Product Perfect

**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)

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

First 20 minutes

General Forth app knowledge and experience

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

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.

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.

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.

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.

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.

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What you’re looking for early on

Does the candidate demonstrate a strong understanding of Forth language?

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

Does the candidate have experience with stack-based programming?

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

Next 20 minutes

Specific Forth 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 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.

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.

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.

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.

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.

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

Wrap-up questions

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

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.

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.

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.

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

Forth Application Conversion Assessment Forth Application Integration Assessment Forth Data & Database Integration Assessment

Custom Forth development projects

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

Hiring guide for Forth Engineers

Forth Developer Hiring Guide

What you’re looking for early on

The ideal back-end app developer

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

Forth Consultive assessments

Custom Forth development projects