Hiring guide for Mercury Engineers

Mercury Developer Hiring Guide

Mercury is a logic/functional programming language that's primarily used for the development of complex software systems. It was developed at the University of Melbourne, Australia in 1995. Mercury is designed to improve programmer productivity by providing high-level declarative constructs while also enabling efficient execution. It combines the clarity and expressiveness of declarative programming with advanced static analysis and error detection features. Its syntax and semantics are based on Prolog, but it has strong typing similar to Haskell and ML, modes (a kind of flow typing), uniqueness types (which ensure that destructive array updates are safe), higher-order programming, type classes, user-definable algebraic data types among others. The Mercury system includes not just a compiler but also a debugger, a profiler, standard library support etc. The compiler can generate C code or Java bytecode as output for portability purposes.

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

First 20 minutes

General Mercury app knowledge and experience

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

How would you install Mercury on a Unix system?
First, you would download the source code from the Mercury website. Then, you would unpack the source code using the tar command. After that, you would configure the build process by running the configure script. Finally, you would build and install the system using the make and make install commands.
What are the basic data types in Mercury?
The basic data types in Mercury include int for integers, float for floating point numbers, char for characters, and string for strings. It also has boolean for Boolean values, list for lists, and pred for predicate types.
How would you define a predicate in Mercury?
A predicate in Mercury is defined by its name, argument types, and determinism. For example, the predicate ':- pred is_even(int)::out.' defines a predicate is_even that takes an integer as input and outputs a Boolean value.
What is the purpose of the main predicate in Mercury?
The main predicate in Mercury is the entry point of the program. When the program is run, the main predicate is the first one that gets executed.
Describe the difference between a predicate and a function in Mercury.
In Mercury, a predicate is a procedure that can succeed or fail, while a function always succeeds and returns a value. A function is essentially a predicate with one additional output argument, which is the return value.
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What you’re looking for early on

Does the candidate demonstrate a solid understanding of Mercury programming language?
Has the candidate shown problem-solving skills during the interview?
Is the candidate able to communicate effectively?
Does the candidate have experience with projects similar to what your company is working on?

Next 20 minutes

Specific Mercury 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 errors in Mercury?
Errors in Mercury can be handled using exceptions. You can throw an exception using the throw predicate and catch it using the catch predicate. Alternatively, you can use the 'maybe' type to represent computations that may fail.
What is determinism in Mercury and how does it affect program execution?
Determinism in Mercury refers to the number of possible outcomes of a predicate or function. It can be deterministic, semi-deterministic, multi-deterministic, or non-deterministic. The determinism of a predicate or function affects how it can be called and how its results can be used.
Describe the difference between a list and a tuple in Mercury.
A list in Mercury is a homogeneous collection of elements, while a tuple is a heterogeneous collection of elements. In other words, all elements of a list have the same type, while elements of a tuple can have different types.
How would you implement recursion in Mercury?
Recursion in Mercury can be implemented by defining a predicate or function that calls itself. The base case is defined by a clause that does not call itself, and the recursive case is defined by a clause that does call itself.
What are type classes in Mercury and how would you define one?
Type classes in Mercury are similar to interfaces in other languages. They define a set of predicates or functions that a type must implement to be a member of the class. A type class is defined using the ':- typeclass' declaration.
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The ideal back-end app developer

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

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

What does this simple Mercury code do?
:- pred hello_world(io::di, io::uo) is det.

hello_world(!IO) :-
    io.write_string("Hello, World!
", !IO).
This code defines a predicate 'hello_world' that writes the string 'Hello, World!' to the standard output.
What does this Mercury code do?
:- pred add(int::in, int::in, int::out) is det.

add(X, Y, Z) :-
    Z = X + Y.
This code defines a predicate 'add' that takes two integers as input and outputs their sum.
What does this Mercury code do?
:- pred reverse_list(list(T)::in, list(T)::out) is det.

reverse_list([], []).
reverse_list([H|T], Rev) :-
    reverse_list(T, RevT),
    list.append(RevT, [H], Rev).
This code defines a predicate 'reverse_list' that takes a list as input and outputs the reverse of that list.
What does this Mercury code do?
:- pred concurrent_map(func(T) = U, list(T), list(U)).
:- mode concurrent_map(in, in, out) is det.

concurrent_map(_, [], []).
concurrent_map(F, [H|T], [F(H)|U]) :-
    concurrent_map(F, T, U).
This code defines a predicate 'concurrent_map' that applies a function concurrently to each element of a list.

Wrap-up questions

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

Describe the difference between higher-order and first-order predicates in Mercury.
First-order predicates in Mercury are those that do not take other predicates or functions as arguments, while higher-order predicates do. Higher-order predicates allow for more flexible and reusable code.
How would you implement concurrency in Mercury?
Concurrency in Mercury can be implemented using the 'spawn' predicate, which creates a new thread of execution. You can also use the 'wait' predicate to wait for a thread to finish.
What is the Mercury standard library and what does it provide?
The Mercury standard library is a collection of modules that provide useful predicates and functions. It includes modules for input and output, string manipulation, list manipulation, file handling, and more.

Mercury application related

Product Perfect's Mercury development capabilities

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