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> > It's exception safe. There are no runtime exceptions. All exceptions have defined semantics, and they are always handled.

> This seems contradictory. Are there exceptions or not? EDIT. Apparently there are (http://tutorial.ponylang.org/expressions/exceptions/). But then I don't understand what the above means.

I think the author is using a terminological Java-ism. In Java, a "runtime exception" (RuntimeException and subclasses) is an exception that doesn't have to be handled, and therefore may be thrown unhandled at runtime. Pony has exceptions, but no unhandled exceptions, i.e. no "runtime exceptions" in Java speak.

Yes, that's probably what is meant.

So the point is that a program will never crash due to an exception.

>> It's deadlock free. This one is easy, because Pony has no locks at all!

> This is nonsense. Deadlock happens when multiple processes/threads are waiting for each other. It is not caused by locks.

Actually deadlocks happen when 2+ resources are not locked in the same order by 2+ processes/threads... As far as I know, in Pony there's no way to send a blocking message to another actor, so there's no lock, so there's no deadlock.

Please correct me if I'm wrong

I shall refer to a higher authority:

> A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause.

That is from Modern Operating Systems, 4th ed. (2015), by A.S. Tanenbaum & H. Bos, page 439. (By "process" they really mean "process or thread".)

One way for thread A to wait on thread B is as you said: thread A can request a lock that thread B currently owns; then A waits for B. So requesting locks in opposite orders can result in deadlock. But of course if there are no locks, then no such waiting occurs, and so deadlock from this cause is impossible.

But there are other ways to wait. If var_a and var_b are shared variables, then code like the following might result in deadlock (The following code is in C; to simplify things, I'm assuming that data races and memory-access ordering are not issues.)

Thread A does:

    var_a = 1;
    while (var_b != 0) ;
    var_a = 0;

And thread B does:

    var_b = 1;
    while (var_a != 0) ;
    var_b = 0;

If Pony is capable of executing code like the above, then deadlock is possible.

There are many ways code can grind and thrash to a halt, like your code shows. But a deadlock refers specifically to the idea of two processes waiting until the other is done before they proceed. It's a scheduling problem, and a lock is used for synchronization. If there are no locks (spinlocks, semaphores, etc), there are no deadlocks by name (just other problems).

> ... there are no deadlocks by name (just other problems).

Okay, supposing we have a definition of deadlock that agrees with your usage: sure. But then these "other problems" you mention will have exactly the same practical effect as deadlock, right? A program will hang, sometimes unpredictably, based on more or less random decisions made by some low-level scheduling algorithm.

And in that case, Pony's guarantee of no deadlock is (1) completely correct, and (2) completely meaningless, from the point of view of a programmer whose concern is that his code will behave as required.

Am I wrong? EDIT. Maybe: https://news.ycombinator.com/item?id=10903876

Pony prevents you from writing that code. All communication between actors is by message passing, and there are no global variables. Check out reference capabilities.

Wow, so maybe the no-deadlock guarantee really does say something nontrivial. Thanks for the heads up.

You can deadlock in purely message passing code, even if there are no blocking sends. See an example in Erlang: http://james-iry.blogspot.com/2009/04/erlang-style-actors-ar...

That anti-pattern isn't possible in Pony, because Pony has no blocking receive call.

Of course, and Pony prevents programmer from writing code that waits for anything else. Sure. We're supposed to believe this?

  > How is `None` so different from this "null" that we want to avoid?

The difference is the defaults. Languages "with null" generally assume that any value can be null. Other languages, like Pony, do not allow any type to be null.

Pony's "None" type is an example of a "unit type": a type with only one possible value. It's a totally different thing.

you may be safe from "dereferencing null", but you'll still have to check for those edge cases, whether it's called Null or None

No, because random values cannot be None. The only thing that can be None is something with the type "None".

I see many examples where they do exactly what I described, checking for "None" as a parameter value. Sure, it's explicitly named as a possible value, but you STILL have to make the check yourself.

http://tutorial.ponylang.org/pattern-matching/match/#matchin...

I see how it can be used in new and different ways, and is definitely safer, but it's not like the idea of "that thing you want isn't here" goes away by introducing a None type.

I think I prefer the Option type to using a union with None here, for the reason that you state. I'm coming at this from a type theory / language design perspective: unit types are very different than nullable types, though if you write a union type with unit as one of the possibilities, well...

EDIT: Sorry, I was thinking of sum vs product here, union types are sum types. So this is the same thing as Option. Sounds good. The advantage of using a union type is that you _have_ to check. This is significantly better than nulls.

They're both sums, but Pony (and e.g. Typed Racket) has unlabelled unions, while Haskell/ML have labelled unions. Pony's (X | None) is not like Haskell's Maybe X in that in Haskell, X could be unit, while in Pony that would collapse the type to just None. In other words, in Haskell, Maybe (Maybe X) makes sense; in Pony and Typed Racket, it doesn't.

Yet another terminology issue: do you mean tagged vs untagged? IE, there's an extra field for the discriminant. Or rather, Pony does _not_ have a discriminant, but Haskell/ML do?

(I am 99% sure that's what you mean, but since I already screwed up terms once in this thread...)

That's roughly it. I was referring to a difference at the type system level, rather than the runtime level, though.

Disjoint unions aka sum types (as seen in Haskell/ML etc) entail some runtime way of keeping track which side of the sum any given inhabitant lies on. Tags are one way to do this, but e.g. in a language where pointers/references are never the zero machine word, and unit is represented as the zero word, no tag would be needed for the type Maybe String.

Union types (as seen in Pony, apparently, and also in Typed Racket), by contrast, /don't/ preserve information about which side of the union an inhabitant came from.

So, disjoint types: A + A ≠ A but union types: A ∪ A = A

So expanding (Maybe X) into (X + 1), we have that (Maybe (Maybe X)) = ((X + 1) + 1) ≠ X + 1,

but if instead (Maybe X) expands to (X ∪ 1), we have that (Maybe (Maybe X)) = ((X ∪ 1) ∪ 1) = X ∪ 1 = (Maybe X).

This is one of the problems with uses of null in languages that have it.

Ah, I guess I misunderstood. From the tutorial:

> Pony often uses the primitive None to indicate that something has "no value".

So that is not a special value that we can assign to any variable.

"None" would seem to be pretty pointless all by itself, but I guess we could do a union of any old type with "None", to get a result much like Haskell's "Maybe". And then the Pony type system would give us much the same guarantees as Haskell's would; so we get a nullable variable that still allows for typesafe operations -- e.g., dereferencing, if applicable. (I guess ....)

Yeah, when I read the parent comment, I thought that None was part of a type like Maybe, but the next sentence describes it as a unit type:

  > Of course, it does have a value, so that you can check what it is, and the value is the single instance of None.

Yes, but, as I noted, Pony has type unions (http://tutorial.ponylang.org/types/type-expressions/). So you can roll your own Maybe types.

From the page I linked above:

> var x: (String | None)

> Here we have an example of using a union to express an optional type, where x might be a String, but it also might be None.

That's pretty clearly a Maybe.

Maybe (at least in Haskell/Rust) is a sum type, not a union type, though.

EDIT: Ugh, sorry, those are the same, I was thinking of sum vs _product_ type, ignore me.

Actually, I think you've noted a significant issue: Pony looks to have been designed, at least in part, based on a sound theoretical foundation provided by modern type theory, semantics, etc. However, the documentation does not use the standard terminology of these fields. As a result, it is difficult to evaluate some of the claims that are made.

Is a union type (at least in Haskell/Rust) structured like a union in C, where alternative types of values (e..g. int, float, char) occupy the same bytes, just not at the same time? Or is separate space allocated for the different types of values? Asking because, if a function returns a union type, with one type within it representing an integer (which can have any value), and if the function also wants to return an error condition, then no valid integer value can be used to indicate the error, because all integer values are valid return values from the function. In this case I would think that a union type should have separate space allocated for the integer value and the error-signalling value. (Something like what golang does with its return values?)

There are two major forms of sum type implementations that I'm aware of: tagged and untagged. What you're describing is a tagged union, for exactly the reason you state: there's an extra field for a discriminant which lets you choose the tag. Untagged unions are C's unions, where there's no discriminant, and you just have to know which type is valid at a given time.

Rust supports tagged unions, but can also optimize them to untagged in certain circumstances. We will probably add some kind of untagged union as well, mostly for FFI, but they're very unsafe, for the above reasons.

Got it, thanks. Interesting about Rust's design goals and tradeoffs.

Similarly, Pony's union (ie sum) type implementation always optimises to untagged, but required "boxing" raw machine-words when they're in a union type.

The Haskell void type () is also a None, and it's pretty helpful. The easiest example is `putStrLn :: String -> IO ()`.

I found the statement regarding runtime exceptions confusing as well. Perhaps what is meant by that statement is that Pony lacks unchecked (i.e. Java) rather than runtime exceptions.

Yes, that's probably it.

Or to put it more simply: an exception will never crash a program.