Forth Developer Hiring Guide

Hiring Guide for Forth Engineers

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

**Forth** is a stack-based computer programming language designed in the 1970s by Charles Moore. It is known for its simplicity, elegance, and efficiency. Forth is a compiled language, which means that it is translated into machine code before it is executed. This makes Forth very fast, but it also means that it is not as portable as interpreted languages like Python or Java. Forth is often used in embedded systems, where speed and efficiency are essential. **Sources:** * [Forth Wikipedia page](https://en.wikipedia.org/wiki/Forth_(programming_language)) * [Forth Language Reference Manual](https://www.forth.org/fortran/forthref.html)

First 20 minutes

General Forth 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 does the command 'SWAP' do in Forth?

'SWAP' command in Forth is used to swap the top two items on the stack. If the top two items are 3 and 4, after executing 'SWAP' the top item would be 3 and the second item would be 4.

How would you use the 'DUP' command in Forth?

'DUP' is used to duplicate the top item on the stack. For example, if the top item on the stack is 3, after executing 'DUP', the stack would have two 3's on top.

Describe the difference between immediate and non-immediate words in Forth.

Immediate words in Forth are executed as soon as they are encountered during the compilation process, while non-immediate words are not executed until the program is run.

What are the two main types of words in Forth?

The two main types of words in Forth are defining words and executing words. Defining words are used to create new words and executing words perform a specific operation.

How would you define the stack in Forth?

The stack in Forth is a LIFO (Last In, First Out) data structure that is used for temporary storage of data and return addresses.

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What youre looking for early-on

Has the candidate kept up-to-date with the latest developments in Forth and related technologies?

The tech industry is always evolving. A good developer should stay updated with the latest advancements to ensure they are using the best and most efficient methods.

What is the candidate's ability to work in a team or collaborate with others?

Software development is often a team effort. The candidate should be able to collaborate effectively with others, share ideas, and take constructive criticism.

How familiar is the candidate with problem-solving and debugging in Forth?

Problem-solving is a crucial skill for developers. The candidate should be able to identify, troubleshoot, and fix issues in the code.

Does the candidate have experience with stack-based programming?

Since Forth is a stack-based language, having experience with this type of programming is crucial to effectively use the language.

Can the candidate write efficient, readable, and maintainable code in Forth?

The ability to write quality code is essential for any developer. It ensures that the code can be easily understood, maintained, and optimized by others.

Does the candidate demonstrate a strong understanding of Forth language?

A proficient Forth developer should have a strong understanding of the language, its syntax, and how to use it effectively.

Next 20 minutes

Specific Forth 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.

What are the uses of 'CREATE' and 'DOES>' commands in Forth?

'CREATE' is used to define a new word and 'DOES>' is used to give it a runtime behavior. For example, 'CREATE ARRAY 100 ALLOT DOES> @' creates a 100-element array and defines its behavior at runtime.

How would you implement a loop in Forth?

A loop in Forth can be implemented using the 'DO' and 'LOOP' commands. For example, '5 0 DO I . LOOP' will print the numbers 0 to 4.

Describe the difference between 'POSTPONE' and '[COMPILE]' in Forth.

'POSTPONE' and '[COMPILE]' are both used to compile a word at runtime. However, 'POSTPONE' works with both immediate and non-immediate words, while '[COMPILE]' only works with non-immediate words.

What is the purpose of the 'DROP' command in Forth?

The 'DROP' command in Forth is used to remove the top item from the stack.

How would you create a new defining word in Forth?

You can create a new defining word in Forth using the ':' command. For example, ': SQUARE DUP * ;' would define a new word called 'SQUARE' that squares a number.

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The ideal back-end app developer

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

A skilled Forth engineer should demonstrate deep knowledge of the Forth language, problem-solving capabilities, and strong debugging skills. Red flags would include inability to explain complex Forth concepts, lack of hands-on experience, and poor communication skills.

Digging deeper

Code questions

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

What does the following simple Forth code do?

: SQUARE ( n -- n^2 ) DUP * ;

This code defines a new word, SQUARE, which takes the top item on the stack, duplicates it using DUP, and multiplies the top two items on the stack using *. The result is that it squares the input number.

What will be the output of the following Forth code?

: GREETING ( -- ) 'Hello, World!' . ; GREETING

The output of the code will be 'Hello, World!'. The code defines a new word, GREETING, which pushes the string 'Hello, World!' onto the stack and then prints it using the . operator. The word GREETING is then invoked.

What does the following Forth code do that involves array manipulation?

: ARRAY ( n1 n2 -- array ) CREATE CELLS ALLOT ; 2 3 ARRAY myArray

This code defines a new word, ARRAY, which creates an array with the specified number of cells. The word takes two inputs from the stack, multiplies them using CELLS (which gives the total number of cells needed), and reserves that many cells using ALLOT. The word ARRAY is then used to create a new array, myArray, with 2*3=6 cells.

Can you explain what the following Forth code does that involves threading?

: PARALLEL ( xt1 xt2 -- ) 2DUP 2>R ['] THREAD >R EXECUTE R> EXECUTE ;

This code defines a new word, PARALLEL, which takes two execution tokens (xt1 and xt2) from the stack, duplicates them using 2DUP, and moves them to the return stack using 2>R. It then fetches the execution token of the THREAD word using ['] and pushes it to the return stack using >R. The EXECUTE word is then used to start the two threads in parallel. The R> EXECUTE sequence is used to fetch and execute the second thread.

What does the following Forth code do that involves class design?

: CLASS ( -- ) CREATE DOES> @ ; : METHOD ( xt class -- ) >BODY ! ; 3 CLASS MyClass ' + MyClass METHOD

This code defines two new words, CLASS and METHOD. The CLASS word creates a new class (or object) and sets its behavior using DOES> @, which fetches the class's behavior when it's invoked. The METHOD word sets a method (or behavior) of a class. It takes an execution token and a class from the stack, fetches the body of the class using >BODY, and stores the execution token in the class using !. The code then creates a new class, MyClass, and sets its method to the '+' operation.

What does the following advanced Forth code do?

: FIB ( n -- n1 ) DUP 2 < IF DROP 1 ELSE DUP 1- RECURSE SWAP 2 - RECURSE + THEN ;

This code defines a new word, FIB, which calculates the Fibonacci number of the input number. The word checks if the input number is less than 2 using DUP 2 <. If it's true, it drops the input number and pushes 1 onto the stack. If it's false, it duplicates the input number, decrements it by 1, and recursively calls FIB. It then swaps the top two items on the stack, decrements the top item by 2, recursively calls FIB, and adds the two results. The THEN ends the IF-ELSE structure.

Wrap-up questions

Final candidate for Forth role questions

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

Describe how multitasking is achieved in Forth.

Multitasking in Forth can be achieved using the 'PAUSE' command which allows another task to run, and the 'RESUME' command which resumes a paused task. There is also a 'FORGET' command to remove a task.

How would you handle errors in Forth?

Errors can be handled in Forth using the 'CATCH' and 'THROW' commands. 'CATCH' executes a word and returns an error code, while 'THROW' stops execution and returns the error code if it is not zero.

What is the purpose of the 'SEE' command in Forth?

The 'SEE' command in Forth is used to decompile a word and display its definition.

Describe the difference between 'LEAVE' and 'EXIT' in Forth.

'LEAVE' is used to prematurely end a 'DO' loop, while 'EXIT' is used to prematurely end a word definition.

How would you implement recursion in Forth?

Recursion can be implemented in Forth using the 'RECURSE' command. For example, ': FACTORIAL DUP 1 > IF DUP 1- RECURSE * THEN ;' defines a recursive factorial function.

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Forth application related

Product Perfect's Forth development capabilities

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

Forth Consultive assessments

Custom Forth development projects

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