Forth

Forth

Hard

Instructions

Implement an evaluator for a very simple subset of Forth.

Forth is a stack-based programming language. Implement a very basic evaluator for a small subset of Forth.

Your evaluator has to support the following words:

  • +, -, *, / (integer arithmetic)
  • DUP, DROP, SWAP, OVER (stack manipulation)

Your evaluator also has to support defining new words using the customary syntax: : word-name definition ;.

To keep things simple the only data type you need to support is signed integers of at least 16 bits size.

You should use the following rules for the syntax: a number is a sequence of one or more (ASCII) digits, a word is a sequence of one or more letters, digits, symbols or punctuation that is not a number. (Forth probably uses slightly different rules, but this is close enough.)

Words are case-insensitive.

Track Specific Notes

The input is presented as a list of strings. Definitions are presented as : var x ... ; where var is bound to what follows. Otherwise the string represents a sequence of stack manipulations.

Running and testing your solutions

From the command line

Simply type make chez if you're using ChezScheme or make guile if you're using GNU Guile. Sometimes the name for the scheme binary on your system will differ from the defaults. When this is the case, you'll need to tell make by running make chez chez=your-chez-binary or make guile guile=your-guile-binary.

From a REPL

  • Enter (load "test.scm") at the repl prompt.
  • Develop your solution in forth.scm reloading as you go.
  • Run (test) to check your solution.

Failed Test Cases

If some of the test cases fail, you should see the failing input and the expected output. The failing input is presented as a list because the tests call your solution by (apply forth input-list). To learn more about apply see The Scheme Programming Language -- Chapter 5

Last updated 28 July 2022
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