18 October 2011

Thoughts on the structure of Klink Libraries

Klink Libraries

Previously

I blogged some ideas about Library paths for Klink, the Kernel implementation I wrote. I listed several desiderata, based on lessons from the past. I also blogged about how I'd like bridge the gap between package as used by developer and package as used by user.

Additional desiderata

  • (The desiderata from the previous blog post, plus:)
  • Should co-operate well with development. Switching from use to development shouldn't require gross changes or compete with library support.
  • Can fetch libraries automatically with reasonable power and control
    • In particular, automatable enough to support "remote autoloading" but ultimately should be under the user's control.
  • Support clean packaging

Fetching libraries: mostly lean on git

The well-loved version manager git provides most of what I'd want, out of the box:

  • Co-operates well with development (More than co-operates, that's what it's usually for)
  • Reasonably compact for non-development. You can clone a repo with depth=1
  • Fetching
    • Via URL (git protocol or otherwise)
    • Doesn't treat URLs as sexps - only a mild problem.
  • Finding out what's there to be fetched, in the sense of available versions (eg, looking for latest stable release) 
    git ls-remote --tags URL
    • But we have to distinguish tags and tags, which AIUI don't refer to versions.
  • Secure digital signatures are easy
  • Excluding local customizations from being updated
    • This is possible with .gitignore and some care
    • But customizations will live somewhere else entirely (See below)
  • Practices supporting stable releases. git-flow (code and practices) does this.
  • NOT a well-behaved heterogenerated tree of libraries.

Of course git does not support knowing that a repo is intended as Kernel code. Looking at filename extensions does, but that seems to require fetching the repo first. For the same reason, it can't easily be any file that "lives" in the repo. It should be something about the repo itself.

So the convention I propose is that the presence of a branch named --kernel-source-release indicates a branch of stable Kernel code. Tags on that branch would indicate available versions, so even if coders are working informally and doing unstable work on "master", only tagged versions would be offered.

But does keeping --kernel-source-release up to date require extra effort for the maintainer? IIUC, git can simply make --kernel-source-release track "master", so if a coder's workflow is organized, he needn't make any extra effort beyond issuing a one-time command. Branch tracking is intended for remotes, but seems to support this.

Should there be other branches, like --kernel-source-unstable or --kernel-source-development? I suspect they're not needed, and any use of unstable branches should be specifically configured by the daring user.

I'm not proposing to permanently tie Klink (much less Kernel) specifically to git forever. But it serves so well and is so well supported that I'm not concerned.

Where to put it all?

That addressed how we can fetch code. In doing so, it put some restrictions on how we can organize the files on disk. So I should at least sketch how it could work on disk.

The easy part

Of course one would configure directories for libraries to live in. Presumably one would distinguish system, local, and user.

Path configuration

But the stow approach still left issues of where exactly to stow things. We can't solve it in the file system. That would result in one of two ugly things:

  • Making each project represent the entire library filespace, with its real code living at some depth below the project root.
  • Making each project physically live in a mirror of the target filespace. This would have all the problems we were avoiding above plus more.

So I propose per-project configuration data to tell stow about paths. I'd allow binding at least these things:

prefix
The library prefix, being a list of symbols.
parts
List of sub-parts, each being a list, being:

For example, 

((prefix (std util my-app))
   (parts
      (
         (source
            [,,src,,]
            ())
         (source
            [,,tests,,]
            (tests))
         (info
            [,,doc,,]
            ())
         (default-customizations
            [,,defaults,,]
            ())
         (public-key
            [,,pub_key.asc,,]
            ()))))

That would live in a file with a reserved name, say "%kernel-paths" in the repo root. As the example implies, the contents of that file would be sexps, but it wouldn't be code as such. It'd be bindings, to be evaluated in a "sandbox" environment that supported little or no functionality. The expressions seem to be just literals, so no more is required.

Dependencies and version identity

Surfeit of ways to express version identity

There are a number of ways to indicate versions. All have their strengths:

  • ID hash
    • Automatic
    • Unique
    • Says nothing about stability and features
  • Release timestamp
    • Easily made automatic
    • Time ordered
    • Nearly unique, but can mess up.
    • Says nothing about stability and features
  • Version major.minor.patch
    • Just a little work
    • Expresses stability
    • Expresses time order, but can be messed up.
  • Test-satisfaction
    • Lots of work
    • Almost unused
    • Automatically expresses stability and features
    • No good convention for communicating the nature of tests
  • `stable', `unstable', `release', `current'.
    • Expresses only stability and currency
  • By named sub-features
    • Just a little work
    • Expresses dependencies neatly
    • Expressive
    • Not automatic

I chose sub-feature names, based on how well that works for emacs libraries, a real stress test. That is, I choose for code to express dependencies in a form like: 

(require (li bra ry name) (feature-1 feature-2))

Co-ordinating sub-features with version identity

The other forms of version identity still exist as useful data: ID hash, version tags, results of tests. What makes sense to me is to translate them into sets of provided features. Do this somewhere between the repository and the require statement. require would still just see sets of features.

Desiderata for this translation:

  • Shouldn't be too much work for the developer.
    • Probably easiest to support automatic rules and allow particular exceptions. With a git-flow workflow, this could almost be automatic. As soon as a feature branch is merged into "master", that version and later versions would be deemed to have a feature of that name.
  • Should be expressable at multiple points in the pipeline, at least:
    • Annotations in the source code itself
    • In the repo (In case the source code annotations had to be corrected)
    • Stand-alone indexes of library identities. Such indexes would be libraries in their own right. Presumably they'd also record other version-relevant attributes such as signature and URL.
    • Locally by user
  • Should be derivable from many types of data, at least:
    • Branches (eg, everything on "master" branch has the feature stable)
    • Tag text (eg, all versions after (2 3 3) provide foo-feature)
    • Tag signature (eg, check it against a public key, possibly found in the repo)
    • Source code annotations (eg, after coding foo-feature, write (provide-features ear lier fea tures foo-feature))
    • Tests (eg, annotate foo-feature's (sub)test suite to indicate that passing it all means foo-feature is provided)
    • ID
      • To express specific exceptions (eg, ID af84925ebdaf4 does not provide works)
      • To potentially compile a mapping from ID to features
    • Upstream data. Eg, the bundles of library identities might largely collect and filter data from the libraries
  • Should be potentially independent of library's presence, so it can be consulted before fetching a version of a library.
  • Should potentially bundle groups of features under single names, to let require statements require them concisely.

Dependencies

With sub-features, we don't even need Scheme's modest treatment of dependencies, at least not in require. Instead, we could avoid bad versions by indicating that they lack a feature, or possibly possess a negative feature.

The usual configuration might implicitly require:

  • works
  • stable
  • trusted-source
  • all-tests-passed

The set of implicitly required features must be configurable by the user, eg for a developer to work on unstable branches.

Library namespace conventions

On the whole, I like the CPAN namespace conventions. I'd like to suggest these additional (sub-)library-naming conventions:

raw
This interface provides "raw" functionality that favors regular operation and controllability over guessing intentions.
dwim
This interface provides "dwim" functionality that tries to do what is probably meant.
test
This sub-library contains tests for the library immediately enclosing it
testhelp
This sub-library contains code that helps test libraries that use the library immediately enclosing it. In particular, it should provide instances of objects the library builds or operates on for test purposes.
interaction
This library has no functionality per se, it combine one or more functional libraries with an interface (keybindings, menus, or w/e). This is intended to encourage separation of concerns.
inside-out
This library is young and has not yet been organized into a well-behaved namespace with parts. It can have sub-libraries, and their names should evolve to mirror the overall library organization so that it can become a real library. 
(inside-out new-app)
user
This user is providing a library that doesn't yet have an official "home" in the namespace. The second component is a unique user-name. 
(user tehom-blog/blogspot.com inside-out new-app)
(user tehom-blog/blogspot.com std utility new-util)
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13 October 2011

Mutability And Signals

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).

Circularity

My blog readers, being smart, will already be wondering what happens when replaced is propagated in a circular structure. Does it propagate forever, crashing the interpreter? That might go something like this: The signal propagates from pair to pair, repeatedly reaching its original location, pointlessly representing more new objects that differ only from the k*N nth cdr, and never finishing because there's always an (k+1)*N nth cdr to represent.

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEh9r91P85LNzBKPGgc22Ihrb9vM6I10c2gPpxQKpSXPLVlc_U_OTSEx06KvMkNz79yh-JNMfmpy7ikiecIzXHxQpuZx-e7j_i-P2k84-wZ9U9EOK9-BOndYogEHaCIVP4S0ISbaBnVYTpQ/

For instance, in the circular list above, suppose we gave object C a new value. It emits (replaced New-C C), which B receives in new-cdr. B emits (replaced New-B C) which both A and old C receive. Leaving aside A's reaction, old C then emits (replaced New-Old-C C) which old B again receives. A is going to be deluged with infinite signals, and we will keep making new copies of old B and old C. Worse, we make many new-Bs when we only have a place for one.

The too-easy answer would be to block all repetitions between the same two objects. That fails, because we re-emit on new copies. So also block repetitions where the objects are the same except that one is replaced by an updated copy?

So as a first draft, I propose that these built-in signal emissions:

  • Be each associated with a unique ID
  • Refuse to propagate that ID circularly

The ID would be an opaque ID equal? only to itself. It would be unique to a given cascade of emissions. That is, set-car! etc would make fresh IDs but new=car etc would use the ID of the replaced signal that provoked it.

One subtle point: It suffices for the signalled object itself to recognize repititions. Originally I thought repititions would have to be recognized across a transitive closure. But only the original object is connected to by its contents, and connections from new parts to new object don't receive any signal to propagate.

What about structural information?

The above is progress, but it discards structural information.

Consider an application that displays the first N items from a possibly-circular list on screen, and redisplays them just as needed (Say they are formatted in some time-consuming way and different screen locations can't share this work) The display would want to be notified of all the nth cdr occurences of a changed element. The approach above wouldn't permit that.

So when there's circularity, let's switch tactics and use polling to retrieve information as needed. Let any circular object provide a stream of all the replacements that occured in it for a given emission-ID. Applications only read the stream as far as they need to and the rest is not really generated.

But often a slot would prefer to get a simple new object, without replaying all the detailed changes.

Desiderata

To sum up the desiderata on this:

  • We want slots to be able to replay detailed changes.
  • We want slots to be able to just get a new object
  • We don't want mutated objects to have to guess what their target slots want, which may not even be the same for different slots.
  • We don't want mutated objects to have to do inordinate (even infinite) work preparing these signals.
  • We don't want to make assumptions about the topology of the mutated object. It can be an arbitrary directed graph.

A thought-experiment sequence of operations

The sequence of operations on the circular list above would go something like this. Here I use FOO.N to indicate the nth version of FOO.

  • We gave object C.1 a new car, V (we emit replaced.1 to it) Its change is:
    • "Replace car with object V". We'll represent this as
      • The trivial accessor identity.
      • A promise to construct V (Trivial since we started off knowing it)
  • replaced.1 reaches C
  • We emit replaced.2 for C. Its change is:
    • A disjunction (or) of
      • "Replace what you're holding with C.2"
        • identity
        • A promise to construct C.2
      • "Replace car of it with V"
        • car
        • Promise of V
  • replaced.2 reaches B
  • We emit replaced.3 for B.1. Its change is:
    • A disjunction (or) of
      • "Replace this with B.2"
        • identity
        • A promise to construct B.2
      • A disjunction (or) of
        • "Replace cdr of this with C.2"
          • cdr
          • A promise to construct C.2
        • "Replace cddr of this with V"
          • cadr
          • Promise of V
  • replaced.3 reaches A.1
    • We'll say the promises are not forced yet. If they are, that still forces these following operations to occur first.
  • replaced.3 also reaches C.1. Here we encounter a repitition. C.1 has already encountered a signal from this trigger. We don't want to emit another consequence of it to the same places. Instead we want to fold the signals together.

Let's stop operations and look at it

  • If we made replaced.4, its change would be:
    • A disjunction (or) of
      • "Replace this with C.2"
        • identity
        • A promise to construct C.2
      • A disjunction (or) of
        • "Replace cdr with B.2"
          • cdr
          • A promise to construct B.2
        • A disjunction (or) of
          • "Replace cddr of this with C.2"
            • cddr
            • Again, a promise to construct C.2
          • "Replace cadr of this with V"
            • caddr
            • Promise of V

So replaced.4 resembles replaced.2. The big change is that instead of "Replace car of this with V", we have a cdr replacement. It pushes "replace with V" down to another cycle of the loop. That doesn't support the "display circular list" example, which was our motivation for doing this. So:

  • We need to support something that can mention both the car replacement and the cdr replacement of C.1
  • We need to effectively be able to adjust previous signals instead of emitting more.

I first considered and discarded an approach of rewriting the second disjunct into a conjunction.

Now the most tempting approach is to rethink the car replacement operation. It did not fully describe the object, it just described the new car, V.

Let's say that all such operations are in fact constructors, and in the case of "Replace car of it with V", we let the cdr argument default to the current cdr.

Timing issues:

  • The default can't be committed to until the signal and its consequents have stopped propagating.
  • We know this by Emission-Id.
  • So forcing one of these promises first causes the signal to propagate until it's done.

This sounds quite like what I would expect from a solution. Constructors "stand for" the objects they make, as is often the case in RT languages. We are getting a structure that's isomorphic to the original object (circular list of period 2). "Just mutate contents" remains a valid optimization as long as we're in a closed system (And a tempting one; I have to keep reminding myself that we're conceptually replacing mutation with signals so it won't do)

The new structure of replacement info

So replaced.2's replacement info actually looks like:

  • A disjunction (or) of
    • "Replace what you're holding with C.2"
      • identity
      • A promise to construct C.2
    • "Construct it with V as car"
      • Arguments
        • car : Promise of V
        • cdr : C.2's cdr value, defaulting to C.1's cdr value.

Default arguments

The action of replaced.3 on C.1 just provides a value for the cdr argument. Re-emission is not needed, nor wanted since at C's cdr the signal has reached a join.

So the default cdr argument is held in a sort of limbo until it's given or not. That's a lot like mutation in itself. So it too is managed by signals:

  • "All done". Signal has fully propagated, use defaults for anything not filled.
  • "Use this" Accessor Value. After receiving "use this", "All done" has no effect.

These are of course quasi-mutating the stream argument of replace. There's an infinite regress there, but since it is all behind promises that delay evaluation until after this is done, I think we can consider it as if the stream had had one unchanging value forever.

Structure of the change stream

So each "cell" of the stream of operations changes consists of a disjunction on constructors and values, rather than a cons cell. Each disjunct can be:

  • A promise of a value
  • A promise of a constructor parameterized by promises of values

So it's like an n-ary tree-stream.

Note that this naturally orders the change stream from shallow to deep. If it was ordered from deep to shallow we couldn't find elements in finite time.

This doesn't preclude specialized representations that (say) compact multiple simple cdr accesses to an nthcdr access. These could be represented as additional constructors in the disjunct.

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.
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Thinking about Immutability in Klink

Previously

Klink is my stand-alone implementation of Kernel by John Shutt.

Types of immutability

There are several types of immutability used or contemplated in Klink.

Complete immutability
Not much to be said.
Pair immutability
A pair that forever holds the same two objects, though those objects' contents may change.
List immutability
For instance, a list where you can change the contents, but not the number of elements.
Recursive structural immutability
An immutable tree of mutable non-pair objects.
Administrative immutability
Eg, what ports typically have. When you read or write to a port, it remains "the same thing" administratively.

Non-recursive structural immutability?

If you read section 4.7.2 of the Kernel report (copy-es-immutable), you may notice that Pair immutability and List immutability are actually extensions. So I figured I should at least advance a rationale for them.

Is non-recursive immutability worth supporting? ISTM it's already strongly suggested by Kernel.

Implied by an informal type
Some combiners take finite lists as arguments; all applicatives require a finite list as argobject. That distinguishes the finite list as at least an informal type. There's a predicate for it, finite-list?, but pairs that "are" finite lists can "become" other sorts of lists (dotted or circular), so it falls short of being a formal type. This seems like an irregularity to me. Structural immutability would solve it.
Implied by implied algorithms
For some combiners (eg reduce), any practical algorithm seems to require doing sub-operations on counted lists. That implies a structurally immutable list, because otherwise the count could theoretically become wrong; in practice it's saved from this by writing the algorithm carefully. So there are at least ephemeral, implied structurally immutable lists present.
Vectors
John once told me that he would eventually like to have vectors in Kernel. Vectors are optimized structurally immutable finite lists.
Opportunity for optimization
There's an opportunity for other optimizations where list immutability is used and recognized. In particular, caching a list's element count is often nice if one can be sure the list count won't change.

Comparison table

Type of immutabilityPairListTree
Special care needed
for shared objects?NoOnly own tailYes
Exists in Klink?YesNoYes
Exists in Kernel?NoNoYes
What might be mutable?car, cdrelementsleaves
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21 September 2011

(Long) I'd like to see a Linux package manager co-operate with development

An understandable choice

Now, this is all understandable. They don't call development versions "the bleeding edge" for nothing. If a package manager has to choose between blindly trusting locally built software and blindly ignoring it, it should ignore it.

And trying to make it all work together would raise problems of communication and coordination. How is a package manager supposed to know what it needs to know? And even if it knows, how is it supposed to coordinate with "make install"?

Does CheckInstall fix it?

Not for me. CheckInstall wants to watch "make install" and create a package (deb, RPM, others). The idea is that then you use that package to install and uninstall.

  • Creating a package every time I "make install" is a very heavy mechanism. It's not really for developers, it's for small distributors.
  • I'd forget to use it when I "make install", then what have I got?
  • The way I work would confuse it. I typically configure to install into usr/local/stow. Which leads me to the next section.
  • It's a roundabout way of doing things.

Stow could help

What Stow does

If you know about Stow, you probably should skip to Could a package manager work thru stow?

Stow is a Perl package by Bob Glickstein that helps install packages cleanly. More cleanly than you might think possible, if you're familiar with traditional installation. It works like this:

  • You install the package entirely inside one directory, usually a subdirectory of usr/stow or usr/local/stow We'll say it's in usr/local/stow/foo-1.0 No part of it is in bin or usr/bin or usr/doc etc. Every file lives under usr/local/stow/foo-1.0

    With most builds you can arrange for "make install" to do this by passing ./configure an argument like 

    --prefix=/usr/local/stow/foo-1.0/
  • To complete the install, just: 
    cd /usr/local/stow/ 
stow foo-1.0
  • That makes symlinks from usr/doc, usr/bin, etc into the stow/foo-1.0 directory tree. No file is physically moved or copied out of stow/foo-1.0
  • Now the package is available just as if it had been installed.
  • Want it gone? Just 
    cd /usr/local/stow/ 
stow -D foo-1.0
  • Want it completely gone from your disk? After the above, you can just: 
    rm -r foo-1.0

This is neat in every sense of the word. It also can manage multiple versions of a package neatly.

Must it be stow?

Maybe you don't like Perl. I don't either, though I've coded in it and I do like CPAN and a fair bit of stuff that's written in Perl.

stow is not the only tool for this, it's just the first. There are a number of variants or offshoots: Graft, ln_local, Reflect, Sencap, Toast. They mostly seem to be in Perl as well. One exception is Reflect, which requires only bash and coreutils. Unfortunately, Reflect appears to be abandoned.

So the idea here is not so much an application as a file-location protocol. Another tool could do the same job on the same directory. You could even change tools later on.

General approach

It's easy for me to say "it uses stow", but that leaves a lot of little issues. I'll tackle them one by one below, but first I'll outline a general approach.

Desiderata:

  • It needs to work safely even if interrupted. So all the relevant information needs to be stored in the filesystem.
  • It shouldn't require any component to understand details that are not its own concern, following the general rule that components should do one thing well.
  • If possible, it shouldn't require any component to do much more than it does now.
  • It should place little or no extra constraint on development
    • Especially it should place no constraint on how to develop packages that one's package manager is not interested in.
  • It should not make security holes.

File-naming scheme

So I will base this mostly on a file-naming scheme:

  • A package named "FOO" relates uniquely to /usr/stow/FOO/
  • The files of version N.N of a package FOO live under /usr/stow/FOO/N.N/
    • It's a slightly stronger version of the usual stow subdirectory naming practice. The difference from usual practice is just a level of subdirectory; it's FOO/N.N/ instead of FOO-N.N/
    • Almost no format is assumed for N.N, except that it can't start with a single colon.
    • I'm reserving double or more initial colons for situations where the version name wants to start with a colon.
  • FOO-related files and directories that are not themselves installed ("Magic" files) live in /usr/stow/FOO/
    • Their names all start with a single colon.

Specifically unaffected so far:

  • Most stow functionality.
  • Most package manager functionality. It mostly needs to build paths accordingly.
  • Naive local packages that live as usr/stow/FOO-N.N (no slash). They just are not understood as versions of FOO and thus receive no benefit.

stow2

I will talk about stow2, a hypothetical variant of stow. It's old stow with a few minor extra behaviors. It doesn't aim to be a package manager. It doesn't watch out for system coherence as a package manager does. In this vision, it's merely the part of the package manager that handles physically adding or removing a package. It just aims to do software-stowing for either a package manager, a developer or both without inviting problems.

Specific issues

What about shared config?

I mentioned sharing config above. Config shouldn't disappear or get overwritten when changing versions, or when changing how versions are managed.

So configuration data that should persist across versions will live under /usr/stow/FOO/:config/. It's stowed or unstowed just like a normal stow directory. There difference is in how it's used:

  • It is meant to contain data that ought to remain unchanged across versions, such as user preferences.
  • It initially contains very little. I am tempted to say nothing at all, but the case of config triggers makes me unsure.

Unaffected:

  • Unaware packages will just operate on config data where they think it lives, which is where it's stowed from /usr/stow/FOO/:config/.
  • Developers can treat config data for their builds by their own lights.
  • stow2 can stow this like any other directory tree.
  • Maybe package format, which would not need a separate space for config data if there isn't any.

Altered responsibility: A package manager should:

  • When installing or updating
    • Create the FOO/:config/ directory if it doesn't already exist.
    • If it already exists, don't alter it.
    • In any case, arrange for the package to update the config data, eg to add settings for new flags.
  • When told to remove a package's config data, unstow that directory before deleting it.

How the updating can be done is another issue, and this post is already long. I'll just say that I have a vision of farming off the (re)config control role entirely to make, at some stage higher than stow2, in such a way that developers and package managers can both use it.

What if a user unstows an installed package?

OK, everything works neat while the package manager is the only thing moving. But what happens when the user/developer starts using stow as it was meant to be used? He doesn't hurt anything by stowing new packages, but what happens when he unstows an installed package. . .:

I propose solving the unstow case by adding:

  • New flag: Whether a given stow directory is under control of a package manager or not.
  • New behavior:
    • Let stow2 refuse to unstow if that flag is set, unless forced.
    • If forced, inform the package manager.

So reserve the filename /usr/stow/FOO/:managed-by

  • If it doesn't exist, that means no package manager considers FOO installed.
  • If it exists, it is a symlink to a package manager that considers FOO installed.
    • I don't specify what part of the package manager. It might be useful to be a link to an executable.
    • A package-manager, as package itself, has to manage versions of where that symlink points
      • Maybe by pointing it towards an unchanging location.
      • Maybe by understanding previous installed versions of itself.
  • When stow2 stows, it should create ../:managed-by pointing to itself.
  • stow2 should refuse to unstow if ../:managed-by exists and doesn't point to itself, unless forced.
  • (Stows that require unstowing another version are really the unstow case)
  • If stow2 forces an unstow, it should erase ":managed-by".
  • stow2 should support an filename argument that means "Act for this package manager".

What if a user stows a package?

I said "He doesn't hurt anything by stowing new packages". But we wanted more than harmlessness. We wanted locally built packages to be basically on the same footing as installed packages; at least, we want the user to be able to make that happen without real extra work.

But that invites us into a potentially complex system of dependencies and signing. Again, stow2 shouldn't have to concern itself with that but it also shouldn't make a mess.

For dependencies etc

For some concerns, we're just going to have to bite the bullet and say that the developer must talk to the package manager in its own format. Dependencies are one example.

Let's reserve the directories /FOO/:control for all package control data about FOO. Now, various package managers might have their own formats, and different dependencies etc might apply to different versions. So let's further reserve any file or directory /FOO/:control/PMNAME/N.N for a package manager named PMNAME with regard to package FOO version N.N. Here we can't do much about name clashes, but it's just between a few package managers.

Sometimes the same control data applies to many versions of the same package, up to version identity data. Particularly in development, the version might change frequently but the developer shouldn't have to create new control data each time. So let's reserve /FOO/:control-default and /FOO/:control-default/PMNAME.

So a conforming package manager PMNAME:

  • Must recognize files of the form /FOO/:control/PMNAME/N.N and /FOO/:control-default/PMNAME even if FOO is not a package it knows about.
  • Similarly, must recognize files of /FOO/:control-default/PMNAME
    • For them, find all FOO/N.N and consider each a version of FOO.
  • Must treat all such package versions as available or installed, as appropriate.
    • Even where this requirement conflicts with security measures it uses for external packages, such as digital signing.
  • May use its own internal control format, except insofar as it conflicts with other requirements. For instance:
    • The requirement to associate :control-default/PMNAME data with multiple versions, which might conflict with a "version" field in control data.
    • The requirement to treat such packages as available, which might conflict with digital signing.
  • Is allowed but not required to distinguish such packages, for instance by:
    • presenting them differently
    • presenting their dependency chains differently
  • Is recommended to provide a means in control data for indicating particular packages as unstable. Generally package managers already provide this.

But this still requires that the developer create control data in the package manager's format. Let's make it a little easier for him and require any package manager that has FOO-N.N to create /FOO/:control/PMNAME/N.N on request. Generally that's simple. Then the developer can adapt that instead of starting from scratch.

Digital signatures

A package manager generally guarantees package integrity by checking signatures. They are considered part of the control data.

We could require the developer to cause each "make install" to be signed. But this would not make /usr/stow/ even slightly more secure. Someone already put the code in question into it (usually by "sudo make install") Any misbehavior they wanted to do was already possible.

So I just say, trust the local versions. They belong there - or if somehow they don't, it's beyond what a package manager can provide security against.

The exception is if some user:

  • Has write permission into /usr/stow/
  • Does not have permission to stow
  • Does have permission to run the package manager
  • Can stow via the package manager. Perhaps it has the sticky bit, or he can sudo it but can't sudo stow.

So a package manager ought not to consider local versions installable by a user who can't directly run stow. It seems difficult for that situation can arise.

What about name collision?

What if two packages have the same name? That wasn't an issue when we assumed a package manager, which assumes a naming authority that can manage the namespace. And what about the flip side, if one package has two names at various times?

There's often no problem, but when there is, the developer has all the power to solve it. He can name his local stow subdirectories anything he pleases (directly or thru ./configure --prefix=). So it's up to the developer to manage the stow namespace in harmony with his favorite package manager.

Now, if more than one package manager is operating on the same system, there could namespace problems. But that wasn't even possible before, so nothing is lost.

What about bootstrapping?

How do you use stow2 if it's sitting in /usr/stow/stow2/1.1 unstowed? This is a solved problem: To bootstrap, call stow2 by its absolute path. Its support apps (Perl interpreter or w/e) also would be used by absolute path if they haven't been installed. That's all.

What about system bootstrapping?

Some Linux boot systems require some boot software such as the kernel to live in a small1 (<100 Mb) partition at the front of the hard disk. You can't symlink from /boot/ into /usr/stow/, but there's already a well-known solution. You make a stow directory there, eg /boot/stow/, and you put such software in it.

One would have to define a mapping that the package manager understands, and affected systems would have to be configured, and affected packages tagged, but for most packages and systems it's no work.

What about an unbuilt package having requirements?

So I solved most of the obstacles, including "they cannot satisfy requirements", but not "they cannot have requirements". There's a place for them in control data, but:

  • It pertains to the wrong stage. Generally you need packages for the build stage, not after installation. If you as a developer only find you need some other package after installation, you just fetch it.
  • At the time /FOO/:control/PMNAME/N.N is read, FOO/N.N doesn't exist and the package presumably isn't available thru the package manager. So it would be fruitless for a package manager to try to install FOO-N.N.
  • It misses an opportunity to learn requirements from the configure stage.
  • It misses a wider opportunity for other apps to communicate their needs to a package manager. For instance, as a simple way for an app to pull in optional support that's not bundled with it (hopefully only with user approval)

"BUILD" packages don't suit development requirements either.

This one is unrelated to anything stow2 should do. It is properly the domain of package managers as requirement managers. Here I suggest:

  • A common file format for representing requirements, not particular to any package manager.
  • A canonical flag to invoke a package manager with regard to such a file, like: 
    aptitude2 --wanted autoconf2-201109212117-1.requirements

The format seems to require at least these fields:

  • Name of requesting app
  • Date requested
  • List of desired packages, each giving:
    • Name
    • Version required
    • What exactly is wanted, eg app, static lib, dynamic lib, dev headers.

Because there is no one package manager in control, name collision and name splits are now an issue. In particular, virtual package names have little to go on.

One approach is to allow the package name to be represented in multiple ways, each with respect to some managed namespace. Like: 

(("debian" "sawfish") ("redhat" "sawmill"))

Total requirements

So totalling up the requirements,

  • A file-naming scheme, consisting of:
    • One or more STOW directories
      • Zero or more package names
        • Zero or more version names (each a directory tree)
        • :managed-by (a symlink)
        • :installed (a symlink)
        • :config (a directory tree)
        • :control
          • Zero or more package-manager names
            • Zero or more version names
        • :control-default
          • Zero or more package-manager names
        • :stowing (a symlink)
  • A format for listing required packages, not specific to one project manager.
  • stow2 must
    • Respect ../:managed-by wrt any directory it (un)stows from.
    • Impersonate a given package-manager, by filename argument, to ../:managed-by.
    • Otherwise behave like stow
  • Package managers must
    • Respect /FOO/:managed-by
    • Recognize /FOO/:control-default/PMNAME
    • Recognize /FOO/:control/PMNAME/N.N
    • Create /FOO/:control/PMNAME/N.N on request
    • Obey a tag-to-stow-directory mapping, part of its own config.
    • Understand the above required packages format
    • Obey command-line flag --wanted
  • A common means for developers and package managers alike to cause config updates. I suggest leaning on make.
  • That's all.

Footnotes:

1 I just called 100 Mb small. There was a time when we all called that "huge".

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17 September 2011

A prediction from PDM

A prediction from Parallel Dark Matter

Previously

Parallel Dark Matter is my (possibly entirely wrong) theory that dark matter is explained by 5 "parallel universes" that exchange only gravitational force with the visible universe. I presumptively call these parallel universes "branes" because they fit with braneworld cosmology.

Recent news and PDM

Recently astronomers found supermassive black holes growing in surprisingly small galaxies 1.
If PDM is true, a black hole in one brane is a black hole in all 6 branes and is fed by all 6 branes. This suggests that in the other branes the associated galaxy is bigger, by two arguments:
  • A "Where did the mass for the hole come from?" argument
  • A regression-to-means argument across branes

A prediction and possible test of PDM

So if PDM is true, we should find that these galaxies have disproportionate dark matter. If this is not true, then PDM will simply be wrong.
Quantitatively, I'd expect the amount of dark matter to be nearly in line with the size of the black holes. Ie, we should find that:
  1. The ratio of dark matter mass to black hole mass is nearly the same between these galaxies and massive early galaxies.
  2. The ratio of dark matter mass to whole galaxy mass is not at all the same between these galaxies and massive early galaxies.
But I don't understand galaxy formation well enough, so I'll play it safe and merely predict that factor (1) is more predictive of amount of dark matter than factor (2) is.
I'd also predict that, if it be established in the future that some otherwise similar galaxies have no central massive black hole, those galaxies will not have as much dark matter as these will (in ratio to galactic mass). No quantitative prediction here, just "less".

Possible confounding factors

Galaxies tend to be smaller the further back you go. As I read the article, these particular galaxies are even much smaller than the norm back then. If I misread that, then ignore this post.
These galaxies are about 10 billion years old, so the universe was about 1/4 its current age. If it were much younger, the parallel galaxies probably wouldn't have had time to form or to grow very big, and the above logic wouldn't hold (But then we'd barely be seeing visible galaxies either). AIUI 10 billion years ago is old enough.

Footnotes:

1 University of California - Santa Cruz (2011, September 16). Small distant galaxies host supermassive black holes, astronomers find. ScienceDaily. Retrieved September 17, 2011, from http://www.sciencedaily.com/releases/2011/09/110915131601.htm
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10 September 2011

Could backquote be adapted sensibly for Kernel?

Backquote vs Kernel

If you look at Kernel source, you'll quickly see constructions that a Lisper or Schemer would immediately use backquote for, like: 

(eval ($if (null?/1 bindings)
           (list* $let () body)
           (list $let
                 (list (car bindings))
                 (list* $let* (cdr bindings) body)))
      env)

Quote and backquote are discouraged in Kernel. They tend to detach symbols from the environments they pertain to. They're not needed much, because in Kernel it's usually not neccessary to pass bare symbols around unevaluated.

But backquote is a lot more convenient than managing tree structure manually. It's more WYSIWYG. It lets you read and write something like: 

(eval ($if (null?/1 bindings)
         ($` $let () . body)
         ($` $let
            (#unquote(car bindings))
            ($let* (cdr bindings) . body)))
   env)

Can we have it both ways? Yes.

Is it possible to have a variation of backquote that only exposes the sort of things manual list construction exposes, forms that don't naturally include symbols?

Yes. For instance, one could construct a form as traditional backquote does and evaluate that form in the current environment, without exposing the quoted form. The result would usually be evaluated a second time.

That gives the right result, but some pieces of the sexp get evaluated early (the low-quoted pieces), some get evaluated late (but still internally). It works but it seems dodgy.

With an equivalent result, one could say that backquote internally builds a form that, when evalled, recovers the original structure of the tree, except that low-quoted pieces are copied literally. Then it evals that form. In this formulation, there are no new quoted parts even internally, and all evaluation wrt the current environment happens in the final stage of backquote.

Any new machinery needed? Not much.

The backquote itself can be just an operative. The existing primitives supply enough functionality. The one piece of new language machinery needed is a means to introduce the low quote without causing special cases.

This rules out using a combiner, because backquote doesn't ordinarily evaluate combiners until it has settled the form structure, and low quote must be seen before that.

So low quote needs to be something that backquote understands particularly, which implies a unique object defined with backquote. But we can't convey this object by the usual means of binding it and writing its symbol. The logic is similar to the above: We'd need to use it before we evalled it and found it was special.

So we need a special way to read this object, thus the "#unquote" above. And that's about it.

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06 September 2011

Are the stances really in that order?

Previously

I'm basically continuing this from comments that John Shutt and I exchanged on The Obligational Stance. He questions whether the stances are really in that order or even in that topology.

My answer (Repeating myself from a comment)

I'm not sure I have a compelling argument that the hierarchy is in that order, or in that topology. But my thinking, albeit somewhat loose, is "How could stance N+1 mean anything if the machinery for stance N wasn't already in place?", for each of the 3 cases.

In the case of N = intentional, how could an entity see obligations if it didn't have beliefs about the world?

Trying for a counterexample

Would (say) an ATM be a counterexample? As a machine, it naturally fits the design stance and does not naturally fit the intentional stance. Yet it's "obligated" to give you your money on demand, and generally does so.

But is it really seeing an obligation? ISTM no, not any more than it has beliefs when it prints out in English "Your bank balance is X". Its designers have the beliefs and see the obligations. The ATM is competent to realize the designers' beliefs and obligations schematically, but is not competent to treat them qua beliefs and obligations. For instance, it's not capable of improvising with them or of extending them to new situations.

One can try to stretch the stances to cover it. It, like a thermostat, has tiny, rickety, mini-beliefs and mini-sees obligations in a tiny, rickety way. Can it mini-see obligations without having mini-beliefs? My intuition says no.

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