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What Rhai Isn't
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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:
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No classes. Well, Rust doesn't either. On the other hand...
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No traits... so it is also not Rust. Do your Rusty stuff in Rust.
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No structures/records - define your types in Rust instead; Rhai can seamlessly work with any Rust type.
There is, however, a built-in [object map] type which is adequate for most uses. It is possible to simulate [object-oriented programming (OOP)][OOP] by storing [function pointers] in [object map] properties, turning them into methods.
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No first-class functions - Code your functions in Rust instead, and register them with Rhai.
There is, however, support for simple [function pointers] allowing runtime dispatch by function name.
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No garbage collection - this should be expected, so...
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No closures - do your closure magic in Rust instead; turn a Rhai scripted function into a Rust closure.
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No byte-codes/JIT - Rhai has an AST-walking interpreter which will not win any speed races. The purpose of Rhai is not to be extremely fast, but to make it as easy as possible to integrate with native Rust programs.
Due to this intended usage, Rhai deliberately keeps the language simple and small by omitting advanced language features such as classes, inheritance, first-class functions, closures, concurrency, byte-codes, JIT etc.
Avoid the temptation to write full-fledge program logic entirely in Rhai - that use case is best fulfilled by more complete languages such as JavaScript or Lua.
Therefore, in actual practice, it is usually best to expose a Rust API into Rhai for scripts to call. All your core functionalities should be in Rust. This is similar to some dynamic languages where most of the core functionalities reside in a C/C++ standard library.