Factor Developer Hiring Guide

Hiring Guide for Factor Engineers

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

Factor is a stack-based, concatenative programming language that was initially developed by Slava Pestov in 2003. It draws inspiration from Forth, Joy, and Lisp languages while incorporating modern features such as dynamic types and garbage collection. The language's design emphasizes simplicity, extensibility, and practicality for real-world applications. Factor is known for its powerful metaprogramming capabilities and its unique approach to code organization through vocabularies. Its development continues under an open-source model with contributions from a global community of programmers.

First 20 minutes

General Factor knowledge and experience

The next 20 minutes of the interview should attempt to focus more specifically on the development questions used, and the level of depth and skill the engineer possesses.

What is the role of quotations in Factor?

In Factor, quotations are anonymous functions or blocks of code. They are often used with combinators, which are words that manipulate or apply quotations. They can also be used to create new words or to implement control structures.

How would you debug a Factor program?

Factor provides a number of tools for debugging, including the listener, the inspector, and the debugger. The listener is a REPL that allows you to interactively run Factor code. The inspector lets you examine and modify the state of objects, and the debugger allows stepping through code and inspecting the stack.

Describe the difference between stack-based and register-based programming.

In stack-based programming, like Factor, operations are performed on a stack data structure. The operands are pushed onto the stack, and operations pop their operands from the stack and push their results back onto it. In contrast, register-based programming involves storing values in a fixed number of variables or 'registers'. Operations act directly on these registers.

What are the core principles of Factor?

Factor is based on the principles of simplicity, flexibility, and expressiveness. It operates on a stack, which simplifies the language structure. It also has a flexible and extensible syntax, and it is highly expressive with its use of high-level language features.

How would you define Factor programming language?

Factor is a stack-based, concatenative programming language with high-level features including dynamic types, extensible syntax, macros, and garbage collection. It allows direct manipulation of the stack and uses a unique data structure called a 'word' for its operations.

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What youre looking for early-on

Does the candidate show an ability to learn and adapt?

Technology and programming languages are constantly evolving. A good candidate should demonstrate an ability to learn new technologies and adapt to changes.

Can the candidate demonstrate experience or knowledge of working in a team environment?

Software development is often a collaborative endeavor. Experience or ability to work effectively in a team is crucial for project success.

Is the candidate able to communicate their thought process clearly?

Clear communication is key in any role, but especially in development where the candidate may need to explain their code or problem-solving process to others.

Does the candidate exhibit strong problem-solving abilities?

Regardless of the specific language used, strong problem-solving abilities are crucial in software development to create efficient solutions and troubleshoot issues.

How comfortable is the candidate with stack-based programming?

Factor is a stack-based language, so comfort with this programming paradigm is necessary to effectively develop and troubleshoot code.

Does the candidate demonstrate a deep understanding of Factor language?

This is crucial as the role specifically requires proficiency in Factor programming. A deep understanding of the language will allow the candidate to effectively write and debug code.

Next 20 minutes

Specific Factor development questions

The next 20 minutes of the interview should attempt to focus more specifically on the development questions used, and the level of depth and skill the engineer possesses.

How would you handle exceptions in Factor?

Factor provides several words for exception handling, including 'throw', 'catch', and 'rethrow'. 'Throw' signals an exception, 'catch' establishes a dynamic extent in which exceptions can be caught, and 'rethrow' re-signals an exception to be caught by an outer catcher.

What is the purpose of the 'vocab' keyword in Factor?

The 'vocab' keyword in Factor is used to define a new vocabulary, or namespace. Vocabularies are used to organize words and to prevent name clashes. A word's full name includes the vocabulary, like 'math.sin' or 'io.print'.

Describe the difference between 'push' and 'pop' operations in Factor.

'Push' and 'pop' operations in Factor are related to the manipulation of the data stack. 'Push' adds an element to the top of the stack, while 'pop' removes the top element from the stack. These operations are fundamental to Factor's stack-based paradigm.

What are combinators in Factor and how are they used?

Combinators in Factor are higher-order functions that manipulate or apply other functions, often quotations. They are used to implement control structures and to reduce code duplication. Examples of combinators include 'map', 'reduce', 'each', and 'if'.

How would you implement a loop in Factor?

Factor does not have traditional loop constructs like 'for' or 'while'. Instead, it uses recursion and combinators. For example, to iterate over a sequence, one might use the 'each' combinator, providing a quotation that describes what to do with each element.

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The ideal back-end app developer

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

A skilled Factor engineer should demonstrate a deep understanding of Factor programming language, strong problem-solving skills, and excellent communication abilities. Red flags would include inability to explain complex concepts clearly, lack of detail in their responses, or unfamiliarity with Factor's unique features and syntax.

Digging deeper

Code questions

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

What does the following Factor code do?

10 20 + .

This code adds the numbers 10 and 20 together and prints the result, which is 30.

What will be the output of the following Factor code?

1 2 3 4 5 [ 2 * ] map .

This code multiplies each element in the array [1, 2, 3, 4, 5] by 2. The output will be the array [2, 4, 6, 8, 10].

What does the following Factor code do?

1 10 [ 2dup * ] 2map .

This code generates a sequence of squares from 1 to 10. It duplicates each number in the sequence and multiplies the duplicates together, resulting in the square of each number.

What will be the output of the following Factor code?

1 1000 [ ] [ 2dup * ] bi@ .

This code uses the 'bi@' combinator to apply the '[ ]' quotation (which does nothing) and the '[ 2dup * ]' quotation (which squares a number) to the numbers 1 through 1000. The output will be the squares of the numbers 1 through 1000.

What does the following Factor code do?

: person ( -- class )  ; person boa{ } define

This code defines a new class named 'person' with no instance variables or methods. The 'boa{' '}' construct is used to define the class's constructor, which in this case takes no arguments.

What will be the output of the following Factor code?

: fib ( n -- n )  dup 2 < [ drop 1 ] [ 2dup - recurse swap 1 - recurse + ] if ; 10 fib .

This code defines a recursive function to compute the nth Fibonacci number. It then computes the 10th Fibonacci number, which is 55, and prints it.

Wrap-up questions

Final candidate for Factor role questions

The final few interview questions for a Factor candidate should typically focus on a combination of technical skills, personal goals, growth potential, team dynamics, and company culture.

How would you optimize a Factor program for performance?

Optimizing a Factor program for performance could involve several strategies, including avoiding unnecessary memory allocation, using efficient data structures and algorithms, taking advantage of Factor's support for parallelism, and using Factor's profiling tools to identify bottlenecks. However, as with any language, the most important thing is to write clear, correct code first, and only optimize when necessary.

Describe the difference between 'curry' and 'compose' in Factor.

'Curry' and 'compose' are both combinators in Factor. 'Curry' takes a quotation and some values and returns a new quotation that, when called, will push the provided values onto the stack and then call the original quotation. 'Compose' takes two quotations and returns a new quotation that, when called, will call the first quotation, discard its results, and then call the second quotation.

What is the role of the garbage collector in Factor?

The garbage collector in Factor automatically reclaims memory that is no longer in use. This helps to prevent memory leaks and reduces the burden on the programmer. Factor's garbage collector is generational, meaning it optimizes for the case where most objects die young.

How would you implement multithreading in Factor?

Factor provides a vocabulary called 'threads' for multithreading. This includes words for creating and managing threads, such as 'spawn', 'sleep', 'kill', and 'join'. Factor's threads are mapped to native OS threads.

What are some use cases for tuples in Factor?

Tuples in Factor are used to group related data together. They are similar to structs in C or objects in JavaScript. Use cases for tuples might include representing a point in a 2D space with an 'x' and 'y' coordinate, or representing a person with a 'name' and 'age'.

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Factor application related

Product Perfect's Factor development capabilities

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