Honestly having looked at the memcached clients available for Java recently, I don't think any of the options could be considered feature-complete. None of the main ones support the meta protocol at all, meaning most of the advanced features aren't possible (and these are things that can't be emulated on the client side).

Hell, the main feature I needed (bulk CAS get) didn't even require the meta protocol or recent memcached features - spymemcached just never bothered to implement it. I ended up abandoning the change I was working on, because the upstream never looked at my PR and it wasn't worth forking over (bigco bureaucracy etc).

There are also quite a few legitimate bugs open for years that haven't had so much as a comment from maintainers.

This is a little bit tricky because the parser has to distinguish between:

  for x in arr (something ())
           \                 /-- function call

and

  for x in arr (something ())
               \            /-- loop body

This is consequence of combining "blocks" and "precedence" into the same construct ().

A more fitting example would be to support:

  for x in arr do set z += x;
  for x in arr do something x;

IIRC these both currently require an explicit block in my parser.

I see, thanks for the clear example of the ambiguity you need to avoid! Fun stuff.

This is similar but not quite the same as persistent data structures. In particular:

- We can avoid quite a few allocations in loops by mutating lists/dicts in place if we hold an exclusive reference (and after the first mutation, we always will). Updates to persistent data structures are relatively cheap, but they're a lot more expensive than an in-place update.

- Herd has syntax sugar for directly modifying nested values inside lists/dicts. E.g. `set foo.bar.[0].baz = 1;`.

In practice, is this faster than a different implementation of the same semantics using persistent data structures and a tracing GC? That will depend on your program.

> 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.

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's only semantically a pass-by-value, in reality a reference is passed and the data is only copied if needed (i.e. value is mutated while shared).

So the language has reference semantics, and (per the edit) for every object (like in Python)?

(Ah, no, your example elsewhere in the thread suggests that the referred-to structures get implicitly copied all over the place.)

Nope, it's value semantics (unlike Python), the references are just an internal optimization. Implicit copies happen when a list/dict with more than one reference is mutated.

> the references are just an internal optimization

Optimization specifically for function calls, or... ?

Because if you're doing all this copy-on-write anyway, the indirection seems to be a needless cost in other contexts.

This applies everywhere, and it fundamentally wouldn't be possible for just function calls.

> needless cost

Are you comparing to a language with mutable references or a our functional language? A language with mutable references will of course be faster, but this is more intended as an alternative to pure functional languages (since functions are referentially transparent).

In this case, the cost of the indirection is approximately zero (relative to the cost of just doing reference counting), since passing a reference just requires a bump to the refcount. And most of the time the refcount increments are skipped by "moving" instead of copying the reference.

I'm comparing to the same language implemented without the supposed internal optimization, according to my understanding of what you're doing.

But I think at this point I'd have to read and analyze the code to have a proper understanding.

... Although granting that you already have paid the cost of reference-counting GC (and, I assume, per-object allocation), it probably is indeed insignificant. And special-casing where the reference count == 1 is also kinda neat. (E.g. CPython doesn't "move references" per se if I'm thinking clearly; but it does detect cases where it can safely mutate the underlying implementation of a type that's "immutable" from the perspective of language syntax.)

> 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.

There is some prior work on mitigating the performance cost of immutability that you might be interested in. For example, Clojure's persistent vectors allow fast modifications without destroying the original vector, because internally they're wide trees rather than just linear arrays of memory. This allows for assignments to be implemented without a copy of the full vector. https://hypirion.com/musings/understanding-persistent-vector...

> Use immutable pass by reference. Make a copy only if mutability is requested in the thread.

This is essentially what Herd does. It's only semantically a pass by value, but the same reference counting optimizations still apply.

In fact, Herd's approach is a bit more powerful than this because (in theory) it can remove the copy entirely if the caller doesn't use the old value any more after creating the thread. In practice, my optimizations aren't perfect and the language won't always detect this.

The big downside is that we have to use atomic reference counts for _everything_. From memory this was about a 5-15% performance hit versus non-atomic counters, though the number might be higher if other bottlenecks were removed.

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

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.