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Language Atlas (mini-courses for more languages) Module 12

Elm and The Elm Architecture interview questions

Interview questions on elm and The Elm Architecture: short warm-ups, then what each harder question tests, the first step of a good answer and its follow-ups.

12 questions: 4 warm-ups and 8 with model answers · Levels: Fresher to staff

Warm-up questions

Short questions with short answers, each answered in full.

Fresher to mid

Where does an Elm application record the versions of its packages, and what is ~/.elm for?

Answered in the lesson Elm 0.19.3: install, build and run headlessly
Fresher to mid

What do Maybe and Result replace in JavaScript, and how do you get a value out of one?

Answered in the lesson Elm for JavaScript developers
Fresher to mid

What are the parts of The Elm Architecture, and who calls them?

Answered in the lesson The Elm Architecture, JSON and ports
Fresher to mid

Why does the splitter work in whole paise instead of rupees with decimals?

Answered in the lesson Project: a bill-splitter app in Elm

Questions with model answers

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

Fresher to senior

Your Elm app's CI starts every job on a fresh machine. On some days the build fails at its first step with PROBLEM LOADING PACKAGE LIST, while builds on laptops work. Nobody changed elm.json. What is going on, and how do you make the CI build reliable?
What it tests

Knowing what elm init and elm make fetch, the role of ~/.elm and ELM_HOME, elm.json as its own lock file, and caching dependencies in CI without weakening reproducible builds.

First step

Separate the versions from the files. elm.json already fixes every version, direct and indirect, so nothing is being resolved differently; what fails is the download. A fresh machine starts with an empty ~/.elm and must reach package.elm-lang.org before it can build, while a laptop's cache is full, which is why the same project builds there even offline.
Follow-ups
- How do you cache the packages without risking a stale or poisoned cache?
- What changes if the CI machines may never reach the internet?
- A teammate adds elm/http. What happens on the next CI run?
From the lesson Elm 0.19.3: install, build and run headlessly
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In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 4 minutes
Mid to staff

Your admin dashboard is a large JavaScript app. A team wants to write a new multi-step refund flow in Elm, and the flow must use the dashboard's date picker and its signed-in session. Do you agree, and how do you introduce Elm so that the risk stays small?
What it tests

Weighing Elm's compile-time guarantees against the cost of the JavaScript border, embedding Browser.element in an existing page, designing flags and ports, and planning a rollout that can be undone.

First step

Start from what Elm buys here: a flow with many states that must never mix is where custom types and the compiler's checks pay most. Then count the border crossings: the date picker and the session are JavaScript, so each needs a flag, a port or a custom element. If they are few and stable, embed one Browser.element on the refund page only.
Follow-ups
- How does the Elm element get the user's session, and what must it never receive?
- How do you use the existing date picker from Elm?
- What result would make you stop the experiment?
From the lesson Elm 0.19.3: install, build and run headlessly
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Fresher to senior

The refunds screen of your Elm app shows a refund of 5 paise as 0.95 rupees, while positive amounts look right and every existing test passes. The formatter uses paise // 100 and modBy 100 paise. Explain the bug, fix it, and say which tests would have caught it.
What it tests

Knowing Elm's truncating // against the floored modBy, handling the sign when formatting money, and choosing test values at the edges: negative amounts, zero and amounts under one rupee.

First step

Work -5 through by hand. -5 // 100 is 0, because // drops the fraction towards zero, while modBy 100 -5 is 95, because modBy takes the sign of the modulus. The two halves of the amount come from different rounding rules, and they agree only for amounts of 0 and more.
Follow-ups
- Why did the existing tests pass?
- Would remainderBy instead of modBy fix it?
- How would you make a negative amount impossible where none is allowed?
From the lesson Elm for JavaScript developers
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Mid to staff

After a release, the browser console of your Elm app shows "Compiled in DEV mode", and a Debug.log line prints customer totals. The CI log of the deploy ends with Success!. What happened in the pipeline, and how do you fix it so that it cannot happen again?
What it tests

Reading DEBUG REMNANTS, knowing that a failed elm make leaves the earlier output in place, trusting exit statuses over log text, and designing a build step that fails closed.

First step

Find the optimised build in the CI log. If it printed DEBUG REMNANTS, it failed with exit status 1 and wrote nothing, even though the screen ended with Success!. Then check what was uploaded: the file an earlier development build left in the output folder, which still holds the DEV mode warning and the Debug.log call.
Follow-ups
- Why did the screen end with Success!?
- How do you keep Debug calls from reaching the main branch at all?
- The console showed customer totals. What else do you do?
From the lesson Elm for JavaScript developers
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Mid to staff

Your Elm app shows a payment status that a third-party JavaScript library reports through a port. In production some sessions stop updating, and the console shows "Trying to send an unexpected type of value through port". What is going on, and how do you make that border robust?
What it tests

Knowing how typed ports check values, why the exception lands in the sender's JavaScript, ports of type Json.Decode.Value with decoders, and keeping evidence of bad values.

First step

Read where the exception is thrown: in the JavaScript that called send, because the value did not match the port's declared type. The library sometimes sends another shape, such as a string or an object where the port wants an Int, and the throw also skips whatever came after send in that callback, which is why the sessions stop updating.
Follow-ups
- Why does the message end in [object Object]?
- How do you change the port without breaking the rest of the app?
- How do you learn which shapes the library really sends?
From the lesson The Elm Architecture, JSON and ports
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Mid to staff

The orders API is moving to a new response format with prices in a nested object, and for a few weeks the server will send both formats. Your Elm app decodes orders. How do you change the decoders, test them and roll out without errors for either format?
What it tests

Versioned decoders with andThen or oneOf, strict optional fields, tests on recorded documents of both formats, and the order of a rollout between a server and its clients.

First step

Agree on how a document says which format it is: a version field is best, read first, as the lesson's decoder does. Then add a decoder for the new shape beside the old one, both producing the same Order, and keep both strict, so that an unexpected document fails with a path instead of being half read.
Follow-ups
- The server cannot add a version field. What then?
- How do you test the decoders before the server ships?
- When can the old decoder be deleted?
From the lesson The Elm Architecture, JSON and ports
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes
Mid to staff

Users of your Elm bill splitter notice that the diner whose name comes first always pays the odd paise, a few paise every week. Change the rule so that it is fair over time without breaking the guarantee that the shares add up, and say how you would test it.
What it tests

Separating the sum guarantee from the allocation rule, designing a deterministic rotation, keeping old bills reproducible, and testing both properties with fuzz tests.

First step

Keep the part that makes the shares add up, equal whole parts plus a leftover from 0 to n - 1 that is always given out, and change only who receives the leftover. A rotation that starts each item where the previous item's leftover stopped spreads the odd paise, and it gives the same answer for the same bill every time.
Follow-ups
- Old bills were split with the alphabetical rule. What happens to them?
- Which properties do the new fuzz tests check?
- Would giving the odd paise out at random be fairer?
From the lesson Project: a bill-splitter app in Elm
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 4 minutes
Fresher to senior

A diner says the splitter's total is 2 paise more than the restaurant's printed bill. The restaurant applies tax once to the subtotal, while the splitter rounds tax on each item. Which is right, and how do you make the app match the receipt without breaking the shares?
What it tests

Understanding rounding per item against rounding per bill, how far apart they can be, making the rounding rule a choice of the bill, and reconciling a total with a printed document.

First step

Show that both follow a rule: rounding each item's tax half up can differ from rounding the subtotal's tax by up to half a paisa per item, so four items of 1450 paise at 5 percent give 4 × 73 = 292 paise of tax when each item is rounded, but 290 on the subtotal of 5800. The receipt is what the diners pay, so the app should be able to follow it.
Follow-ups
- How do you share a tax that is computed on the whole bill?
- Which test proves the new mode matches a receipt?
- Should the diners choose the mode?
From the lesson Project: a bill-splitter app in Elm
Model answer, Pro.
In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 3 minutes

The lessons of this module

Elm 0.19.3: install, build and run headlessly Free lesson, Pro answers: 25 minutes
Elm for JavaScript developers Free lesson, Pro answers: 20 minutes
The Elm Architecture, JSON and ports Free lesson, Pro answers: 20 minutes
Project: a bill-splitter app in Elm Free lesson, Pro answers: 25 minutes

All Language Atlas (mini-courses for more languages) interview questions · The Language Atlas (mini-courses for more languages) track

Warm-up questions

Where does an Elm application record the versions of its packages, and what is ~/.elm for?

What do Maybe and Result replace in JavaScript, and how do you get a value out of one?

What are the parts of The Elm Architecture, and who calls them?

Why does the splitter work in whole paise instead of rupees with decimals?

Questions with model answers

Your Elm app's CI starts every job on a fresh machine. On some days the build fails at its first step with PROBLEM LOADING PACKAGE LIST, while builds on laptops work. Nobody changed elm.json. What is going on, and how do you make the CI build reliable?

Your admin dashboard is a large JavaScript app. A team wants to write a new multi-step refund flow in Elm, and the flow must use the dashboard's date picker and its signed-in session. Do you agree, and how do you introduce Elm so that the risk stays small?

The refunds screen of your Elm app shows a refund of 5 paise as 0.95 rupees, while positive amounts look right and every existing test passes. The formatter uses paise // 100 and modBy 100 paise. Explain the bug, fix it, and say which tests would have caught it.

After a release, the browser console of your Elm app shows "Compiled in DEV mode", and a Debug.log line prints customer totals. The CI log of the deploy ends with Success!. What happened in the pipeline, and how do you fix it so that it cannot happen again?

Your Elm app shows a payment status that a third-party JavaScript library reports through a port. In production some sessions stop updating, and the console shows "Trying to send an unexpected type of value through port". What is going on, and how do you make that border robust?

The orders API is moving to a new response format with prices in a nested object, and for a few weeks the server will send both formats. Your Elm app decodes orders. How do you change the decoders, test them and roll out without errors for either format?

Users of your Elm bill splitter notice that the diner whose name comes first always pays the odd paise, a few paise every week. Change the rule so that it is fair over time without breaking the guarantee that the shares add up, and say how you would test it.

A diner says the splitter's total is 2 paise more than the restaurant's printed bill. The restaurant applies tax once to the subtotal, while the splitter rounds tax on each item. Which is right, and how do you make the app match the receipt without breaking the shares?