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.

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.

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.

> 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.

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.

  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

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.

There are two ways to define a variable binding:

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