## Mutability and Signals

Recently I've been working on Rosegarden, the music sequencer. It uses Qt which uses signals.

Signals implement the Observer pattern, where an object notifies "observers" via signals. A signal is connected to one or more "slots" in other objects. The slots are basically normal methods, except they return nothing (void). When a signal is emitted, Qt arranges for the slots to be called, other than those of deleted objects. So far, I find it works easily and elegantly.

This made me wonder: Could signals take the place of mutability in Scheme? And might that give us both referential transparency and reactiveness simultaneously?

There's not much support for signals for Lisp and Scheme. There's Cells, but it seemed to be conceived of as just a super-spreadsheet. I want to go much further and use signals to re-imagine the basics of object mutation.

## Quasi-mutation: The general idea

Basically, we'd use signals between constant objects to fake mutability.

• Objects can't mutate.
• "Slots" are closures.
• Signals are emitted with respect to particular objects.
• Not "by objects". Again, we're not object-oriented. We're just indexing on objects.
• Ability to emit is controlled by access to objects and signal types.
• Indexing on one particular argument seems overly special, so I contemplate indexing on any relevant arguments. This is again similar to generic functions.
• Signals can be connected to slots.
• The signals go to where the object's respective signal is connected. They are indexed on objects.
• Constructors connect the signal replaced from the parts to the constructed object.
• More precisely, to a closure that knows the object.
• The closure would fully represent the objects' relation. For instance, mutable pairs might have the slots new-car and new-cdr with the obvious meanings.
• But not for immutable objects. Immutable objects' slots would not be new-car and new-cdr, they would raise error.
• The constructed object can access its part objects, in appropriate ways by its own lights. For instance, a pair object could retrieve its car and cdr objects.
• This particular signal replaced is not exposed.
• The details of replaced will be refined below.
• Slots such as new-car will typically:
• Construct a near-copy of the object, with the new part in the old part's place. This effectively connects a new version of the object to the new part and disconnects it from the old part.
• Emit replaced with respect to the object, propagating the change.
• "Normal" setters such as set-car! emit replaced wrt the old object with the new object as value.
• That's just the classical way. There's plenty of room to do clever new things with signals.
• As above, doing this to immutable objects causes error.
• Constructed objects behaving this way would include at least:
• Mutable pairs (and therefore mutable lists and trees)
• Environments
• Continuations. While not often considered object-like, continuations have parts such as the current environment and their parent continuations.
• External-world-ish objects such as ports react to signals in their own appropriate way, not neccessarily propagating them further.

## As if

Literally implementing ever pair with at least two signals between itself and its car and cdr seems prohibitive if not impossible. Physically, it couldn't be the only mechanism of mutation. So I'm talking about a mechanism that acts as if it's continuous down to basics like pairs and lists, but really uses a more modest mechanism where it can (presumably containment, as now).

## Object identity

Above, I said "constructed object can access a part object", not "a part's value". Since objects no longer ever change value, the difference is subtle. It's just this: the object has a single set of signal connections. So it has a single identity. So there is a trace of object identity remaining.

One could represent identity value-wise by saying that values consist of a (classical) value and an "object-identity" value, and that object-identity values are opaque and unique except as shared by this mechanism. So signals are connected with respect to object-identity values.

It has a flavor of "the long way around", but it lets us treat objects entirely as values.

### Object versioning

Different versions of an object have different Object-IDs. Imperatively this wouldn't be anything, since two simultaneous references to the same object can't have different values. But here, one can both mutate an object as far the the world is concerned and hold onto its old value. But it should never be the case that objects are eq? but not equal?. So different versions have different Object-IDs.

### The equality predicates

Equality:

equal?
is what it normally is in Scheme or Kernel: The objects have the same current value.
eq?
A and B are eq? just if
• (equal? identity-of-a identity-of-b)
=?
is just an optimization of equal?

## Signals and immutability

I mentioned that I was thinking about immutability in regard to this. So far, I've just described how to duplicate mutability with signals.

For immutable objects, some or all slots would still get connections, but would raise error instead of propagating mutation.

But that's only half of the control we should have over mutation. We'd also like to guarantee that certain evaluations don't mutate even mutable objects they have access to, eg their arguments, the environment, and dynamic variables. The "foo!" convention indicates this (negatively) but doesn't enforce anything. "foo!" convention notwithstanding, we'd like to guarantee this from outside an arbitrary call, not from inside trusted combiners.

### Blocking signals

So we'd like to sometimes block signals. If signals were emitted anyways, they'd be errors and would not reach their destinations. So if replaced is blocked, code either doesn't try to mutate objects or tries and raises an error. Either is consistent with immutability.

ISTM the simplest way to block signals is to disconnect their existing connections and connect them to error combiners. When the call is done, restore their original connections. However, that doesn't play well with asynchrous execution.

Instead, we'll make a copy of the original object that will (probably lazily) infect its parts with "don't mutate me in this scope".

### Scope

For a traditional imperative language, where flow of control and scope are structurally the same, we could block signals in specific scopes, recursively. But for Scheme and Kernel, that won't suffice. What would happen if an object is passed to a continuation and mutated there? We've broken the guarantee that the object wouldn't be mutated. Any time we let objects be passed abnormally, this can happen.

We might try to:

1. raise error if affected objects are passed to continuation applications, or
2. "infect" the other scope with the signal restrictions.

Neither is appealing. In this mechanism, continuing normally is also passing to a less restrictive scope. And continuing normally should behave about the same way as continuing abnormally to the same destination. We also don't want error returns to permanently "freeze" objects.

So ISTM we must distinguish between continuing to a (not neccessarily proper) parent of the restricting scope (normally or otherwise) and continuing elsewhere. Signal blocks are removed just if control reaches a parent. This is essentially how Kernel guards reckon continuation parentage.

### Doing this for all object parts

We'd usually want to say that no part of any argument to a combiner can be mutated. It's easy enough to treat signal connections from the root argobject. But we need to "infect" the whole object with immutability, and not infect local objects, which may well be temporary and legitimately mutable.

Since these arguments are ultimately derived from the root argobject, what we can do is arrange for accessors to give immutable objects in their turn. But they have to be only temporarily immutable - blocked, as we said above. And we'd prefer to manage it lazily.

So I propose that accessing a blocked object gives only objects:

• whose replaced signals are re-routed to error, as above, until control "escapes normally" (an improper parent continuation is reached)
• which are in turn blocked, meaning they have the same property infecting all of their accessors.

#### Non-pair containers

Non-pair containers are not treated by mechanisms like copy-es-immutable. But we want to treat immutably fully, so they have to be treated too. This is the case even for:

• Environments
• Encapsulation types. In this case, their sole accessor is required to do this, as all accessors are.
• Ports. Administrative state or not, they can be immutable.
• Closures. Anything they return is considered accessed and automatically gets this blockingness. Their internal parts (static environment, etc) need not be blocked.
• Continuations. Like closures, what they return is considered accessed and automatically gets this blockingness.

#### Exemptions

We'd like to be able to exempt particular objects from this. Some combiners mutate an argument but shouldn't mutate anything else. There'd probably be a signal-block spec that would specify this.

### Blocking signals to keyed dynamic objects

We can easily extend the above to deal with dynamic environment, but keyed dynamic objects are not so simple. Their accessors would be covered by the above if derived from the argobject or the dynamic environment, but they need not be.

So we need an additional rule: Keyed dynamic objects are blocked if accessed in the dynamic scope of the blocking. That's recursive like other blockings. Keyed rebindings in the dynamic scope aren't, because one might bind a temporary that's legitimately mutable.

Side note: I'd like to see a stylistic convention to differentiate between combiners that mutate their arguments ("foo!") and combiners that mutate something else, meaning either their dynamic environment or a dynamic variable (Say "foo!!")

## Interface to be defined

I've already written a lot, so I'll leave this part as just a sketch. To support all this, we need to define an interface.

• A means of defining new signal types
• Returns a signal emitter
• Returns a signal identifier for connect' to use.
• A signal scheduler. By general principles, the user should be able to use his own, at least for exposed signals. It's not too hard to write one with closures and continuations.
• Means for the user to emit signals
• Means for the user to connect and disconnect signals
• Not exposed for many built-in objects such as pairs.
• "capture all current", as for blocking
• "disconnect all"
• block-all-signals: connects all signals to error continuation
• Possible argument except'
• (Maybe) block-signal: connects given signal to error continuation
• A "dirty-flag" mechanism, often useful with signals.
• Possibly a priority mechanism.

## Some possibilities this raises

• Use the signal scheduling mechanism for managing constraints. Since when we can enforce immutability, constraints become very tempting.
• Provide general signals
• Provide an interface to Lone-COWs, a sort of copy-on-write object optimized for usually being uniquely referenced/owned.
• Supplying "out-of-band" signals to a debugger or similar. They really do need to be out-of-band.
• Provide a broadly applicable interface for redo/undo. It could basically just capture historical copies of objects.