This repository contains an implementation of MLstruct, the Boolean-algebraic core of MLscript. It is relatively faithful to the OOPSLA 2022 paper MLstruct: Principal Type Inference in a Boolean Algebra of Structural Types. To obtain MLstruct, we started from the full MLscript language prototype and removed most of the extra features/bells and whistles, and made some extra simplifications.

What would TypeScript look like if it had been designed with type inference and soundness in mind?

We provide one possible answer in MLscript, an object-oriented and functional programming language with records, generic classes, mix-in traits, first-class unions and intersections, instance matching, and ML-style principal type inference. These features can be used to implement a form of extensible variants that is as powerful as but also more flexible than unification-based approaches using row variables.

MLstruct supports union, intersection, and complement (or negation) connectives. We make sure they form a Boolean algebra and add enough structure to derive a sound and complete type inference algorithm. For more details, please see the paper mentioned above.

Running the tests only requires the Scala Build Tool and NodeJS installed. In the terminal, run sbt mlscriptJVM/test.

To run the regression tests continuously as you develop, launch the SBT shell first with sbt and then type ~mlscriptJVM/testOnly mlscript.DiffTests.

To run the web demo on your computer, compile the project with sbt fastOptJS, then open the local_testing.html file in your browser.

You can make changes to the type inference code in shared/src/main/scala/mlscript, have it compile to JavaScript on file change with command sbt ~fastOptJS, and immediately see the results in your browser by refreshing the page with F5.