14 great preprocessors for developers who love to code

As much as we love them, programming languages can often feel like straitjackets. They’re a complex bundle of syntactic rules, and if we break them–even once–the compiler starts screaming out error messages. There are conventions to define every little thing, such as the best way to name variables or how to indent your code. The language designers claim these constraints are a feature, not a bug.

But programming languages don’t need to be that way. Over the years, clever developers have come up with sneaky and not so sneaky ways to write code in their own idiosyncratic styles. Preprocessors can bridge the gap, jumping into the pipeline before the code is compiled to fix all the strange twists and personal styles that keep coding fun.

Preprocessors aren’t new. Languages like C have relied on them for years. But lately they’ve been growing in popularity, as developers create more expressive ways to let programmers write software however they like. When it comes time to compile, all that unique style is quietly removed and replaced, so the final version fits the narrow rules of the language.

In the interest of programmers slipping out of the straitjacket, here’s our list of ways to preprocess your code. Included are language-specific preprocessors, ones the bridge the gap between data scientists and developers, and even one for converting American-style English to something more palatable to our colleagues across the pond.

LESS and SASS

The power and responsibilities given to CSS are now so great that we need a way to bring order to some of the more elaborate layouts seen on modern sites. Both LESS (Leaner CSS) and SASS (Syntactically Awesome StyleSheets) are preprocessors that let you behave more like a programmer by deploying variables and other functions to simplify a CSS layout. If your website design has a dominant color, you’ll only need to define it once. SASS is the more powerful of the two, with more complex options like loops. While it’s leaner, LESS is still powerful enough to get the job done. Either tool lets you bring a programmer’s touch and sensibility to cleaning up the seemingly endless list of CSS layout options.

AbsurdJS

Some people like consistency and prefer to work with one particular language. If you’re a fan of JavaScript and you want to use its power to craft your CSS, then AbsurdJS is the preprocessor for you. The letters JS are in the name, but the goal is CSS. With AbsurdJS, you can use the power of a programming concept like inheritance to produce elaborate CSS layouts. Like LESS and SASS, this is a preprocessor that lets you think like a programmer instead of a designer.

Bython

Some developers like using braces to define blocks of code. Some prefer hitting the spacebar and the tab key. Python was designed for the programmers who love a good indent. Now that it’s more powerful and ubiquitous, some of the curly-brace-loving crowd may want to use Python libraries and tools. Bython is a preprocessor that lets you keep your curly-braces and your Python libraries, too. You just code like you normally would and Bython does the rest. It will automatically replace your curly braces with indents, so you never have to hit the spacebar.

Pypreprocessor

The C language has long offered C coders the chance to make complex decisions about code with preprocessing statements like #ifdef, which turns big blocks of code on and off. Now Python programmers can do the same thing with Pypreprocessor, a dynamic library that lets you use flags and metavariables to make code disappear and reappear at will.

TypeScript

JavaScript was originally designed for web programmers who needed to add short blocks of code to websites that were mostly built from HTML. If you didn’t want to spell out the type of a variable, it was no big deal because the JavaScript code block was small and easy to understand. That’s changed, and now many developers build elaborate and very dynamic sites out of thousands and thousands of lines of JavaScript.

Given the range of JavaScript, some developers now want the assurance that comes from strongly typed code. TypeScript is the answer, and it’s an amazing compromise. Generic JavaScript continues to be acceptable to TypeScript, which means that all the type information you add is optional. TypeScript’s preprocessing stage double checks for errors when it can find them, then spits out something the generic JavaScript engine can process. Some of the most popular JavaScript frameworks, like Angular, now rely on TypeScript for strong typing.

CoffeeScript

For every Python programmer who yearns to be writing with a C-style syntax, there’s a JavaScript programmer who desires the freedom and simplicity of writing Python. CoffeeScript is the answer. There are now a number of variants like ToffeeScript, Civet, Storymatic, and CoffeeScript II: The Wrath of Khan, along with more than a dozen others. All these languages save us from the incredibly onerous task of lifting a right pinkie finger to press the semicolon key. They also offer neat features like asynchronous grammars and elaborate mechanisms for metaprogramming. The result is cleaner code with less punctuation, which is—in the eyes of CoffeeScript programmers, at least—much easier to read.

Handlebars and Pug

Modern code typically contains many blocks of text with messages for eventual human users. These are often filled with many insertions and customizations. Templating systems like Handlebars and Pug help to speed up writing these blocks of human readable text. There’s no need to write the low-level code required to glue together strings. You just write the text and the templating system handles the chore of patching together all the parts.

AWK

The Unix command-line tool is one of the simplest and most powerful tools for working with pure text. Named for its three original creators, Alfred V. Aho, Peter J. Weinberger, and Brian W. Kernighan, AWK links together a number of commands for extracting data from lines and sorting and filtering it. It is possible to build full reports using AWK. Programmers also use it to clean up raw data in the processing pipeline before the main program imports it.

Vapour

R is a powerful language that was largely created by statisticians, who generally think like mathematicians not computer programmers. That’s not a bad thing, but it can be a hurdle for using all the great libraries in R, which lack some of the great advances in programming design. Vapour is a preprocessor that lets R users think like programmers, specifically, programmers who love using type systems to catch bugs and enforce structure. Vapour’s developers say it’s still in an early alpha stage, so they may be adding new features and tweaking the syntax. The goal is to let the tool evolve quickly with the needs of users.

Spiffing

Not all English speakers use the language in the same way, particularly across continents and cultures. Spiffing (also see Spiffing) is a preprocessor that translates code written in American English to British English. It’s a bit of a goof, but that doesn’t mean it’s not useful or capable of bridging a cultural divide. If it catches on, maybe one day, developers will build out the preprocessor so that it converts the rather direct American-style diction into a more reserved British style. Instead of if-then statements, we could have perchance-otherwise syntax, for example.

Linting preprocessors

Not all preprocessors convert code. Some clean up after us and look for the bugs we’ve missed. The original Unix command-line tool, lint, has metastasized and now its functions are found as preprocessors in many language development stacks. These linting tools, or linters, fix formatting, enforce naming conventions, and even fix some syntactic and semantic errors. Some enforce rules that flag potential security flaws from bad logic. Popular versions include RuboCop for Ruby code, Pylint for Python, and ESLint for JavaScript (ECMAScript).

Preprocessors for documentation

Some preprocessors produce something other than runnable code. Tools like Sphinx, MkDocs, and Doxygen analyze your files and create an annotated and cross-referenced set of documentation files directly from code. These tools are designed to work with multiple languages but almost all the languages have their own official preprocessor. Popular examples include Javadoc, Rustdoc, Godoc, and JSDoc.

Preprocessors for integrated data reporting

Data scientists don’t just speak R. They also write out complex data reports in a human language filled with charts, tables, and graphs created by R. Over the years, data scientists have created complex preprocessors for not only R but also LaTeX, the typesetting language. The scientist writes everything in R and human language, then the preprocessor splits it up, sending the computational instructions to R and the typesetting instructions to LaTeX. At the same time, it juggles the parts so the pictures produced by R end up in the right place in your document. LaTeX then folds them into the eventual PDF that’s produced from the human language parts of the file. It does all this while also organizing page references and illustration numbers to be consistent.

There are a variety of options with different strengths and advantages. R Markdown is a variation of common markdown that can also merge computational and data analysis. It can also merge in results from languages like Python or SQL to produce slides, documents, books, and websites. Knitr and its precursor Sweave are two closely aligned preprocessors that are both well-supported by Rstudio. For those who want to merge Python with LaTeX, there’s also Pweave. One day there may be a meta version that merges all of them into one big preprocessor.

Using AI for preprocessing

All preprocessors require some configuration. Why not just get an AI to do it? Some are already uploading their preprocessor to an AI LLM and asking it to fix everything that’s wrong. In one example, some bean counters hit the roof after the developers told them that it would cost more than $1 million to rewrite their Agda compiler to make it current. Someone had the bright idea to just upload all 500+ files from their codebase to Anthropic’s Sonnet-3.5. And Voilà! The compiler was converted to TypeScript in the blink of an eye. The developers reported that most of the code ran fine with no intervention. The LLMs aren’t perfect, but they’re bringing us closer to a world where we can just wave our hands around and the machines do our bidding, like magic.