Practice BEAM concurrency until the failure modes make sense
For backend developers who can already write basic Elixir and want to learn OTP and concurrency. The first wave follows Mailbox and Supervision simulator exercises. Elixir, Erlang, and Lua code submissions require separate approval.
LetItCrash is for people 18 or older.
Free for invited learners. Tell us where the simulators help or confuse you.
This is probably for you if
- You can read and write basic Elixir, but OTP still feels uncertain.
- You want to reason through message ordering, crashes, and restarts.
- You'd rather struggle with one hard problem than watch someone else solve an easy one.
- You're fine with rough edges if it means your feedback shapes what gets built next.
The feedback loop
Wrong answers teach. Passes schedule your return.
A wrong answer starts a conversation
def sum([]), do: 0
def sum([head | tail]), do: sum(tail)Mockratis — You dropped the head. Walk it by hand: what does sum([1, 2]) return with your second clause?
This example shows our code practice. The current OTP/concurrency wave uses Mailbox and Supervision simulators; Elixir code submissions require separate approval.
A pass is a promise to return
Solving something schedules a return visit — sooner if it was a struggle, later once it sticks. Your dashboard always shows what's due today, so retention is built into the practice, not bolted on.
The path
Start with mailboxes and supervision
Our first learner wave starts with message ordering, selective receive, and supervision decisions. Other subjects remain visible, but this is the path we want you to test.
Mailboxes
Predict how messages arrive, match, and wait. Start with ordering, then test selective receive and mailbox growth.
Supervision
Choose restart behavior for failing processes and inspect which parts of a running system recover.
Why it works
Practice built for retention, not streak-chasing
Decisions you can test
Try a mailbox or supervision decision and see the simulated outcome. The first wave focuses on these deterministic exercises.
Hints that never reveal
A cantankerous old tutor meets you where your reasoning broke and asks the one question that gets you unstuck — so next month you can re-derive it, not just recall it.
Spaced repetition, everywhere
Every exercise, drill, and memory card feeds one SM-2 scheduler. Struggle shortens the interval; mastery stretches it. You never guess what to practice next.
Help us test how backend developers learn OTP
What you get
- Free access to Mailbox and Supervision simulators. Other catalog content remains visible; some code submissions require separate approval.
- A direct line to the developer — your feedback goes straight into the roadmap.
What we ask
- Honest feedback — especially when something confuses or frustrates you.
- Patience with rough edges — this is an alpha, and things will break.
LetItCrash is for people 18 or older.
Invitations are limited to 5–8 learners in this wave. No card is required.