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Haskell is a purely functional programming language, named after the logician Haskell Curry, and was first introduced in 1990 by a committee of academics. It is renowned for its high degree of abstraction and mathematical accuracy, making it ideal for complex computational tasks. The language's design emphasizes correctness and predictability, with features like static typing and lazy evaluation. Haskell has influenced numerous other programming languages including Scala, Swift, and Rust. Its development is managed by the open-source community under the stewardship of the Haskell.org committee.

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

First 20 minutes

General Haskell app knowledge and experience

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

What is the role of the main function in Haskell?

The main function in Haskell is the entry point of the program. It's where the execution of the program starts.

How would you define a function in Haskell?

A function in Haskell is defined by providing a function name, followed by parameters, an equals sign, and then the function body. For example: add x y = x + y.

What are the types of data in Haskell?

Haskell has several types of data including Integer, Float, Double, Bool, Char, and String.

What does it mean that Haskell is a statically typed language?

In Haskell, being a statically typed language means that the type of every expression is known at compile time, which leads to safer code and fewer runtime errors.

How would you use a list in Haskell?

In Haskell, lists are a homogenous data structure. They can be created by enclosing the elements in square brackets, for example: [1, 2, 3, 4, 5].

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What you’re looking for early on

Does the candidate have a strong understanding of functional programming concepts?

Has the candidate demonstrated the ability to solve complex problems using Haskell?

Is the candidate familiar with Haskell's type system?

Can the candidate work with monads?

Next 20 minutes

Specific Haskell 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.

What is a tuple in Haskell and how is it different from a list?

A tuple in Haskell is a sequence of elements of different types. Unlike lists, tuples can store multiple data types and their size is fixed.

What is currying in Haskell?

Currying in Haskell is the technique of translating the evaluation of a function that takes multiple arguments into evaluating a sequence of functions, each with a single argument.

How would you implement recursion in Haskell?

Recursion in Haskell is implemented by defining a function that calls itself. For example, the factorial function can be implemented recursively in Haskell as: factorial 0 = 1; factorial n = n * factorial (n - 1).

What is a monad in Haskell?

A monad in Haskell is a design pattern that allows for computations to be chained together in a sequence, each passing its result onto the next. They are used to handle side effects such as IO, state, or exceptions.

Describe the difference between 'let' and 'where' in Haskell.

'let' and 'where' are both used to define local bindings in Haskell. The difference is in their scope and order of operations. 'let' bindings are expressions themselves, while 'where' bindings are declarations that are part of other declarations.

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At this point, a skilled Haskell engineer should demonstrate strong problem-solving abilities, proficiency in Haskell 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 Haskell.

What does the following Haskell code do?
fizzBuzz n = map fb [1..n] where
  fb x | x `mod` 15 == 0 = "FizzBuzz"
       | x `mod` 3  == 0 = "Fizz"
       | x `mod` 5  == 0 = "Buzz"
       | otherwise = show x
This code defines a function 'fizzBuzz' that takes an integer 'n' and returns a list of strings. Each string represents a number from 1 to 'n', but if the number is divisible by 3, it's replaced with 'Fizz', if it's divisible by 5, it's replaced with 'Buzz', and if it's divisible by both, it's replaced with 'FizzBuzz'.
What will be the output of the following Haskell code?
let x = 5 in let y = 6 in x * y
The output of this code will be 30. This code declares two variables 'x' and 'y' with values 5 and 6 respectively, and then multiplies them.
What does the following Haskell code do?
import Data.List
import Data.Char
digitSum :: Int -> Int
digitSum = sum . map digitToInt . show
This code defines a function 'digitSum' that takes an integer and returns the sum of its digits. It converts the integer to a string, maps each character to its integer value using 'digitToInt', and then sums the resulting list of integers.
What does the following Haskell code do?
import Control.Concurrent
main = forkIO (putStrLn "Hello from a new thread!") >> putStrLn "Hello from the main thread!"
This code creates a new thread that prints 'Hello from a new thread!', and then the main thread prints 'Hello from the main thread!'. The order in which these messages are printed is not deterministic, because the new thread and the main thread are running concurrently.

Wrap-up questions

Final candidate for Haskell 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 Haskell application deployments. Inquire about their experience in handling system failures and their approach to debugging and troubleshooting.

How would you handle exceptions in Haskell?

Exceptions in Haskell are handled using the 'catch' function. This function takes two arguments: the computation where an exception might be thrown, and a function that takes an exception and returns a computation.

What are type classes in Haskell?

Type classes in Haskell are a way of defining certain behavior for types. They are similar to interfaces in other languages. For example, the 'Eq' type class is used for types that support equality testing.

How would you implement polymorphism in Haskell?

Polymorphism in Haskell is implemented through type classes. By defining a type class, you can define behavior that can be used across multiple types.

Haskell application related

Product Perfect's Haskell development capabilities

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

Haskell Consultive assessments

Haskell Application Conversion Assessment Haskell Application Integration Assessment Haskell Data & Database Integration Assessment

Custom Haskell development projects

Custom Haskell App Conversion Haskell Custom App Development Process App UI Design for Haskell Facelifts

Hiring guide for Haskell Engineers

Haskell Developer Hiring Guide

What you’re looking for early on

The ideal back-end app developer

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

Haskell Consultive assessments

Custom Haskell development projects