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Pattern matching and control flow interview questions

Interview questions on pattern matching and control flow: warm-ups, then what each harder question tests and how a good answer starts.

7 questions: 3 warm-ups and 4 with model answers · Levels: Fresher to mid

Warm-up questions

Short questions with short answers, each answered in full.

Fresher

Is = assignment in Elixir? What does it do, and what is the pin operator for?

Answered in the lesson Pattern matching, the match operator and the pin
Fresher

When do you use case, when cond and when if in Elixir?

Answered in the lesson case, cond, if and unless
Fresher

What is a guard in Elixir, and what may it contain?

Answered in the lesson case, cond, if and unless

Questions with model answers

What each question tests, the first step of a good answer and the follow-ups to expect.

Fresher to mid

A service starts with {:ok, text} = File.read("settings.json"). On a new server it stops at boot with a MatchError showing {:error, :eacces}. A teammate proposes rescuing the error and starting with built-in defaults. What do you recommend, and how should start-up treat a settings file that is missing or wrong?
What it tests

Reading what a MatchError reports, telling a broken setup from an expected failure, and choosing between a crash, a clearer error and handling with case.

First step

Read the value in the error: :eacces means the file is there but the service may not read it, a deployment mistake that no code can repair. Starting with defaults would hide it and run the service with settings nobody chose, so start-up should stop, with a message that names the file and the reason.
Follow-ups
- Would File.read!/1 be better than the match here?
- The file is optional on developers' machines. What changes?
- The file exists, but its JSON is invalid. What should start-up do?
From the lesson Pattern matching, the match operator and the pin
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In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 4 minutes
Fresher to mid

A webhook handler starts with %{"event" => event, "amount" => amount} = payload, the decoded JSON from a payment partner. After the partner adds a refund event without an amount, requests fail with 500, the partner retries, and the crash logs fill with whole payloads, customer emails included. Explain each effect and redesign the handler.
What it tests

Seeing that let it crash is for the unexpected, not for outside input, that a MatchError prints the whole value, and checking untrusted data with case and explicit errors.

First step

Separate the two kinds of failure. A partner's payload is outside input, so a shape the code does not know is an expected event, not a bug. The match turns it into a crash, the crash into a 500 that invites a retry, and the MatchError message prints the whole payload into the log.
Follow-ups
- What status should the handler return for a payload it cannot use?
- How do you keep personal data out of the logs when a request fails?
- How do you test the handler against the partner's next new event?
From the lesson Pattern matching, the match operator and the pin
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Fresher to mid

An order router has the clauses {:ok, order} when length(order.items) > 0 -> ship(order) and {:ok, _order} -> :empty_order. Orders from an old import, whose items field is nil, have been reported as empty for weeks, and nothing ever raised. Explain why, and rewrite the clauses.
What it tests

Knowing that an error inside a guard makes the clause fail silently, matching on the shape of a list instead of its length, and deciding what data of the wrong kind should do.

First step

Run the guard by hand on an order from the import: length(nil) raises ArgumentError, but inside a guard an error only makes the clause fail, so the order falls through to the :empty_order clause. The compiler had already warned about length(order.items) > 0 and suggested a pattern instead.
Follow-ups
- Should an order with items set to nil raise, or be routed somewhere?
- Why does the compiler prefer [_ | _] to length/1 here?
- Which tests would have caught it on the first day?
From the lesson case, cond, if and unless
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Fresher to mid

An HTTP client returns {:ok, %{status: status}} or {:error, reason}. Write the case that decides whether to retry: which statuses and errors are worth another try, in what order the clauses go, and what happens to a status or a reason you did not expect.
What it tests

Writing case clauses with patterns and guards for a real policy, ordering them from specific to general, and choosing between a catch-all and a crash on purpose.

First step

Sort the outcomes first: 2xx is done; 408, 429, 502, 503 and 504 may succeed later; other 4xx and 5xx will fail the same way again; a timeout may succeed later. Then write one clause per group, the specific ones on top, with guards such as status in 400..599 for the general ones.
Follow-ups
- A 429 comes with a Retry-After header. Where does that go?
- Is it safe to retry a POST that timed out?
- How do you keep the clauses in the right order as the policy grows?
From the lesson case, cond, if and unless
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes

All Elixir & Erlang/OTP interview questions · The Elixir & Erlang/OTP track

Warm-up questions

Is = assignment in Elixir? What does it do, and what is the pin operator for?

When do you use case, when cond and when if in Elixir?

What is a guard in Elixir, and what may it contain?

Questions with model answers

An order router has the clauses {:ok, order} when length(order.items) > 0 -> ship(order) and {:ok, _order} -> :empty_order. Orders from an old import, whose items field is nil, have been reported as empty for weeks, and nothing ever raised. Explain why, and rewrite the clauses.

An HTTP client returns {:ok, %{status: status}} or {:error, reason}. Write the case that decides whether to retry: which statuses and errors are worth another try, in what order the clauses go, and what happens to a status or a reason you did not expect.