Rust 'does' lazy evaluation only in the sense that you can implement it if you go out of your way. It's not the default like in Haskell. It's not a functional programming language–having functors, monoids, and monads (in some restricted sense) doesn't make a language FP.
Lazy evaluation is not a requirement of functional programming. There's nothing really in typed lambda calculus requiring any laziness, that's just one possible way of expression evaluation.
The only real requirement of functional programming is the support of first class functions, the possibility to pass and return functions like any other values.
If you want to discuss your own interpretation of what makes a language more or less functional, which could be interesting, please provide a definition first.
Note that I didn't say that lazy evaluation is needed for functional programming. I said that Rust doesn't have lazy evaluation, and that it's not a functional programming language.
IMHO, a functional programming language requires:
- First-class functions and function literal (lambda) syntax support
OCaml doesn't have a language standard afaik, and its implementation(s) support(s) TCO, so imho that's more important.
'Emphasis' just means the 'paved path', i.e. default data structures like records and variants are immutable, default list type is immutable. Of course if you want mutable stuff it's there, but it's not the default.
> The only real requirement of functional programming is the support of first class functions, the possibility to pass and return functions like any other values.
By that definition C language supports functional programming. I think a lot of people would disagree with your definition.
Actually, first class functions also means the ability to form functions (ie, closures). Javascript for instance has that feature. I think that python also supports it. C OTOH doesn't, but it has other advantages.
C++ had support for partial applications with templates (std::bind iirc), and it was possible to implement closures as objects with the () operator implemented. Now it has support for closure with a dedicated syntax.
It would be possible to do something similar in C - a closure is just a function pointer and a bunch of parameters bundled together, but I don't think there's enough flexibility in the language to make it look like regular function calls.
Monads or something like them are vital to be able to recover imperative programming in a functional context. If functional programming means programming with functions (in the mathematical sense) then it means expression-oriented languages/styles, and how you express conventional (procedural) programming idioms in those is a key part of having a functional programming language that you can actually use.
Imo, the advantage of "Haskell's approach" is that it's always obvious when you can and can't use imperative code, and the type system enforces it. I think this is important in many contexts, like STM[1] or Facebook's React[0]. But there are plenty of disadvantages:
* it's a bit more verbose.
* you can accidentally implement a weaker interface than the one you'd want, e.g. by only implementing Functor and not Traversable for your data structure.
* the flip side of the benefits is that adding I/O to a previously pure function changes its interface.
I don't know how it'd compare to OCaml's algebraic effects proposal, though.
You lose the freedom to fearlessly refactor that's the main advantage of functional programming. If it's not clear which parts of the code can be reordered then you either just can't keep the code clean anywhere near as much, or you need much higher test coverage etc. to keep the same level of confidence when you do.
When mathematicians developed monads, they were doing abstract algebra. They were not using them as a kludge to (for example) impose sequence on functional programming.
Having to use monads for sequencing is something that happens in Haskell and not in Rust.
Sure, I/O in Rust is a little easier because it's not wrapped up in an abstract type, but in exchange, Rust needs Try, async/.await and const-fn to solve just a few of the issues that monads solve in more generality. I don't think either choice is a kludge, it's just a bunch of trade-offs.
Just to say, Haskell doesn't need monads to sequence computations, in fact Haskell as a programming language (1989) is older than Moggi's seminal paper on monads applied to programming in 1993.
These three things are mostly Haskell-specific though, at least in relation to functional programming.