In some software systems, we need to handle complexBusiness Rules,Syntax parsingorExpression evaluationIn order to simplify the analysis and processing of these logics,Interpreter PatternIt provides an elegant solution. The interpreter pattern is often used to build a custom language or simplify complex rule engines, such as mathematical expression evaluation, command parsers, configuration file parsing, and other scenarios. This article will introduce the concept of the interpreter pattern, the difference from other patterns, the problems it solves, implementation examples in Golang, and the application and precautions in actual development.

What is the Interpreter Pattern?

Interpreter modeis aBehavioral design patterns, used forDefine and parse the grammatical rules of a languageIt can represent the grammar of a language as a set ofClass or Expression, and parses and evaluates the input data through these classes. Each class represents a grammar rule, and the interpreter combines these classes to complete the interpretation and execution of the entire expression or command.

Components of the Interpreter Pattern

1. Abstract Expression: Defines the interface for the interpretation method, and all specific expressions must implement this interface.
2. Terminal Expression: Implements operations related to the most basic elements of the language, such as constants, variables, etc.
3. Non-terminal Expression: Indicates a more complex grammar rule, usually consisting of multiple sub-expressions.
4. Context: Stores global information when the interpreter is running, such as the value of variables.
5. Client: Build a syntax tree and evaluate it using the context.

Differences between Interpreter Mode and Other Modes

1. Chain of Responsibility Pattern

• Target: The chain of responsibility pattern passes the request along the processing chain until an object handles it.
• the difference: The Chain of Responsibility pattern processes requests sequentially, while the Interpreter pattern is used to parse and evaluate grammatical structures.

2. Command Pattern

• Target: The command pattern encapsulates the request as an object and supports undo and redo of the request.
• the difference: The command mode is mainly used for operation encapsulation, while the interpreter mode focuses on grammar parsing and execution.
  The command mode can be viewed in detail:In-depth analysis of Go design pattern Command Pattern (Command Pattern) implementation and application in Golang

3. Strategy Pattern

• Target: The strategy pattern defines a family of algorithms and allows one of them to be selected at runtime.
• the difference: The strategy pattern is used to dynamically select algorithms, while the interpreter pattern is used for parsing and evaluation of languages.
  The strategy mode can be viewed in detail:In-depth analysis of the implementation and application of Go design pattern strategy pattern in Golang

What problem does the Interpreter pattern solve?

1. Handling complex grammar rules: By encapsulating grammar rules as classes, the code is clearer and easier to maintain.
2. Implementing a custom language: Can be used to build domain-specific languages (DSLs) to simplify the expression and execution of complex business rules.
3. Dynamic Evaluation: Allows dynamic evaluation based on input data at runtime, such as mathematical expression parsing.

Application scenarios of the interpreter pattern

1. Mathematical expression evaluation: Parse and calculate the result of mathematical expressions, such as3 + 5 * 2.
2. Command parser: Parse and execute the commands entered by the user.
3. Configuration file parsing: Parse configuration files in specific formats, such as JSON, YAML, etc.
4. Workflow Engine: Parse and execute complex business processes.

Interpreter pattern implementation in Golang

Next, we will use a specific Golang example to show how to use the interpreter pattern to implement a simpleMathematical expression evaluatorWe will support the four basic operations: addition, subtraction, multiplication, and division.

Example: Mathematical Expression Interpreter

1. Define the abstract expression interface

package main // Expression interface: defines the common interface for all expressions type Expression interface { Interpret() int }

2. Implementing terminal expression

// Number structure: terminal expression, indicating a specific number type Number struct { value int } func (n *Number) Interpret() int { return n.value }

3. Implementing non-terminal expressions

// Add structure: non-terminal expression, indicating addition operation type Add struct { left Expression right Expression } func (a *Add) Interpret() int { return a.left.Interpret() + a.right.Interpret() } // Subtract structure: non-terminal expression, indicating subtraction operation type Subtract struct { left Expression right Expression } func (s *Subtract) Interpret() int { return s.left.Interpret() - s.right.Interpret() }

4. Build expressions and evaluate them

func main() { // Build the expression (5 + 3) - 2 expression := &Subtract{ left: &Add{ left: &Number{value: 5}, right: &Number{value: 3}, }, right: &Number{value: 2}, } // Interpret and calculate the result result := expression.Interpret() fmt.Printf("Result: %d\n", result) }

Output

Result: 6

Code Analysis

1. Expression interface: defines the common interface for all expressionsInterpret, which interprets the expression and returns the result.
2. Number Structure: AchievedExpressionInterface, indicating a specific number.
3. Add and Subtract structures: AchievedExpressionInterface representing addition and subtraction operations.
4. main function: Constructs a mathematical expression(5 + 3) - 2, and calculate the result through interpreter mode.

Application in actual development

1. Command Line Parser: Parse and execute user-entered commands in the CLI tool.
2. Expression Evaluator: Parse mathematical or logical expressions in calculator applications or data analysis tools.
3. Workflow Engine: Analyze the rules and conditions in the business process and perform corresponding operations based on the analysis results.
4. Configuration file parser: Parse and interpret the contents of the configuration file to support dynamic configuration.

Notes on using interpreter mode

1. Performance issues: In complex grammar rules, the performance of parsing and evaluation may become a bottleneck. For complex systems, you can consider usingAbstract Syntax Tree (AST)To optimize the parsing process.
2. readability: If the grammar rules are very complex, the interpreter pattern may lead to a large number of classes and interfaces, increasing the complexity of the code.
3. Applicable scenarios：The interpreter pattern is suitable for scenarios where the grammatical rules are relatively stable. If the grammatical rules change frequently, a more flexible solution may be required.

Comparison between the Interpreter Pattern and the Chain of Responsibility Pattern

Summarize

Interpreter modeIt is a very useful design pattern, especially for scenarios that deal with complex grammar and rules. By encapsulating grammar rules as classes, we can clearly define and parse expressions. In Golang, the interpreter pattern can be easily implemented through a combination of interfaces and structures, and applied to scenarios such as mathematical evaluators, command line parsers, and business rule engines. In actual development, the reasonable use of the interpreter pattern can improve the readability and maintainability of the code, but attention should be paid to performance issues and code complexity. If you need to parse expressions or rules in your project, the interpreter pattern will be a very suitable choice.

Reference Links

• Design Patterns - Interpreter
• Go Design Patterns