Hiring guide for TXL Engineers

TXL Developer Hiring Guide

TXL is a unique programming language specifically designed for prototyping computer languages and transformations on source code. It was initially developed by the University of Ottawa, Canada. TXL stands for "Turing eXtender Language," harking back to its original purpose of extending the capabilities of Turing machines. The key feature that sets TXL apart from other programming languages is its ability to transform one form of source code into another through pattern matching and replacement. This makes it particularly useful for tasks such as program transformation, software renovation, semantic analysis, and rapid prototyping of new languages or language extensions. TXL is a functional rule-based language that uses a term rewriting system. It can parse arbitrary context-free grammars and includes built-in support for all common data types, including lists, trees, strings, integers and floating-point numbers. The transformations in TXL are specified as rewrite rules where the left-hand side specifies a pattern to be matched against the input code while the right-hand side specifies how to replace or rewrite matched instances. Despite its specialized focus on transformation tasks, TXL is Turing complete and can be used for general-purpose programming as well.

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

First 20 minutes

General TXL app knowledge and experience

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

How would you describe the basic structure of a TXL program?
A TXL program consists of a set of transformation rules. Each rule has a pattern and a replacement. The pattern is matched against the input and if it matches, it is replaced by the replacement.
What are the main components of a TXL program?
The main components of a TXL program are the grammar, the transformation rules, and the main function.
Describe the difference between a TXL rule and a TXL function.
A TXL rule is a transformation that is applied to the input. A TXL function is a reusable piece of code that can be called from within a rule or another function.
How would you use TXL to parse a source code file?
You would use the parse command followed by the name of the grammar and the name of the source code file.
What are the steps to create a new TXL program?
First, you define the grammar. Then, you write the transformation rules. Finally, you write the main function that applies the rules to the input.
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What you’re looking for early on

Does the candidate have a strong understanding of TXL programming language?
Can they solve problems using TXL?
Have they demonstrated good communication skills during the interview?
Do they have experience with other programming languages?

Next 20 minutes

Specific TXL 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.

Describe the difference between a TXL parse tree and a TXL abstract syntax tree.
A TXL parse tree represents the structure of the input as it was parsed according to the grammar. An abstract syntax tree is a simplified version of the parse tree that omits unnecessary details.
How would you handle errors in a TXL program?
You would use the fail command to abort the current transformation and return an error message.
What are the different types of TXL variables and how are they used?
There are two types of TXL variables: pattern variables and replacement variables. Pattern variables are used in the pattern of a rule to match any value, while replacement variables are used in the replacement of a rule to insert a value.
Describe the difference between a TXL deconstruct and a TXL reconstruct.
A TXL deconstruct breaks down a value into its constituent parts, while a TXL reconstruct builds a new value from its constituent parts.
How would you use TXL to transform a source code file?
You would use the transform command followed by the name of the TXL program and the name of the source code file.
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The ideal back-end app developer

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

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

What does this simple TXL code do?
rule main replace [program] P [program] with P [program] end rule
This code is a basic TXL rule that matches the entire input program (P) and replaces it with itself. Essentially, it does nothing to the input program.
What does this TXL code do?
rule replaceIntegers replace [expression] E [integer] with '0' end rule
This TXL code defines a rule that replaces all integer expressions (E) in the input program with the integer 0.
What will be the output of this TXL code given an array of integers?
rule sortArray replace [expression] E [array] sort E end rule
This TXL code sorts an array of integers in ascending order. The output will be the input array sorted in ascending order.
What does this TXL code do in terms of threading or concurrency?
rule avoidDeadlock replace [statement] 'lock(A); lock(B);' with 'lock(B); lock(A);' end rule
This TXL code is a rule that changes the order of lock acquisition in an attempt to avoid potential deadlocks. It replaces any instance where lock A is acquired before lock B with an instance where lock B is acquired before lock A.

Wrap-up questions

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

What are the steps to debug a TXL program?
First, you would use the trace command to trace the execution of the program. Then, you would use the debug command to step through the program one transformation at a time.
Describe the difference between a TXL match and a TXL replace.
A TXL match checks if a value matches a pattern, while a TXL replace replaces a value that matches a pattern with a new value.
How would you use TXL to refactor a source code file?
You would write a TXL program that transforms the source code in the desired way, and then use the transform command to apply the program to the source code file.

TXL application related

Product Perfect's TXL development capabilities

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