Bits of .NET

📋 Match Arrays Like Prolog

Match first element, last element, empty list — all with pattern matching. List patterns make array processing declarative.

📝 Basic Patterns

int[] numbers = [1, 2, 3, 4, 5];

// Match first two elements
if (numbers is [1, 2, ..]) {
    Console.WriteLine("Starts with 1,2");
}

// Match last two elements
if (numbers is [.., 4, 5]) {
    Console.WriteLine("Ends with 4,5");
}

// Extract middle elements
if (numbers is [1, .. var middle, 5]) {
    Console.WriteLine($"Middle: {string.Join("", "", middle)}"); // 2,3,4
}

// Empty check
if (numbers is []) {
    Console.WriteLine("Empty array");
}

🎯 Switch with List Patterns

string GetCommandType(string[] args) => args switch
{
    [] => "No arguments",
    ["help"] => "Help command",
    ["build"] => "Build command",
    ["test", ..] => "Test command with arguments",
    ["run", var project, ..] => $"Run project: {project}",
    ["-v" or "--version"] => "Version command",
    [_, _] => "Exactly two arguments",
    [_, .., _] => "At least two arguments",
    _ => "Unknown command"
};

// Nested list patterns
int[][] matrix = [[1, 2], [3, 4], [5, 6]];
if (matrix is [[1, 2], ..]) {
    Console.WriteLine("First row is 1,2");
}

✅ Real-World Example

public static string ParseLogLevel(string[] parts) => parts switch
{
    // ERROR: Database connection failed
    ["ERROR", "Database", ..] => "Database error",
    
    // WARNING: Low disk space on /dev/sda
    ["WARNING", "Disk", .. var details] => $"Disk warning: {string.Join(" ", details)}",
    
    // INFO: User 123 logged in
    ["INFO", "User", var userId, "logged in"] => $"User {userId} login",
    
    // Any error with timestamp [2024-01-15 ERROR] 
    [var timestamp, "ERROR", ..] when timestamp.StartsWith("[") => $"Error at {timestamp}",
    
    // Default
    _ => "Unknown log entry"
};

// Usage in parser
string[] logParts = ["ERROR", "Database", "connection", "timeout"];
string result = ParseLogLevel(logParts); // "Database error"

💡 Benefits

• No more index out of bounds checks (pattern handles length)
• Declarative parsing of command-line arguments
• Destructuring with .. var captures rest of array
• Works with any type that has length/indexer