Examples of what Lisp's macros can be used for

Question

I've heard that Lisp's macro system is very powerful. However, I find it difficult to find some practical examples of what they can be used for; things that would be difficult to achieve without them.

Can anyone give some examples?

Answer 1

Source code transformations. All kinds. Examples:

• New control flow statements: You need a WHILE statement? Your language doesn't have one? Why wait for the benevolent dictator to maybe add one next year. Write it yourself. In five minutes.

• Shorter code: You need twenty class declarations that almost look identical - only a limited amount of places are different. Write a macro form that takes the differences as parameter and generates the source code for you. Want to change it later? Change the macro in one place.

• Replacements in the source tree: You want to add code into the source tree? A variable really should be a function call? Wrap a macro around the code that 'walks' the source and changes the places where it finds the variable.

• Postfix syntax: You want to write your code in postfix form? Use a macro that rewrites the code to the normal form (prefix in Lisp).

• Compile-time effects: You need to run some code in the compiler environment to inform the development environment about definitions? Macros can generate code that runs at compile time.

• Code simplifications/optimizations at compile-time: You want to simplify some code at compile time? Use a macro that does the simplification - that way you can shift work from runtime to compile time, based on the source forms.

• Code generation from descriptions/configurations: You need to write a complex mix of classes. For example your window has a class, subpanes have classes, there are space constraints between panes, you have a command loop, a menu and a whole bunch of other things. Write a macro that captures the description of your window and its components and creates the classes and the commands that drive the application - from the description.

• Syntax improvements: Some language syntax looks not very convenient? Write a macro that makes it more convenient for you, the application writer.

• Domain specific languages: You need a language that is nearer to the domain of your application? Create the necessary language forms with a bunch of macros.

Meta-linguistic abstraction

The basic idea: everything that is on the linguistic level (new forms, new syntax, form transformations, simplification, IDE support, ...) can now be programmed by the developer piece by piece - no separate macro processing stage.

Answer 2

Pick any "code generation tool". Read their examples. That's what it can do.

Except you don't need to use a different programming language, put any macro-expansion code where the macro is used, run a separate command to build, or have extra text files sitting on your hard disk that are only of value to your compiler.

For example, I believe reading the Cog example should be enough to make any Lisp programmer cry.