What Rhai Isn't
===============

{{#include ../links.md}}

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_.

  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]
  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]({{rootUrl}}/engine/call-fn.md).

  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 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.

* **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 [disabling keywords/operators][disable keywords and operators], adding [custom operators],
  and defining [custom syntax].


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.


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.