Coding Challenge #95 - Forth InterpreterThis challenge is to build your own interpreter for a Forth-like programming language and learn some Forth.
Hi this is John with this week’s Coding Challenge. 🙏 Thank you for being one of the 88,421 software developers who have subscribed, I’m honoured to have you as a reader. 🎉 If there is a Coding Challenge you’d like to see, please let me know by replying to this email📧 Coding Challenge #95 - Forth InterpreterThis challenge is to build your own Forth-like interpreter. Forth is a stack-oriented programming language that was designed by Charles H. "Chuck" Moore and first used by other programmers in 1970. It gained an official standard in 1994. Although it’s not an acronym, for much of it like Forth was referred to as FORTH. Why Forth and not Fourth? Here’s why: “The first time I combined the ideas I had been developing into a single entity, I was working on an IBM 1130, a “third-generation” computer. The result seemed so powerful that I considered it a “fourth-generation computer language.” I would have called it FOURTH, except that the 1130 permitted only five-character identifiers. So FOURTH became FORTH, a nicer play on words anyway.” — Charles H. Moore, creator of Forth Forth has been used to write bestselling computer games, firmware, spaceflight software and various embedded systems. For us it’s a fun language to implement an interpreter for and it gives you a chance to learn about Reverse Polish Notation (RPN), stack oriented-programming and writing interpreters. If You Enjoy Coding Challenges Here Are Four Ways You Can Help Support It
The Challenge - Building A Forth InterpreterThis challenge is to build your own Forth-like interpreter. That is an interpreter for a subset of the Forth language, sufficient for you to write and run code to generate the Fibonacci sequence and the coding interview favourite, FizzBuzz. You can build a Forth-like interpreter in any programming language or stack. You could build it as a CLI tool or as an in-browser interpreter with a web based IDE. So this coding challenge will suit frontend, backend, fullstack, data engineer, system, or whatever type of software engineer you consider yourself to be. Me, I built it as a CLI tool just like the other compilers and interpreters I’ve built. Step ZeroAs always, in a throwback to the days when I wrote C, Coding Challenges is zero indexed. In this step your goal is to:
When it comes to learning Forth, there are two good places to start:
N.B.: If you want to go deeper there is a more in depth set of resources on the Coding Challenges blog post: learning Forth. Step 1In this step your goal is to build a simple REPL (Read-Eval-Print Loop) and allow the user to terminate the REPL by entering the Forth word: bye. In Forth, subroutines are referred to as ‘words’. The typical Forth prompt in the REPL is: So when you’ve completed this step, using your interpreter will look something like this: Not very exciting is it, so let’s add enough Forth for us to do simple calculations. Step 2In this step your goal is to handle the user entering integers and some basic mathematical operations. To enable that you need to handle entering an integer. The integer should then be pushed onto the interpreter’s data stack - remember Forth is stack based. In Forth, anything between brackets is a comment, so words are often described using comments, for example, the Here is the full list of mathematical operations:
You should add support for all these operations. To test them you’ll need to also implement a feature of most Forth REPLs, showing the stack from bottom to top before the prompt. For example: Here the integers 1,2, and 2 are pushed onto the stack, then the operations Leaving the result, 5 on the top of the stack. Step 3In this step your goal is to add support for several Forth words used to manipulate the stack: dup, drop, rot, over and swap:
Be sure to test them. Step 4In this step your goal is to implement support for four new words:
Using them looks like this: Step 5In this step your goal is to implement support for defining new words. In Forth words are defined between You should also handle calling a word that doesn’t exist. For that simply print the word and a question mark: Finally add support for comments. Comments are any text between brackets and are not interpreted: Step 6In this step your goal is to add support for conditionals and loops. The conditionals are:
Once you have them you should be able to implement if then blocks: And if else then blocks: Then move on to adding support for do loop: Don’t forget to refer to the Forth book for full details of the syntax if you need clarification. Step 7In this step your goal is to support running Forth-like code from a script file, with the name provided as an argument to the interpreter. Here are two scripts you should be able to run with the language features built so far: To generate the Fibonacci sequence and FizzBuzz: Once that works, have some fun writing your own Forth code to run in your interpreter! Going FurtherIf you’d like to take this further, work through the resources on learning Forth and read the Forth standard, then add the new features that would be of interest to you. You could also explore compiling the code, either directly or leveraging the LLVM backend. Two Other Ways I Can Help You:
Share Your Solutions!If you think your solution is an example other developers can learn from please share it, put it on GitHub, GitLab or elsewhere. Then let me know via Bluesky or LinkedIn or just post about it there and tag me. Alternately please add a link to it in the Coding Challenges Shared Solutions Github repo Request for FeedbackI’m writing these challenges to help you develop your skills as a software engineer based on how I’ve approached my own personal learning and development. What works for me, might not be the best way for you - so if you have suggestions for how I can make these challenges more useful to you and others, please get in touch and let me know. All feedback greatly appreciated. You can reach me on Bluesky, LinkedIn or through SubStack Thanks and happy coding! John Invite your friends and earn rewardsIf you enjoy Coding Challenges, share it with your friends and earn rewards when they subscribe.
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