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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.

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

First 20 minutes

General Factor app knowledge and experience

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

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.

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.

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.

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.

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.

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What you’re looking for early on

Does the candidate demonstrate a deep understanding of Factor language?

How comfortable is the candidate with stack-based programming?

Does the candidate exhibit strong problem-solving abilities?

Is the candidate able to communicate their thought process clearly?

Next 20 minutes

Specific Factor 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 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.

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'.

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 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'.

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.

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

Wrap-up questions

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

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'.

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 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.

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.

Factor Consultive assessments

Factor Application Conversion Assessment Factor Application Integration Assessment Factor Data & Database Integration Assessment

Custom Factor development projects

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

Hiring guide for Factor Engineers

Factor Developer Hiring Guide

What you’re looking for early on

The ideal back-end app developer

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

Factor Consultive assessments

Custom Factor development projects