Notes on a Smaller Rust

I think there is such a thing as liking what's familiar, but there are also design decisions which are simply easier to read or easier to parse. Another one would be semicolons: I have never missed semicolons one bit in a language where they're not required.

I'm trying to learn Rust, working through various programming exercises, but it looks and feels very foreign to me.

One example is the closure syntax. Compared to Go where closures look just like function definitions, closures in Rust look needlessly complex.

I also always find myself wanting to type mut &a instead of the correct &mut a. The former just seems more obvious to me, a reference to a is &a. Mutable a is mut a. So a mutable reference must be mut &a.

Then the lifetime syntax, I don't even know what to think of that, but that's probably more because I'm struggling asking myself "Should I use a lifetime here, or am I doing something else wrong that is preventing the compiler from figuring it out."

Maybe these things make more sense to people coming from C.

There's no winning over everyone. Of course Rust's syntax will be completely jarring if you're coming from Crystal or Ruby.

In my personal experience it's a little jarring at first and then it completely goes away.

There are many things like this in Rust where if you just follow the compiler errors until you can make it happy, things will end up really convoluted and baffling. They'll also probably not perform as well. To really use Rust you have to absorb what's actually going on underneath its protections, and not just the errors that surface from them. This isn't easy, and is probably the dividing line between people who stick with it and people who decide it's not for them.

You get about 50% of this picture for free (thinking in terms of stack, heap, references) if you've done C/C++ before. The other 50% is completely unique to Rust. But the key is to read the Book, follow guides, etc. Rust isn't really something you can learn just by hacking it out on your own, unfortunately.

The Linux kernel is well known to be completely random when considered as a C program: building something useful out of it relies on a particular set of flags and ad-hoc implementation details of GCC (e.g. -fno-delete-null-pointer-checks).

As you make it easier to propagate the error case to the caller, you asymptotically reach checked exceptions.

If you do a lot of error handling close to the point of the error condition being found, the Rust / checked exceptions approach works well. For the kinds of applications that are written in Java, it's not the case. Most error conditions need to be propagated and the action in flight aborted.

If you're interested in which item failed, ideally you'd be using a method that doesn't throw at all, and instead returns an error condition. I like how C# makes this clear, with Parse and TryParse on Int32. You use Parse when you want unwinding and abort behaviour including stack unwinding, and TryParse when you want to handle errors, and you don't use exceptions for expected errors, since expected errors are not exceptional.

(If the map is lazy, you'd expect the materialization in collect() to throw. But the return type wouldn't change.)

IMO, the kind of code you write on a daily basis is the primary determinant of your preference for error codes vs exceptions. If you need to handle error conditions frequently, you'll prefer error codes, because they're data, like all other data, and you can use the general compositional tools at your disposal to manipulate them. If you handle error conditions exceedingly rarely, and mostly just want to abort, unwind, roll back, go back to the main loop and log them, then exceptions are your friend.

I do not think there is a fundamental superiority either way. I think there are tools for purposes. I do think that Java's checked exceptions are half-baked; they're half-way into the compile time type system in a language whose applications are generally better suited to run time exceptions.

Using magic language syntax like checked exceptions in lieu of type system expressiveness is not great, which is why Java's checked exceptions are a bit on the unpleasant side. You can't have one map() that handles functions with and without exceptions, because exceptions aren't part of the type system.

In Rust, exceptions are part of the type system, so you only need one map(). If you also add syntactic sugar for the types such as:

  1: fn foo(i: i32) -> string throws IOException
  2: throw SomeIOException
  3: try a catch IOException b

  1: fn foo(i: i32) -> Result<string, IOException>
  2: return Err(SomeIOException)
  3: match a {
       None => b
       Some(_) => {}
     }

The Rust implementation (syntax I use might vary from reality) of async/await is just this sort of syntactic sugar over a type: "async fn foo() -> X" is sugar for "fn foo() -> Future<X>", and "await!foo()" is (kind of) sugar for "foo().then(rest of the function)".

The only really kind of disappointing thing about this is that exceptions and futures don't get unified: we get ? for exceptions and await! for futures. Both of them ultimately are continuations: exceptions bypass the continuation and return an error immediately, futures call the continuation only when their value becomes available. The value of using a unified interface to continuations (or monads, if you prefer) is that you can use ones that don't have a magic blessed syntax: parsers with automatic backtracking written as simple imperative blocks, non-determinism via iterators where the continuation is a flatten operation, etc.

If you're talking about a language to write drivers or an operating system, etc in then a GC is just a big No. In that domain we don't even have Malloc/Free, let alone GC services. Having the overhead and normative lifestyle assumptions of _any_ kind of runtime is a big Nope.

Expressions evaluate to a value, and semicolons take an expression, throw away its value, and evaluate to () instead. That's it.

The other behaviors you're talking about fall out of these semantics, but are also different than what you've said; functions evaluate to a value, so the final expression's value determines the value it evaluates to, but you cannot "implicitly return" from the middle of a function by dropping a ;, for example.

  if(foo = 5) {
    ...
  }

Only if a1, a2, a3 are also freed at once. In which case, you can just put all three in a struct that is allocated as a single block. Rust doesn't yet support this very well in the general case, because its support for what C++ calls "placement new" and customized allocation in general is not complete or stabilized. But doing this in a more hackish special-cased way is already possible in many cases.

A platform like the JVM disguises this because every read and write (even without 'volatile') use minimal synchronisation to avoid the worst aspects of the danger here. This ensures that programs that do it wrong (e.g. forget a 'volatile') are "only" incorrect, but not unsafe.