Scenario-Based F# Interview Questions and Answers (2025)

 

Scenario-Based F# Interview Questions and Answers

 

Scenario 1: Designing an Immutable Domain Model

Q1: You're building a financial transaction system. How would you model a transaction immutably in F#?

Answer:

type Transaction = {
    Id: Guid
    FromAccount: string
    ToAccount: string
    Amount: decimal
    Timestamp: DateTime
}

Why it matters:
F# favors immutability for safety and concurrency. An immutable domain model prevents accidental state mutation, especially in multi-threaded systems like financial applications.

 

Scenario 2: REST API for a Blog Using Giraffe

Q2: You’re building a blog platform. How would you handle a GET /posts/{slug} route using F# and Giraffe?

Answer (simplified):

let getPostHandler slug =
    fun (next: HttpFunc) (ctx: HttpContext) ->
        task {
            let post = getPostBySlug slug // Assume this is defined
            return! json post next ctx
        }
 
let webApp =
    choose [
        routef "/posts/%s" getPostHandler
    ]

Why it matters:
This tests your ability to build RESTful APIs using Giraffe's functional routing and async handling.

 

Scenario 3: Pipeline for Processing Orders

Q3: You’re processing e-commerce orders in stages (validate → calculate tax → save). How would you design a pipeline in F#?

Answer:

let validateOrder order =
    if order.Amount <= 0m then Error "Invalid amount" else Ok order
 
let calculateTax order =
    let tax = order.Amount * 0.1m
    { order with Tax = tax } |> Ok
 
let saveOrder order =
    // Simulate DB save
    printfn "Saved order"
    Ok order
 
let processOrder order =
    validateOrder order
    |> Result.bind calculateTax
    |> Result.bind saveOrder

Why it matters:
This uses function composition and error handling with Result, which are central to robust F# programs.

 

Scenario 4: Handling Asynchronous Workflows

Q4: You need to fetch user data from a remote API and log it asynchronously. How do you handle this in F#?

Answer:

let fetchUserData userId = async {
    // Simulated async HTTP call
    return $"User {userId}"
}
 
let logData data = async {
    printfn "Logging: %s" data
    return ()
}
 
let handleUser userId = async {
    let! data = fetchUserData userId
    do! logData data
}

Why it matters:
Tests understanding of F#’s async workflows and coordination of side effects.

 

Scenario 5: Updating State with a Discriminated Union

Q5: You're implementing a state machine for an order. How do you model state transitions in F#?

Answer:

type OrderState =
    | Created
    | Paid
    | Shipped
    | Cancelled
 
type OrderEvent =
    | Pay
    | Ship
    | Cancel
 
let transition state event =
    match state, event with
    | Created, Pay -> Paid
    | Paid, Ship -> Shipped
    | _, Cancel -> Cancelled
    | _ -> state // Invalid transitions

Why it matters:
F# excels at modeling state transitions using pattern matching and DUs.

 

Scenario 6: Handling Errors Without Exceptions

Q6: You need to read a config file and parse its contents. How would you handle failure without exceptions?

Answer:

let readConfig path =
    try
        let content = System.IO.File.ReadAllText(path)
        Ok content
    with
    | :? System.IO.IOException as ex -> Error ex.Message

Why it matters:
Tests idiomatic error handling using Result instead of exceptions.

 

Scenario 7: Parallelizing Computation

Q7: You have a list of URLs and need to download their HTML content in parallel. How do you do it in F#?

Answer:

let download url = async {
    use client = new System.Net.Http.HttpClient()
    let! html = client.GetStringAsync(url) |> Async.AwaitTask
    return (url, html)
}
 
let downloadAll urls =
    urls
    |> List.map download
    |> Async.Parallel
    |> Async.RunSynchronously

Why it matters:
F# makes parallelism composable and clean using Async.Parallel.

 

Bonus Tips for Interviews

·         Show understanding of immutability, functional pipelines, and type safety.

·         Use Discriminated Unions and Pattern Matching to model complex logic.

·         Be familiar with Giraffe, Saturn, or Suave for web-based scenarios.

·         Understand how F# handles error propagation (via Result<'a, 'b> or Option<'a>).




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