F# |> BABEL

The compiler that emits JavaScript you can be proud of. Try it out!

Fable brings together the power of the F# compiler and Babel to make JavaScript a true backend for F#. Some of its main features are:

• Works directly on F# source code, no compilation to .NET bytecode needed
• Optimizes F# code to generate as clean JavaScript as possible
• Passes location data to Babel to generate source maps
• Compatible with all Babel plugins and other JS development tools, like Webpack
• Support for most of the F# core library and a bit of .NET Base Class Library
• Tiny core library included (less than 10KB minified and gzipped) with no runtime
• Organizes code using ES6 modules
• Interacts seamlessly with other JavaScript libraries
• Bonus: compile NUnit tests to Mocha

Philosophy

• Clean code: The compiler must emit JavaScript as readable as possible which plays well with other JS code and development tools, even if this means sacrificing a bit F# semantics.
• Unopinionated: The project shouldn't compromise with a specific platform and adapt to standard JavaScript in any environment: browser or node.
• Open to the community: If the project is to survive, it needs the support of the community. For that, maintainers should compromise to keep the project documented and make it easy for people to contribute. A plugin system is also planned.

Installing

The project is still in testing phase and hasn't been packaged yet. If you wan to try it out, make sure you have F# 4.0 with .NET/Mono and an updated version of node.js installed. Download the repo and run:

build.cmd   // on windows    
./build.sh  // on unix

If everything works, follow these instructions to compile a F# project or script file to JS.

Contributing

At the moment, the best way to contribute is to battle-test the project, report issues, create samples and help promoting it. A plugin system is also available to allow you extend Fable according to you needs.

Issues with the label discussion will be also added to ask the opinion of the community on different topics like the logo, roadmap, etc.

Caveats

• Options are erased in compiled code. This has several benefits like removing overhead and interacting with native JS functions in a safer way (null will be None). However, it will lead to unexpected results if you do weird things like wrapping null in Some. For practical purposes, Fable considers null, undefined, None and unit to be the same thing.

• Information about generic types is not included in the generated JavaScript, so code that depends on this information to be known at runtime for method dispatching may have unexpected behaviour.

• At the moment, comparison for non-primitive values defaults to JS reference comparison so there's no structural comparison for records, unions, tuples or collections. This may change in the future upon users' feedback.

• Map and Set default to the new ES6 Map and Set classes which must increase performance, but with the downside that adding and removing operations are mutable.

• Debugging the F# code is not perfect. For example, you may see the debugger jump directly to the last expression in many functions. This is normal and due to the optimizations performed on the generated code.

To know more, read Compatibility.

Acknowledgements

Of course, this project wouldn't have been possible without the fantastic work of the F# compiler and Babel teams. I hope they feel proud seeing how their work has met in a very unexpected way, giving developers even more possibilities to build great apps.

The awesome F# community has played a big role in making this possible. I've met incredible people and it's impossible to list all the names without forgetting anyone, but I'd like to give a particular mention to Zach Bray for his work on FunScript, Don Syme (the fact that the designer of the language himself shows interest in your work, no matter how humble it is, is really a big push!) and Krzysztof Cieślak (I always have to look up the name to spell it correctly) because he's shown that F# is a perfect fit for a big project targeting JS.

And finally I'd like to thank my partner (is it too old-fashioned to say wife?) for bearing with me everyday.