Hmm this is a bit like peeling a banana only to throw the banana and eat the peel. Pass by value reduces the true benefit of copy-on-write.

Use immutable pass by reference. Make a copy only if mutability is requested in the thread. This makes concurrent reads lock-free but also cuts down on memory allocations.

> Have you benchmarked any real workloads?

Nothing "real", just the synthetic benchmarks in the ./benchmarks dir.

Unnecessary copies are definitely a risk, and there's certain code patterns that are much harder for my interpreter to detect and remove. E.g. the nbodies has lots of modifications to dicts stored in arrays, and is also the only benchmark that loses to Python.

The other big performance limitation with my implementation is just the cost of atomic reference counting, and that's the main tradeoff versus deep cloning to pass between threads. There would definitely be room to improve this with better reference counting optimizations though.

Why don’t we just do this by default for threading in most languages? It’s pretty rare for me to actually want to do memory sharing while threading (mostly because of the complexity)

Why exactly is imperative syntax "convenient" specifically in the context of inter-thread communication?

Pass-by-value is already a copy.

Personally I think local mutability is quite a useful property, which was part of the inspiration for making this instead of just another pure functional language:

- All functions are still referentially transparent, which means we get all the local reasoning benefits of pure functions. - We can mutate local variables inside loops (instead of just shadowing bindings), which makes certain things a lot easier to write (especially for beginners). - Mutating nested fields is super easy: `set foo.bar[0].baz = 1;` (compare this to the equivalent Haskell).

> It had a notion of "type"s which were functions that returned a boolean 1 only if given a valid value for the type being defined.

I've got a hobby language that combines this with compile time code execution to get static typing - or I should say that's the plan, it's really just a tokenizer and half of a parser at the moment - I should get back to it.

The cool side effect of this is that properly validating dynamic values at runtime is just as ergonomic as casting - you just call the type function on the value at runtime.

Thanks, I updated the post title based on this and another comment. Thanks for the pointer to Euphoria too, looks like an interesting language with a lot of similar ideas.

IMHO this is both unnecessarily pedantic and not really quite right. Let’s say we accept the premise that “everything is a value” means reduction is impossible. But a value is just the result of reducing a term until it is irreducible (a normal form). So if there is no reduction there can’t really be values either—there is just “prose” (syntax) and you might as well read a book.

I am unable to extract any meaning from your post. You appear to be making a general claim: it is impossible to design a programming language where everything is a value. You at least admit that "data thingies" can be values. Are you claiming that it is not possible for functions to be values? (If we assume that the argument and the result of a function call is a value, then this would mean higher order functions are impossible, for example.) If not that, then what? Please give a specific example of something that can never be a value in any programming language that I care to design.

Thanks, some interesting reading there that I will check out (I wasn't aware of PCF). Perhaps I should've used more precise wording: "All types are value types".

> Standard ML [...] It has all you dream of and more

The main thing here that's missing in Standard ML (and most other functional languages) is the "mutable" part of "mutable value semantics" - i.e., the ability to modify variables in-place (even nested parts of complex structures) without affecting copies. This is different from "shadowing" a binding with a different value, since it works in loops etc.

Excuse me if I didn't get it right, but as a practical example, I'd assume that I can rewrite every program into JavaScript using the usual control structures and, beyond that, nothing but string values (which are immutable). Simple arithmetic would already be kind of a chore but can be done. (Input and output already happens only via serialized values (cf also webworkers) so there's that; for convenience use TypedArrays wrapped in classes that shield you from immutability). It is not obvious to me where `a = '[1,2]'; a1 = JSON.stringify( JSON.parse( a ).push( 3 ) ) );` is fundamentally different from just pushing a value to a copy of `a`. Also, you could write `a1 = a.slice(0,-1) + '3]'` which only uses non-mutating stuff under the hood.

> How to pass a mutable reference into a function, so that it can change the underlying value and the caller can observe these changes?

Just modify the value inside the function and return it, then assign back. This is what the |= syntax is designed for. It's a bit more verbose than passing mutable references to functions but it's actually functionally equivalent.

Herd has some optimisations so that in many cases this won't even require any copies.

> What about concurrent mutable containers?

I've considered adding these, but right now they don't exist in Herd.

Your assumption is somewhat correct, for both Erlang and Elixir, however the phrase under discussion doesn’t mean the same thing for immutable languages. Both are ‘pass-by-value’ but that term is being overloaded in a particular way. As I said in another comment, ‘value’ in the language from TFA means any object that IS NOT a reference. The qualifier that every semantic object is a ‘value’ and that therefore, all arguments to a function call, threads spawn, etc are independent values which are (logically, at least) copied to new values that are then passed to the new context.

However, for Erlang and Elixir ‘pass-by-value’ is otherwise called ‘call-by-value’. In this case, it is a statement that arguments to functions are evaluated before they are passed into the function (often at the call site). This is in opposition to ‘call-by-name/need’ (yes, I know they aren’t the same) which is, for instance, how Haskell does it for sure, and I think Python is actually ‘by-name’ as well.

So, Herd’s usage here is a statement of semantic defaults (and the benefits/drawbacks that follow from those defaults) for arguments to functions, and Elixir’s usage is about the evaluation order of arguments to functions, they really aren’t talking about the same thing.

Interestingly, this is also a pair of separate things, which are both separate from what another commenter was pedantically pointing out elsewhere in the thread. Programming language discussion really does seem to have a mess of terminology to deal with.

Yes, if you modify a nested dict/list entry, all nodes above it will be cloned. Here's an example:

  x = [1, [2]];
  var y = x;
  set y.[0] = 3; // clones the outer array, keeps a reference to inner array
  set y.[1].[0] = 4; // clones the inner array here. Outer array is now exclusive so it doesn't need another clone.

  var z = x;
  set z.[1].[0] = 4; // clones both arrays at once

I wrote the following and then realized maybe it is just a quirk of the example in the reader that the ‘set’/‘=‘ pair comes at the end of the chain. If so, it is just a unique syntax sugar for a function, I don’t think it is, so I’m leaving my comment as I wrote it letting this act as a caveat:

Although I don’t particularly like the ‘|’ to be used for chaining functions, I certainly know that it has been a long term syntax coming from Unix. My only issue with the ‘|=‘ is that it should be unnecessary. The only reason I can see that the special operator is required is that the ‘set’/‘=‘ syntax pair is a non-functional keyword ‘set’ with an (I think) overloaded keyword ‘=‘. If the equal sign was an ordinary function (i.e. a function that take a value, and an identifier, associates the value and the identifier, then returns the new value like the Lisps and derived lands) it could just be used arbitrarily in chains of functions.

I'm not sure if I understand the question?

There are two ways to define a variable binding:

    x = 1; // declares x as immutable
    var y = 2; // declares y as mutable

Not everything in C is pass-by-value. Sure, you can argue that a pointer itself is passed by value, but the data it points to is definitely not.