1. Definition

The Interpreter Design Pattern provides a way to evaluate language grammar or expressions for particular languages. It involves implementing an expression interface that tells how to interpret a particular context.

2. Problem Statement

Imagine you are building a software tool that allows users to define their simple calculations, like "add 3 and 5" or "subtract 4 from 10". Hardcoding every possible calculation isn't scalable. Instead, you need a way to interpret these user-defined expressions.

3. Solution

Define the grammar for the language you want to interpret. Then, for each grammar rule, define a class that represents the rule and how it's interpreted. The hierarchy of these classes will mirror the syntax tree of the language, allowing you to interpret expressions.

4. Real-World Use Cases

1. SQL parsers in databases.

2. Regular expression engines.

3. Formula interpreters in spreadsheet applications.

5. Implementation Steps

1. Define an abstract Expression class that declares an interpret method.

2. For each grammar rule in the language, derive a concrete Expression subclass.

3. Use these subclasses to interpret expressions in the language.

6. Implementation in Python

# Step 1: Abstract Expression
class Expression:
    def interpret(self, context):
        pass
# Step 2: Concrete Expressions
class AddExpression(Expression):
    def __init__(self, left, right):
        self._left = left
        self._right = right
    def interpret(self, context):
        return self._left.interpret(context) + self._right.interpret(context)
class SubtractExpression(Expression):
    def __init__(self, left, right):
        self._left = left
        self._right = right
    def interpret(self, context):
        return self._left.interpret(context) - self._right.interpret(context)
class NumberExpression(Expression):
    def __init__(self, number):
        self._number = number
    def interpret(self, context):
        return self._number
# Client code
expression1 = AddExpression(NumberExpression(3), NumberExpression(5))
expression2 = SubtractExpression(NumberExpression(10), NumberExpression(4))
print(expression1.interpret({}))
print(expression2.interpret({}))

Output:

8
6

Explanation:

1. The abstract Expression class has an interpret method that all concrete expression classes must implement.

2. We've implemented three concrete expressions: AddExpression, SubtractExpression, and NumberExpression. These can be combined to form complex expressions.

3. In the client code, two expressions are constructed: one that adds 3 and 5, and another that subtracts 4 from 10. They are then interpreted to yield the results.