Скачать или смотреть Mastering defconst in Common Lisp with Macros

Learn how to automate the creation of constants in Common Lisp using macros. Dive deep into the `defconst` function and explore a working solution.
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This video is based on the question https://stackoverflow.com/q/63501806/ asked by the user 'BitTickler' ( https://stackoverflow.com/u/2225104/ ) and on the answer https://stackoverflow.com/a/63504420/ provided by the user 'Rainer Joswig' ( https://stackoverflow.com/u/69545/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Mastering defconst in Common Lisp with Macros: A Step-by-Step Guide

In the world of programming, there are often repetitive tasks that can be tedious and time-consuming. If you're working with Common Lisp, you've likely encountered the need to define multiple constants at once, such as in a chessboard scenario where you might want to define positions or square names. This is where the power of Lisp macros can come into play. In this guide, we'll explore how to use a macro to automate the creation of constants with the defconst function, specifically focusing on a scenario where you need to define 64 constants for square names on a chessboard. Let's dive into the problem and its elegant solution!

The Problem: Defining Multiple Constants

Imagine you are building a chess application using Common Lisp, and you need to define 64 constants for the square names, from "SQ-A1" to "SQ-H8". Manually typing these can be error-prone and tedious. You might wonder if you can leverage Common Lisp’s macro capabilities to simplify this process.

A user sought help when their attempt to create these constants using a macro didn’t work as expected. The initial code compiled without errors, but upon execution, none of the constants were defined. Here’s a look at the code that was causing the confusion:

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Understanding the Issue

To break down the problem, it's essential to understand how macros work in Common Lisp and why the original code failed to produce the desired constants. Here are a couple of key points regarding the failed implementation:

Returning Code: Macros need to return executable code. If they return an empty list or nothing, like in the example with dotimes, then there is no action taken.

Use of make-symbol: The original implementation was intended to create symbols for the constant names, but it utilized make-symbol incorrectly, meaning those names were not being generated as intended.

Macros in Action

To visualize this, consider the fact that when the macro defconst-all-square-names is called, it expands to an empty list if not constructed properly. As mentioned in the feedback, the use of dotimes alone lacks the side effects needed to define constants.

The Solution: A Working Macro

After some interaction with fellow developers and a bit of troubleshooting, a working solution was developed. Here’s a breakdown of the correct approach:

1. Creating Square Names

The function square-name remains unchanged and generates the names based on an index:

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2. Modifying the Macro

The macro now uses intern instead of make-symbol, enabling it to generate and define constant names correctly:

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3. Execute and Enjoy

You can run the macro with the following call:

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How it Works

Using progn: This ensures that the expanded forms of the code run sequentially.

Using loop: The loop iterates from 0 to 63, collecting the generated defconstant expressions into a list for execution.

Conclusion

Using macros in Common Lisp can significantly streamline tasks like defining multiple constants. By ensuring that your macros generate executable code and using appropriate functions like intern, you can avoid common pitfalls and automate repetitive tasks efficiently. Whether you are building games, simulations, or any other application where constants are required, mastering macros is a valuable skill in your Lisp toolkit. Happy coding!