Expr-Lang

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A powerful expression evaluation tool for Claude Desktop using the Model Context Protocol (MCP).

Overview

MCP Expr-Lang provides a seamless integration between Claude AI and the powerful expr-lang expression evaluation engine. This tool allows you to perform complex data manipulations, calculations, and transformations directly within your Claude conversations.

With MCP Expr-Lang, you can:

• Evaluate mathematical expressions
• Transform and filter arrays and objects
• Sort complex data structures
• Perform string manipulations
• Use conditional logic
• And much more!

Installation

Installing via Smithery

To install MCP Expr-Lang for Claude Desktop automatically via Smithery:

npx -y @smithery/cli install @ivan-saorin/mcp-expr-lang --client claude

Prerequisites

• Go 1.18 or higher
• Claude Desktop application
• Git

Step 1: Clone the Repository

git clone https://github.com/ivan-saorin/mcp-expr-lang.git
cd mcp-expr-lang

Step 2: Install Dependencies

go mod download

Step 3: Build the Application

For Windows

go build -o bin/server.exe

For macOS/Linux

go build -o bin/server
chmod +x bin/server

Step 4: Configure Claude Desktop

1. Open Claude Desktop
2. Go to Settings
3. Navigate to the "Advanced" section
4. Add the following configuration to your config.json:

For Windows

{
  "mcpServers": {
    "eval": {
      "command": "C:\\path\\to\\mcp-expr-lang\\bin\\server.exe"
    }
  }
}

For macOS/Linux

{
  "mcpServers": {
    "eval": {
      "command": "/path/to/mcp-expr-lang/bin/server"
    }
  }
}

5. Save the configuration and restart Claude Desktop

Usage

Once installed, you can use the eval tool directly in your conversations with Claude. Simply ask Claude to evaluate an expression using the tool.

Basic Example

User: Use eval to calculate "10 + 5 * 3"

Claude: [Uses eval tool]
Result: 10 + 5 * 3 = 25

Complex Example

User: We have users = [{"Name": "John", "Age": 30},{"Name": "Ivan", "Age": 51},{"Name": "Eve", "Age": 15}]
Use eval to calculate "sortBy(users, .Age, "desc")"

Claude: [Uses eval tool]
Result: The expression returns the users sorted by age in descending order:
[
  {"Age": 51, "Name": "Ivan"},
  {"Age": 30, "Name": "John"},
  {"Age": 15, "Name": "Eve"}
]

For more examples please see EXAMPLES

Features

MCP Expr-Lang supports all features of the expr-lang engine, including:

• Arithmetic operations: Addition, subtraction, multiplication, division, modulo, exponentiation
• String operations: Concatenation, length, uppercase/lowercase conversion
• Boolean operations: AND, OR, NOT, comparison operators
• Array operations: Mapping, filtering, reducing, sorting
• Object manipulation: Property access, transformation
• Conditional logic: Ternary operators, if-else expressions
• Built-in functions: Mathematical, string, array, and date functions

For a complete guide to the expression language syntax and capabilities, see the expr-lang documentation.

Troubleshooting

Common Issues

1. Tool not appearing in Claude: Make sure the path in your config.json is correct and Claude Desktop has been restarted.

2. Execution errors: Check that the expression syntax is correct according to the expr-lang documentation.

3. Permission issues (macOS/Linux): Ensure the server binary has execution permissions (chmod +x bin/server).

Getting Help

If you encounter any issues or have questions:

1. Check the Issues section on GitHub
2. Open a new issue if your problem hasn't been reported

Contributing

Contributions are welcome! If you'd like to improve MCP Expr-Lang:

1. Fork the repository
2. Create a feature branch (git checkout -b feature/amazing-feature)
3. Commit your changes (git commit -m 'Add some amazing feature')
4. Push to the branch (git push origin feature/amazing-feature)
5. Open a Pull Request

License

This project is licensed under the MIT License - see the LICENSE file for details.

Acknowledgments

• expr-lang for the powerful expression evaluation engine
• mcp-golang for the Model Context Protocol implementation
• Claude AI for the AI assistant platform

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Made with ❤️ by Ivan Saorin