What Rhai Isn’t

Rhai’s purpose is to provide a dynamic layer over Rust code, in the same spirit of zero cost abstractions. It doesn’t attempt to be a new language. For example:

  • No classes. Well, Rust doesn’t either. On the other hand…

  • No traits… so it is also not Rust. Do your Rusty stuff in Rust.

  • No structures/records/tuples – define your types in Rust instead; Rhai can seamlessly work with any Rust type that implements Clone.

    There is, however, a built-in object map type which is adequate for most uses. It is possible to simulate object-oriented programming (OOP) by storing function pointers or closures in object map properties, turning them into methods.

  • No first-class functions – Code your functions in Rust instead, and register them with Rhai.

    There is, however, support for simple function pointers to allow runtime dispatch by function name.

  • No garbage collection – this should be expected, so…

  • No first-class closures – do your closure magic in Rust instead: turn a Rhai scripted function into a Rust closure.

    There is, however, support for simulated closures via currying a function pointer with captured shared variables.

  • No byte-codes/JIT – Rhai has an optimized AST-walking interpreter which is fast enough for most casual usage scenarios. Essential AST data structures are packed and kept together to maximize cache friendliness.

    Functions are dispatched based on pre-calculated hashes and accessing variables are mostly through pre-calculated offsets to the variables file (a Scope), so it is seldom necessary to look something up by text name.

    In addition, Rhai’s design deliberately avoids maintaining a scope chain so function scopes do not pay any speed penalty. This particular design also allows variables data to be kept together in a contiguous block, avoiding allocations and fragmentation while being cache-friendly. In a typical script evaluation run, no data is shared and nothing is locked.

    Still, the purpose of Rhai is not to be super fast, but to make it as easy and versatile as possible to integrate with native Rust applications. What you lose from running an AST walker, you gain back from increased flexibility.

  • No formal language grammar – Rhai uses a hand-coded lexer, a hand-coded top-down recursive-descent parser for statements, and a hand-coded Pratt parser for expressions.

    This lack of formalism allows the tokenizer and parser themselves to be exposed as services in order to support advanced features such as disabling keywords and operators, dynamically changing tokens during parsing, adding custom operators, defining custom syntax and filtering variables definition.

Do not write the next 4D VR game in Rhai

Due to this intended usage, Rhai deliberately keeps the language simple and small by omitting advanced language features such as classes, inheritance, interfaces, generics, first-class functions/closures, pattern matching, concurrency, byte-codes VM, JIT etc. Focus is on flexibility and ease of use instead of raw speed.

Avoid the temptation to write full-fledge application logic entirely in Rhai – that use case is best fulfilled by more complete languages such as JavaScript or Lua.

Tip: Use Rhai as a thin dynamic wrapper layer over Rust code

In actual practice, it is usually best to expose a Rust API into Rhai for scripts to call.

All the core functionalities should be written in Rust, with Rhai being the dynamic control layer.

This is similar to some dynamic languages where most of the core functionalities reside in a C/C++ standard library.

Another similar scenario is a web front-end driving back-end services written in a systems language. In this case, JavaScript takes the role of Rhai while the back-end language, well… it can actually also be Rust. Except that Rhai integrates with Rust much more tightly, removing the need for interfaces such as XHR calls and payload encoding such as JSON.