Source code transformations. All kinds. Examples:
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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.
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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.
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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.
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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).
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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.
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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.
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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.
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Syntax improvements: Some language syntax looks not very convenient? Write a macro that makes it more convenient for you, the application writer.
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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.