical applications, using tools like parser generators or compilers might be more suitable than handcrafting interpreters using this pattern.