'''[https://www.rust-lang.org/%|%Rust]''' is a [programming language] designed to be performant, safe, and productive. ** Description ** Rust combines concepts from [C], [C++], [Haskell], and other languages. In order to ensure that a program is '''type-safe''' and to compile it to performant machine code, the compiler tracks the [type] of each [variable] and function. Where the compiler can not '''infer''' a type of a variable, parameter, or result, it must be '''annotated with a type'''. Like Tcl, Rust does not employ a [garbage collection%|%garbage collector], but it also does not normally employ [Tcl_Obj%|%reference counting]. Instead, each variable has a '''lifetime''' that begins at the point data is assigned to the variable, and ends when the variable can no longer has any use, which is often at the end of the '''block''' of code the variable is found in. When the lifetime of a variable comes to an end, any data it owns are cleaned up, and resources deallocated accordingly. When data are assigned from one variable to another, they are moved to the new variable, and are no longer associated with or available through the original variable. Likewise, when data are passed to a function, they are '''moved''', to the specified variable in the function, and are no-longer accessible through the original variable. This '''prevents aliasing''', and also means that the scope of a variable might not extend to the end of the block. When a function returns data, the data moves moves to the caller, and if assigned to a new variable is then owned by that variable. Data may also be assigned to a variable or passed to a new function by reference, in which case the original variable continues to '''own''' the data. In both a type annotation and a function call, '''`&`''' indicates that the variable holds a reference. Rust tracks all references and ensures that no reference has a lifetime greater than the lifetime of the variable to which it refers. Where the compiler can not infer the lifetime of a reference, the lifetime must be explicitly annotated using '''`&'`'''. This managment of data lifetimes eliminates a whole class of issues that a [C] program might be vulnerable to. A variable is '''immutable''' by default. '''mut''' declares a variable to be mutable, and '''&mut''' declares a reference to be mutable. Only one mutable reference may exist at any given point. This eliminates '''data races'''. In a program, data is organized into '''structs''', and code is organized into functions. Each field in a struct is a '''primitive value''' or a reference to primitive value, a struct, an enum or a function. An '''enum''' is a type that enumerates any number of possible variants, each of which may be a reference to a struct. A function that accepts an enum uses '''match''' to tailor its operation to each possible variant of the enum. A struct may be specified to have either named or unnamed fields. A '''trait''' declares a group of functions act as an '''interface''' to a struct. '''impl''' provides a set of function definitions that implement a particular trait for a particular type of struct. Any number of traits may be '''implemented''' for a given struct. To maintain '''coherence''', a module may only implement a trait if either the trait or the structure is local to the module. The type of some data may be specified by specifying what traits are required, which allows [duck typing] at compile time. A '''trait object''' provides a function table that may be modified at runtime, which allows duck typing at runtime. Programs and libraries are organized into '''modules''', which contain any of the various programming artifacts Rust provides, namely structures, variables, functions, traits, and trait implementations. Some traits have special meaning for Rust. A struct has the '''copy''' trait is copied when it would otherwise have been moved, meaning that both the original scope and the new scope each own an independent copy of the value. Rust has various features for programming the structure of the program itself, i.e. for '''metaprogramming''': A '''[macro]''' language, '''[closure%|%closures]''', syntax for '''[templating]''' generic structs and functions, and '''pattern matching''' similar to that of [Haskell], on language '''items''' for such purposes as '''destructuring variable assignment''' and '''code branching according to data type'''. Rust is actually segmented into two different languages: '''safe Rust''' and '''unsafe Rust'''. In the safe parts, '''nothing is undefined''', '''nothing is null''', only '''safe type conversions''' are possible, and only '''safe memory operations''' are allowed. There is, however, an escape hatch: A function '''annotated as unsafe''' takes upon itself the responsibility to guarantee the same things that the Rust compiler guarantees. , so the compiler makes no further effort to check that function for conformance. This makes it possible to write such a function in another language such as [C], if needed. One of the design goals of Rust is for the compiler to be able, as much as possible to '''infer the intent''' of the programmer. The compiler also takes a more active role than most compilers in making suggestions for fixing problems that it finds. This helps to smooth the learning curve. ** Memory Management: Tcl Vs Rust ** Both Tcl and Rust provide automatic memory management without using a garbage collector, and both are memory-safe. Where Tcl uses reference counting, Rust uses its '''borrow rules''' to constrain the use of references, and tracks all references at compile time to ensure that a value may be cleaned up immediately when the program exits from the block that owns it. Tcl's internal reference counting relies on the developer to properly increment and decrement references, which is error-prone since proper reference counting can not be checked for correctness. Because Rust would automate the task of reference counting, relieving the developers of Tcl from this burden, it may prove to be an ideal implementation language for Tcl. ** Object Orientation ** Rust does not provide traditional '''object-oriented programming''' facilities. Inheritance has proven to be a liability, so Rust doesnt have it. Instead, the emphasis is on structures and interfaces that those structures support. Each structure has a type, and where inheritance might be used in other languages, it is common instead to create a new type of structure and implement customized traits for it. Composition is achieved by having a field in one structure be a reference to another structure. This design allows for '''zero-cost abstractions''', since all the details of function dispatch are worked out at compile time. Where needed, Rust also provides facilities that provide for runtime dispatch. ** Rust Vs. Tcl ** Both languages are founded on the concept of immutable value, but Tcl values are much more abstract, both semantically and technically. In Tcl, [copy-on-write] can be implemented underneath the surface of the language whereas in Rust, a programmer details directly with such issues. This level of abstraction is the most fundamental difference between the languages. As value-oriented languages, both languages have no concept of '''[null]''', which is anything but a value. Both languages are '''memory-safe''', assuming that the underlying implementation/compiler is free of bugs. Rust is '''type-safe''', both in the sense of memory layout and data structure, and also in the sense of the semantics of the data and the operations on it, as only the intended structure is accepted as the argument of a function or the value of a variable. while Tcl is not type-safe, it is also not type-unsafe: Each procedure may choose to implement the necessary runtime type verification. However, even then a procedure has no way to determine which program component the declared the data type, and therefore can never achieve the level of semantic type safety that Rust does. For a program written in Rust, the additional burden of annotating data types and tracking data mutability and ownership is high compared to practice of taking things at face value in Tcl, and all the type annotation, templating, and boilerplate code makes a program written in Rust far more verbose, and development far slower. Tcl is far more concise than Rust, but far less performant. In addition to performance, another big win for Rust is that all the additional type information a programmer provides in the source code allows the compiler to verify some aspects of its correctness, and probably reduces the size any accompanying test suite by at least the same amount of code and effort. If a program compiles successfully, it's already some indication that the program works as expected. An equivalent program in Tcl would have to ensure the same level of correctness via a series of unit tests. ---- "When you use Rust, it is sometimes outright preposterous how much knowledge of language, and how much of programming ingenuity and curiosity you need in order to accomplish the most trivial things. When you feel particularly desperate, you go to rust/issues and search for a solution for your problem. Suddenly, you find an issue with an explanation that it is theoretically impossible to design your API in this way, owing to some subtle language bug. The issue is Open and dated Apr 5, 2017." [https://hirrolot.github.io/posts/rust-is-hard-or-the-misery-of-mainstream-programming.html] ---- '''[dkf] - 2022-06-15 14:30:02''' I suspect that Rust will not prove to be as ideal a language to implement Tcl as some think. There are a number of concerning aspects, but the limitations on traits and lifetimes would seem to be the main ones (along with the general hostility to dynamic linking); it's not at all clear how one might have third party commands implemented (where those need the equivalent of `ClientData`) and the lifetime management is almost certainly going end up forcing a lot more copies of things being used, which is known to be expensive. The final problem, the one that ''really'' dissuades me, is the sheer cost in developer effort of moving a codebase the size of Tcl and Tk. <> programming language