Hiring guide for F# Engineers

F# Developer Hiring Guide

F# is a functional-first, open-source, cross-platform programming language developed by Microsoft Research and first introduced in 2005. It is a member of the ML language family, heavily influenced by OCaml and other functional languages like Haskell. F# combines the efficiency of compiled languages with the simplicity of scripting languages and supports both functional and object-oriented programming paradigms. It is primarily used in the fields of finance, data science, machine learning, and academia due to its strong type inference, succinct syntax, and immutability. The language's development and maintenance are currently overseen by the F# Software Foundation.

Ask the right questions secure the right F# talent among an increasingly shrinking pool of talent.

First 20 minutes

General F# app knowledge and experience

The first 20 minutes of the interview should seek to understand the candidate's general background in F# application development, including their experience with various programming languages, databases, and their approach to designing scalable and maintainable systems.

How would you define F#?
F# is a functional-first, open source, cross-platform programming language. It is developed by Microsoft and is part of the .NET platform. F# is designed to overcome the complexity of writing software by providing succinct, efficient, and robust programming capabilities.
What are the key features of F#?
F# has features such as strong typing, type inference, pattern matching, interactive scripting and debugging, data-centric programming, and support for both functional and object-oriented programming paradigms.
How would you explain type inference in F#?
Type inference in F# means that the compiler determines the data types of variables and expressions automatically. This allows the developer to write code without having to explicitly declare the type of every variable.
Describe the difference between mutable and immutable data types in F#.
In F#, mutable data types can be changed after they are initially set, while immutable data types cannot be changed after they are initially set. Immutable data types are the default in F#, which helps to make code safer and easier to reason about.
What are discriminated unions in F#?
Discriminated unions in F# provide a way to create a type that can be one of a number of named cases, each possibly with different values and types. They are used to represent data that can be in one of several distinct states.
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What you’re looking for early on

Has the candidate demonstrated a strong understanding of F# syntax and concepts?
Does the candidate have practical experience with F#?
How well does the candidate understand functional programming principles?
Is the candidate familiar with .NET framework?

Next 20 minutes

Specific F# development questions

The next 20 minutes of the interview should focus on the candidate's expertise with specific backend frameworks, their understanding of RESTful APIs, and their experience in handling data storage and retrieval efficiently.

How would you handle exceptions in F#?
In F#, exceptions can be handled using try/catch/finally blocks. The 'try' block contains the code that might throw an exception, the 'catch' block contains the code to execute if an exception is thrown, and the 'finally' block contains the code that is always executed, whether an exception is thrown or not.
What is pattern matching in F#?
Pattern matching in F# is a technique for checking a value against a series of conditions in a concise, readable way. It can be used to destructure data types, handle options and results, and control program flow.
Describe the difference between sequences, lists, and arrays in F#.
In F#, sequences are collections of items that can be enumerated over, lists are immutable linked lists, and arrays are mutable collections of items with a fixed size. Sequences are lazy and can represent infinite collections, while lists and arrays are eager and finite.
What are active patterns in F#?
Active patterns in F# are a way to define custom patterns for use in pattern matching. They allow you to abstract over common pattern matching idioms and to partition data in a way that makes sense for your problem domain.
How would you create a custom operator in F#?
In F#, you can create a custom operator by defining a static member on a type with a special name that starts and ends with a star (*). The body of the operator is a function that implements the operator's behavior.
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The ideal back-end app developer

What you’re looking to see on the F# engineer at this point.

At this point, a skilled F# engineer should demonstrate strong problem-solving abilities, proficiency in F# programming language, and knowledge of software development methodologies. Red flags include lack of hands-on experience, inability to articulate complex concepts, or unfamiliarity with standard coding practices.

Digging deeper

Code questions

These will help you see the candidate's real-world development capabilities with F#.

What does the following F# code do?
let square x = x * x
This code defines a function named 'square' that takes an argument 'x' and returns the square of 'x'.
What will be the output of the following F# code?
let rec factorial n = if n = 0 then 1 else n * factorial (n - 1)
let result = factorial 5
The output of this code will be 120. The code defines a recursive function 'factorial' that calculates the factorial of a number. It then calls this function with the argument 5.
What does the following F# code do?
let list = [1; 2; 3; 4; 5]
let reversed = List.rev list
This code first defines a list of integers. It then uses the 'List.rev' function to create a new list that is the reverse of the original list.
What does the following F# code do?
open System.Threading
let thread = Thread(fun _ -> printfn "Hello, world!")
This code creates and starts a new thread that prints 'Hello, world!' to the console.

Wrap-up questions

Final candidate for F# Developer role questions

The final few questions should evaluate the candidate's teamwork, communication, and problem-solving skills. Additionally, assess their knowledge of microservices architecture, serverless computing, and how they handle F# application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

What is currying in F#?
Currying in F# is the technique of transforming a function with multiple arguments into a function with a single argument that returns another function. It allows for partial application of functions and can make code more concise and flexible.
Describe the difference between eager and lazy evaluation in F#.
In F#, eager evaluation means that expressions are evaluated as soon as they are defined, while lazy evaluation means that expressions are evaluated only when their value is needed. Sequences in F# are an example of a data type that uses lazy evaluation.
What are computation expressions in F#?
Computation expressions in F# are a way to define custom computational workflows in a way that looks like built-in language support. They can be used to simplify code that deals with side effects, asynchronous computations, and other complex scenarios.

F# application related

Product Perfect's F# development capabilities

Beyond hiring for your F# engineering team, you may be in the market for additional help. Product Perfect provides seasoned expertise in F# projects, and can engage in multiple capacities.