Your First Program

Let's build a complete program that demonstrates Lumen's key features.

The Goal

We'll create a simple task manager that:

1. Defines a Task record with validation
2. Uses pattern matching on task status
3. Handles errors with the result type
4. Demonstrates string interpolation

Step 1: Define the Data Model

Create tasks.lm.md:

enum Status
  Todo
  InProgress
  Done
end

record Task
  id: Int where id > 0
  title: String where length(title) > 0
  status: Status
end

The where clauses add runtime validation.

Step 2: Create Helper Functions

cell create_task(id: Int, title: String) -> Task
  return Task(id: id, title: title, status: Todo)
end

cell mark_progress(task: Task) -> Task
  return Task(id: task.id, title: task.title, status: InProgress)
end

cell mark_done(task: Task) -> Task
  return Task(id: task.id, title: task.title, status: Done)
end

Step 3: Pattern Matching

cell status_label(task: Task) -> String
  match task.status
    Todo -> return "⏳ Not started"
    InProgress -> return "🔄 In progress"
    Done -> return "✅ Completed"
  end
end

Step 4: Error Handling

cell find_task(tasks: list[Task], id: Int) -> result[Task, String]
  for task in tasks
    if task.id == id
      return ok(task)
    end
  end
  return err("Task not found: {id}")
end

Step 5: Put It Together

cell main() -> String
  let tasks = [
    create_task(1, "Learn Lumen"),
    create_task(2, "Build an agent"),
    create_task(3, "Deploy to production")
  ]
  
  let result = find_task(tasks, 2)
  
  match result
    ok(task) -> return status_label(task)
    err(msg) -> return "Error: {msg}"
  end
end

Run It

lumen run tasks.lm.md

Output:

🔄 In progress

Complete Code

# Task Manager
# A simple example demonstrating records, enums, pattern matching, and error handling.

enum Status
  Todo
  InProgress
  Done
end

record Task
  id: Int where id > 0
  title: String where length(title) > 0
  status: Status
end

cell create_task(id: Int, title: String) -> Task
  return Task(id: id, title: title, status: Todo)
end

cell mark_progress(task: Task) -> Task
  return Task(id: task.id, title: task.title, status: InProgress)
end

cell mark_done(task: Task) -> Task
  return Task(id: task.id, title: task.title, status: Done)
end

cell status_label(task: Task) -> String
  match task.status
    Todo -> return "⏳ Not started"
    InProgress -> return "🔄 In progress"
    Done -> return "✅ Completed"
  end
end

cell find_task(tasks: list[Task], id: Int) -> result[Task, String]
  for task in tasks
    if task.id == id
      return ok(task)
    end
  end
  return err("Task not found: {id}")
end

cell main() -> String
  let tasks = [
    create_task(1, "Learn Lumen"),
    create_task(2, "Build an agent"),
    create_task(3, "Deploy to production")
  ]
  
  let result = find_task(tasks, 2)
  
  match result
    ok(task) -> return status_label(task)
    err(msg) -> return "Error: {msg}"
  end
end

Key Concepts Learned

Concept	Example
Enum	enum Status ... end
Record	record Task ... end
Where constraint	id: Int where id > 0
Pattern matching	match task.status ... end
Result type	result[Task, String]
String interpolation	"Error: {msg}"
List literal	[task1, task2]

Next Steps

• Tutorial: Basics — Deep dive into syntax
• Pattern Matching — More match patterns
• Error Handling — Working with results