Elixir Agent Error Fix

The Hook

You will learn how to fix common otp agent errors. This matters because understanding this concept is essential for productive development. Built by the developers of Doda Browser, DodaZIP, and Durga Antivirus Pro.

Quick Fix

Agent

The wrong approach and the correct fix are shown below:

Wrong:

{:ok, agent} = Agent.start_link(fn -> %{} end)

Output:

Right:

{:ok, agent} = Agent.start_link(fn -> %{} end)
Agent.update(agent, &Map.put(&1, :key, "val"))
value = Agent.get(agent, & &1)
Agent.get_and_update(agent, fn state ->
  {state[:key], Map.put(state, :key, "new_val")}
end)

Output:

Prevention

Agent wraps state in a process. Simpler than GenServer for single state.

Common Mistakes with otp agent

1. Using head and tail instead of pattern matching, causing runtime errors on empty lists
2. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks
3. Using return to exit a function early instead of wrapping a pure value in the monad

These mistakes appear frequently in real-world ELIXIR code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ