[palmtree3000]

I came to rust from the other direction, as it were: from Haskell.

There are a lot of similarities! The type systems are similar, with some name changes (sum types -> enums, typeclasses -> traits). Pattern matching is basically the same. Haskell uses (lazy) lists mostly the same way rust uses iterators. Ownership is new, but imposes some of the same requirements that immutability does (no cycles without shenanigans[0]). Rust requires that values are aliasable XOR mutable: Haskell does too (they're always aliasable and never mutable).

[0] https://wiki.haskell.org/Tying_the_Knot

[duped]

The thing is that mutability is a useful and common property used by most programmers. It takes a bit of buy in to be convinced immutability is a good thing that solves bugs. For example:

Here's a reasonable program to write in Python (wave hands here, my python is rusty)

    queue = [root]
    for node in queue:
      if !node.visited:
        # ... visit the node ...
        node.visited = True
        for child in node.children:
          queue.insert(child)

Here's that in Rust.

    let mut queue = VecDeque::new();
    queue.push_back(root);
    
    for node in &mut queue {
      if !node.visited {
        // ... visit the node ...
        node.visited = true;
        for child in &mut node.children {
          queue.push_back(child);
        }
      }
    }

This will not compile, at all. You need to do a lot of work to restructure the data structures in Rust to get that reasonable program to run soundly. Now that's a good thing, because this would be invalid in C/C++ too due to iterator invalidation and lifetime issues, but it's the kind of thing that people who haven't seen a block of code segfault due to a `push_back` before would be confused by.

It's rather rare for a programmer to need to learn about memory safety when compilers in managed languages just solve it for you, and in other systems languages they assume you already know it (or don't care that you can write unsafe programs).

There's just that additional conceptual burden when learning Rust and why programs that seem syntactically correct and safe are forbidden.

[msbarnett]

TBH I'm surprised that even works in Python – modifying a collection you're in the middle of iterating seems like a bad habit to get into in any language.

edit: in fact, if you specifically use a deque in Python the way you are in Rust, Python will throw a "RuntimeError: deque mutated during iteration". This is just a bad approach in any language, honestly.

[duped]

Agreed, python may be a contrived example here. It is possible to pull this off in a few languages where object relocation isn't a problem and mutating a collection during iteration is natural.

[chakkepolja]

For someone who understands C++11, I don't think FP concept _in rust_ will be hard.

But someone who understands Haskell, may still find some memory stuff clumsy.