urbit/doc/spec/u3.md

u3: noun processing in C.

u3 is the C library that makes Urbit work. If it wasn't called u3, it might be called libnoun - it's a library for making and storing nouns.

What's a noun? A noun is either a cell or an atom. A cell is an ordered pair of any two nouns. An atom is an unsigned integer of any size.

To the C programmer, this is not a terribly complicated data structure, so why do you need a library for it?

One: nouns have a well-defined computation kernel, Nock, whose spec fits on a page and gzips to 340 bytes. But the only arithmetic operation in Nock is increment. So it's nontrivial to compute both efficiently and correctly.

Two: u3 is designed to support "permanent computing," ie, a single-level store which is transparently snapshotted. This implies a specialized memory-management model, etc, etc.

(Does u3 depend on the higher levels of Urbit, Arvo and Hoon? Yes and no. u3 expects you to load something shaped like an Arvo kernel, and use it as an event-processing function. But you don't need to use this feature if you don't want, and your kernel doesn't have to be Arvo proper - just Arvo-compatible. Think of u3 as the BIOS and Arvo as the boot kernel. And there are no dependencies at all between Hoon the language and u3.)

c3: C in Urbit

Under u3 is the simple c3 layer, which is just how we write C in Urbit.

When writing C in u3, please of course follow the conventions of the code around you as regards indentation, etc. It's especially important that every function have a header comment, even if it says nothing interesting.

But some of our idiosyncrasies go beyond convention. Yes, we've done awful things to C. Here's what we did and why we did.

c3: integer types

First, it's generally acknowledged that underspecified integer types are C's worst disaster. C99 fixed this, but the stdint types are wordy and annoying. We've replaced them with:

/* Good integers.
*/
  typedef uint64_t c3_d;  // double-word
  typedef int64_t c3_ds;  // signed double-word
  typedef uint32_t c3_w;  // word
  typedef int32_t c3_ws;  // signed word
  typedef uint16_t c3_s;  // short
  typedef int16_t c3_ss;  // signed short
  typedef uint8_t c3_y;   // byte
  typedef int8_t c3_ys;   // signed byte
  typedef uint8_t c3_b;   // bit

  typedef uint8_t c3_t;   // boolean
  typedef uint8_t c3_o;   // loobean
  typedef uint8_t c3_g;   // 5-bit atom for a 32-bit log.
  typedef uint32_t c3_l;  // little; 31-bit unsigned integer
  typedef uint32_t c3_m;  // mote; also c3_l; LSB first a-z 4-char string.

/* Bad integers.
*/
  typedef char      c3_c; // does not match int8_t or uint8_t
  typedef int       c3_i; // int - really bad
  typedef uintptr_t c3_p; // pointer-length uint - really really bad
  typedef intptr_t c3_ps; // pointer-length int - really really bad

Some of these need explanation. A loobean is a Nock boolean - Nock, for mysterious reasons, uses 0 as true (always say "yes") and 1 as false (always say "no").

Nock and/or Hoon cannot tell the difference between a short atom and a long one, but at the u3 level every atom under 2^31 is direct. The c3_l type is useful to annotate this. A c3_m is a mote - a string of up to 4 characters in a c3_l, least significant byte first. A c3_g should be a 5-bit atom. Of course, C cannot enforce these constraints, only document them.

Use the "bad" - ie, poorly specified - integer types only when interfacing with external code that expects them.

An enormous number of motes are defined in i/c/motes.h. There is no reason to delete motes that aren't being used, or even to modularize the definitions. Keep them alphabetical, though.

c3: variables and variable naming

The C3 style uses Hoon style TLV variable names, with a quasi Hungarian syntax. This is weird, but works really well, as long as what you're doing isn't hideously complicated. (Then it works badly, but we shouldn't need anything hideous in u3.)

A TLV variable name is a random pronounceable three-letter string, sometimes with some vague relationship to its meaning, but usually not. Usually CVC (consonant-vowel-consonant) is a good choice.

You should use TLVs much the way math people use Greek letters. The same concept should in general get the same name across different contexts. When you're working in a given area, you'll tend to remember the binding from TLV to concept by sheer power of associative memory. When you come back to it, it's not that hard to relearn. And of course, when in doubt, comment it.

Variables take pseudo-Hungarian suffixes, matching in general the suffix of the integer type:

c3_w wor_w;     //  32-bit word

Unlike in standard Hungarian, there is no change for pointer variables. C structure variables take a _u suffix.

c3: loobeans

The code (from defs.h) tells the story:

#     define c3y      0
#     define c3n      1

#     define _(x)        (c3y == (x))
#     define __(x)       ((x) ? c3y : c3n)
#     define c3a(x, y)   __(_(x) && _(y))
#     define c3o(x, y)   __(_(x) || _(y))

In short, use _() to turn a loobean into a boolean, __ to go the other way. Use ! as usual, c3y for yes and c3n for no, c3a for and and c3o for or.

u3: land of nouns

The division between c3 and u3 is that you could theoretically imagine using c3 as just a generic C environment. Anything to do with nouns is in u3.

u3: a map of the system

There are two kinds of symbols in u3: regular and irregular. Regular symbols follow this pattern:

prefix    purpose                      .h         .c
-------------------------------------------------------
u3a_      allocation                   i/n/a.h    n/a.c
u3e_      persistence                  i/n/e.h    n/e.c
u3h_      hashtables                   i/n/h.h    n/h.c
u3i_      noun construction            i/n/i.h    n/i.c
u3j_      jet control                  i/n/j.h    n/j.c
u3m_      system management            i/n/m.h    n/m.c
u3n_      nock computation             i/n/n.h    n/n.c
u3r_      noun access (error returns)  i/n/r.h    n/r.c
u3t_      profiling                    i/n/t.h    n/t.c
u3v_      arvo                         i/n/v.h    n/v.c
u3x_      noun access (error crashes)  i/n/x.h    n/x.c
u3z_      memoization                  i/n/z.h    n/z.c
u3k[a-g]  jets (transfer, C args)      i/j/k.h    j/[a-g]/*.c
u3q[a-g]  jets (retain, C args)        i/j/q.h    j/[a-g]/*.c
u3w[a-g]  jets (retain, nock core)     i/j/w.h    j/[a-g]/*.c

Irregular symbols always start with u3 and obey no other rules. They're defined in i/n/aliases.h. Finally, i/all.h includes all these headers (fast compilers, yay) and is all you need to program in u3.

u3: noun internals

A noun is a u3_noun - currently defined as a 32-bit c3_w.

If your u3_noun is less than (1 << 31), it's a direct atom. Every unsigned integer between 0 and 0x7fffffff inclusive is its own noun.

If bit 31 is set in a u3_noun and bit 30 is 1 the noun is an indirect cell. If bit 31 is set and bit 30 is 0 the noun is an indirect atom. Bits 29 through 0 are a word pointer into the loom - see below. The structures are:

typedef struct {
  c3_w mug_w;
  c3_w len_w;
  c3_w buf_w[0];    //  actually [len_w]
} u3a_atom;

typedef struct {
  c3_w    mug_w;
  u3_noun hed;
  u3_noun tel;
} u3a_cell;

The only thing that should be mysterious here is mug_w, which is a 31-bit lazily computed nonzero short hash (FNV currently, soon Murmur3). If mug_w is 0, the hash is not yet computed. We also hijack this field for various hacks, such as saving the new address of a noun when copying over.

Also, the value 0xffffffff is u3_none, which is never a valid noun. Use the type u3_weak to express that a noun variable may be u3_none.

u3: reference counts

The only really essential thing you need to know about u3 is how to handle reference counts. Everything else, you can skip and just get to work.

u3 deals with reference-counted, immutable, acyclic nouns. Unfortunately, we are not Apple and can't build reference counting into your C compiler, so you need to count by hand.

Every allocated noun (or any allocation object, because our allocator is general-purpose) contains a counter which counts the number of references to it - typically variables with type u3_noun. When this counter goes to 0, the noun is freed.

To tell u3 that you've added a reference to a noun, call the function u3a_gain() or its shorthand u3k(). (For your convenience, this function returns its argument.) To tell u3 that you've destroyed a reference, call u3a_lose() or u3z().

(If you screw up by decrementing the counter too much, u3 will dump core in horrible ways. If you screw up by incrementing it too much, u3 will leak memory. To check for memory leaks, set the bug_o flag in u3e_boot() - eg, run vere with -g. Memory leaks are difficult to debug - the best way to handle leaks is just to revert to a version that didn't have them, and look over your code again.)

(You can gain or lose a direct atom. It does nothing.)

u3: reference protocols

THIS IS THE MOST CRITICAL SECTION IN THE u3 DOCUMENTATION.

The key question when calling a C function in a refcounted world is what the function will do to the noun refcounts - and, if the function returns a noun, what it does to the return.

There are two semantic patterns, transfer and retain. In transfer semantics, the caller "gives" a use count to the callee, which "gives back" any return. For instance, if I have

{
  u3_noun foo = u3i_string("foobar");
  u3_noun bar;

  bar = u3f_futz(foo);
  [...]
  u3z(bar);
}

Suppose u3f_futz() has transfer semantics. At [...], my code holds one reference to bar and zero references to foo - which has been freed, unless it's part of bar. My code now owns bar and gets to work with it until it's done, at which point a u3z() is required.

On the other hand, if u3f_futz() has retain semantics, we need to write

{
  u3_noun foo = u3i_string("foobar");
  u3_noun bar;

  bar = u3f_futz(foo);
  [...]
  u3z(foo);
}

because calling u3f_futz() does not release our ownership of foo, which we have to free ourselves.

But if we free bar, we are making a great mistake, because our reference to it is not in any way registered in the memory manager (which cannot track references in C variables, of course). It is normal and healthy to have these uncounted C references, but they must be treated with care.

The bottom line is that it's essential for the caller to know the refcount semantics of any function which takes or returns a noun. (In some unusual circumstances, different arguments or returns in one function may be handled differently.)

Broadly speaking, as a design question, retain semantics are more appropriate for functions which inspect or query nouns. For instance, u3h() (which takes the head of a noun) retains, so that we can traverse a noun tree without constantly incrementing and decrementing.

Transfer semantics are more appropriate for functions which make nouns, which is obviously what most functions do.

In general, though, in most places it's not worth thinking about what your function does. There is a convention for it, which depends on where it is, not what it does. Follow the convention.

u3: reference conventions

The u3 convention is that, unless otherwise specified, all functions have transfer semantics - with the exception of the prefixes: u3r, u3x, u3z, u3q and u3w. Also, within jet directories a through f (but not g), internal functions retain (for historical reasons).

If functions outside this set have retain semantics, they need to be commented, both in the .h and .c file, with RETAIN in all caps. Yes, it's this important.

u3: system architecture

If you just want to tinker with some existing code, it might be enough to understand the above. If not, it's probably worth taking the time to look at u3 as a whole.

u3 is designed to work as a persistent event processor. Logically, it computes a function of the form

f(event, old state) -> (actions, new state)

Obviously almost any computing model - including, but not limited to, Urbit - can be defined in this form. To create the illusion of a computer that never loses state and never fails, we:

• log every event externally before it goes into u3
• keep a single reference to a permanent state noun.
• can abort any event without damaging the permanent state.
• snapshot the permanent state periodically, and/or prune logs.

u3: the road model

u3 uses a memory design which I'm sure someone has invented somewhere before, because it's not very clever, but I've never seen it anywhere in particular.

Every allocation starts with a solid block of memory, which u3 calls the loom. How do we allocate on the loom? You're probably familiar with the Unix heap-stack design, in which the stack grows downward and the heap (malloc arena) grows upward:

0           brk                                          ffff
|   heap     |                                    stack    |
|------------#################################+++++++++++++|
|                                             |            |
0                                             sp         ffff

A road is a normal heap-stack system, except that the heap and stack can point in either direction. Therefore, inside a road, we can nest another road in the opposite direction.

When the opposite road completes, its heap is left on top of the opposite heap's stack. It's no more than the normal behavior of a stack machine for all subcomputations to push their results on the stack.

The performance tradeoff of "leaping" - reversing directions in the road - is that if the outer computation wants to preserve the results of the inner one, not just use them for temporary purposes, it has to copy them.

This is a trivial cost in some cases, a prohibitive cost in others. The upside, of course, is that all garbage accrued in the inner computation is discarded at zero cost.

The goal of the road system is the ability to layer memory models. If you are allocating on a road, you have no idea how deep within a nested road system you are - in other words, you have no idea exactly how durable your result may be. But free space is never fragmented within a road.

Roads do not reduce the generality or performance of a memory system, since even the most complex GC system can be nested within a road at no particular loss of performance - a road is just a block of memory.

Each road (u3a_road to be exact) uses four pointers: rut is the bottom of the arena, hat the top of the arena, mat the bottom of the stack, cap the top of the stack. (Bear in mind that the road "stack" is not actually used as the C function-call stack, though it probably should be.)

A "north" road has the stack high and the heap low:

0           rut   hat                                    ffff
|            |     |                                       |
|~~~~~~~~~~~~-------##########################+++++++$~~~~~|
|                                             |      |     |
0                                            cap    mat  ffff

A "south" road is the other way around:

0           mat   cap                                    ffff
|            |     |                                       |
|~~~~~~~~~~~~$++++++##########################--------~~~~~|
|                                             |      |     |
0                                            hat    rut  ffff

Legend: - is durable storage (heap); + is temporary storage (stack); ~ is deep storage (immutable); $ is the allocation frame; # is free memory.

Pointer restrictions: pointers stored in + can point anywhere. Of course, pointing to # (free memory) would be a bug. Pointers in - can only point to - or ~; pointers in ~ only point to ~.

To "leap" is to create a new inner road in the ### free space. but in the reverse direction, so that when the inner road "falls" (terminates), its durable storage is left on the temporary storage of the outer road.

u3 keeps a global variable, u3_Road or its alias u3R, which points to the current road. (If we ever run threads in inner roads - see below - this will become a thread-local variable.) Relative to u3R, + memory is called junior memory; - memory is normal memory; ~ is senior memory.

u3: explaining the road model

But... why?

We're now ready to understand why the road system works so logically with the event and persistence model.

The key is that we don't update refcounts in senior memory. A pointer from an inner road to an outer road is not counted. Also, the outmost, or surface road, is the only part of the image that gets checkpointed.

So the surface road contains the entire durable state of u3. When we process an event, or perform any kind of complicated or interesting calculation, we process it in an inner road. If its results are saved, they need to be copied.

Since processing in an inner road does not touch surface memory, (a) we can leave the surface road in a read-only state and not mark its pages dirty; (b) we can abort an inner calculation without screwing up the surface; and (c) because inner results are copied onto the surface, the surface doesn't get fragmented.

All of (a), (b) and (c) are needed for checkpointing to be easy. It might be tractable otherwise, but easy is even better.

Moreover, while the surface is most definitely single-threaded, we could easily run multiple threads in multiple inner roads (as long as the threads don't have pointers into each others' memory, which they obviously shouldn't).

Moreover, in future, we'll experiment more with adding road control hints to the programmer's toolbox. Reference counting is expensive. We hypothesize that in many - if not most - cases, the programmer can identify procedural structures whose garbage should be discarded in one step by copying the results. Then, within the procedure, we can switch the allocator into sand mode, and stop tracking references at all.

u3: rules for C programming

There are two levels at which we program in C: (1) above the interpreter; (2) within the interpreter or jets. These have separate rules which need to be respected.

u3: rules above the interpreter

In its relations with Unix, Urbit follows a strict rule of "call me, I won't call you." We do of course call Unix system calls, but only for the purpose of actually computing.

Above Urbit, you are in a normal C/Unix programming environment and can call anything in or out of Urbit. Note that when using u3, you're always on the surface road, which is not thread-safe by default. Generally speaking, u3 is designed to support event-oriented, single-threaded programming.

If you need threads which create nouns, you could use u3m_hate() and u3m_love() to run these threads in subroads. You'd need to make the global road pointer, u3R, a thread-local variable instead. This seems perfectly practical, but we haven't done it because we haven't needed to.

u3: rules within the interpreter

Within the interpreter, your code can run either in the surface road or in a deep road. You can test this by testing

(&u3H->rod_u == u3R)

ie: does the pier's home road equal the current road pointer?

Normally in this context you assume you're obeying the rules of running on an inner road, ie, "deep memory." Remember, however, that the interpreter can run on surface memory - but anything you can do deep, you can do on the surface. The converse is by no means the case.

In deep memory, think of yourself as if in a signal handler. Your execution context is extremely fragile and may be terminated without warning or cleanup at any time (for instance, by ^C).

For instance, you can't call malloc (or C++ new) in your C code, because you don't have the right to modify data structures at the global level, and will leave them in an inconsistent state if your inner road gets terminated. (Instead, use our drop-in replacements, u3a_malloc(), u3a_free(), u3a_realloc().)

A good example is the different meaning of c3_assert() inside and outside the interpreter. At either layer, you can use regular assert(), which will just kill your process. On the surface, c3_assert() will just... kill your process.

In deep execution, c3_assert() will issue an exception that queues an error event, complete with trace stack, on the Arvo event queue. Let's see how this happens.

u3: exceptions

You produce an exception with

/* u3m_bail(): bail out.  Does not return.
**
**  Bail motes:
**
**    %exit               ::  semantic failure
**    %evil               ::  bad crypto
**    %intr               ::  interrupt
**    %fail               ::  execution failure
**    %foul               ::  assert failure
**    %need               ::  network block
**    %meme               ::  out of memory
**    %time               ::  timed out
**    %oops               ::  assertion failure
*/
  c3_i
  u3m_bail(c3_m how_m);

Broadly speaking, there are two classes of exception: internal and external. An external exception begins in a Unix signal handler. An internal exception begins with a call to longjmp() on the main thread.

There are also two kinds of exception: mild and severe. An external exception is always severe. An internal exception is normally mild, but some (like c3__oops, generated by c3_assert()) are severe.

Either way, exceptions come with a stack trace. The u3 nock interpreter is instrumented to retain stack trace hints and produce them as a printable (list tank).

Mild exceptions are caught by the first virtualization layer and returned to the caller, following the behavior of the Nock virtualizer ++mock (in hoon.hoon)

Severe exceptions, or mild exceptions at the surface, terminate the entire execution stack at any depth and send the cumulative trace back to the u3 caller.

For instance, vere uses this trace to construct a %crud event, which conveys our trace back toward the Arvo context where it crashed. This lets any UI component anywhere, even on a remote node, render the stacktrace as a consequence of the user's action - even if its its direct cause was (for instance) a Unix SIGINT or SIGALRM.

u3: C structures on the loom

Normally, all data on the loom is nouns. Sometimes we break this rule just a little, though - eg, in the u3h hashtables.

To point to non-noun C structs on the loom, we use a u3_post, which is just a loom word offset. A macro lets us declare this as if it was a pointer:

typedef c3_w       u3_post;
#define u3p(type)  u3_post

Some may regard this as clever, others as pointless. Anyway, use u3to() and u3of() to convert to and from pointers.

When using C structs on the loom - generally a bad idea - make sure anything which could be on the surface road is structurally portable, eg, won't change size when the pointer size changes. (Note also: we consider little-endian, rightly or wrongly, to have won the endian wars.)

u3: API overview by prefix

Let's run through the u3 modules one by one. All public functions are commented, but the comments may be cryptic.

u3m: main control

To start u3, run

/* u3m_boot(): start the u3 system.
*/
  void
  u3m_boot(c3_o nuu_o, c3_o bug_o, c3_c* dir_c);

nuu_o is c3y (yes, 0) if you're creating a new pier, c3n (no, 1) if you're loading an existing one. bug_o is c3y if you want to test the garbage-collector, c3n otherwise. dir_c is the directory for the pier files.

u3m_boot() expects an urbit.pill file to load the kernel from. This is specified with the -B commandline option.

Any significant computation with nouns, certainly anything Turing complete, should be run (a) virtualized and (b) in an inner road. These are slightly different things, but at the highest level we bundle them together for your convenience, in u3m_soft():

/* u3m_soft(): system soft wrapper.  unifies unix and nock errors.
**
**  Produces [%$ result] or [%error (list tank)].
*/
  u3_noun
  u3m_soft(c3_w sec_w, u3_funk fun_f, u3_noun arg);

sec_w is the number of seconds to time out the computation. fun_f is a C function accepting arg.

The result of u3m_soft() is a cell whose head is an atom. If the head is %$ - ie, 0 - the tail is the result of fun_f(arg). Otherwise, the head is a term (an atom which is an LSB first string), and the tail is a (list tank) (a list of tank printables - see ++tank in hoon.hoon). Error terms should be the same as the exception terms above.

If you're confident that your computation won't fail, you can use u3m_soft_sure(), u3m_soft_slam(), or u3m_soft_nock() for C functions, Hoon function calls, and Nock invocations. Caution - this returns just the result, and asserts globally.

All the u3m_soft functions above work only on the surface. Within the surface, virtualize with u3m_soft_run(). Note that this takes a fly (a namespace gate), thus activating the 11 super-operator in the nock virtualizer, ++mock. When actually using the fly, call u3m_soft_esc(). Don't do either unless you know what you're doing!

For descending into a subroad without Nock virtualization, use u3m_hate() and u3m_love respectively. Hating enters a subroad; loving leaves it, copying out a product noun.

Other miscellaneous tools in u3m: u3m_file() loads a Unix file as a Nock atom; u3m_water() measures the boundaries of the loom in current use (ie, watermarks); and a variety of prettyprinting routines, none perfect, are available, mainly for debugging printfs: u3m_pretty(), u3m_p(), u3m_tape() and u3m_wall().

It's sometimes nice to run a mark-and-sweep garbage collector, u3m_grab(), which collects the world from a list of roots, and asserts if it finds any leaks or incorrect refcounts. This tool is for debugging and long-term maintenance only; refcounts should never err.

u3j: jets

The jet system, u3j, is what makes u3 and nock in any sense a useful computing environment. Except perhaps u3a (there is really no such thing as a trivial allocator, though u3a is dumber than most) - u3j is the most interesting code in u3.

Let's consider the minor miracle of driver-to-battery binding which lets u3j work - and decrement not be O(n) - without violating the precisely defined semantics of pure Nock, ever.

It's easy to assume that jets represent an architectural coupling between Hoon language semantics and Nock interpreter internals. Indeed such a coupling would be wholly wrongtious and un-Urbit. But the jet system is not Hoon-specific. It is specific to nock runtime systems that use a design pattern we call a core.

u3j: core structure

A core is no more than a cell [code data], in which a code is either a Nock formula or a cell of codes, and data is anything. In a proper core, the subject each formula expects is the core itself.

Except for the arbitrary decision to make a core [code data], (or as we sometimes say, [battery payload]), instead of [data code], any high-level language transforming itself to Nock would use this design.

So jets are in fact fully general. Broadly speaking, the jet system works by matching a C driver to a battery. When the battery is invoked with Nock operator 9, it must be found in associative memory and linked to its driver. Then we link the formula axis of the operation (a in [9 a b]) to a specific function in the driver.

To validate this jet binding, we need to know two things. One, we need to know the C function actually is a perfect semantic match for the Nock formula. This can be developed with driver test flags, which work, and locked down with a secure formula hash in the driver, which we haven't bothered with just yet. (You could also try to develop a formal method for verifying that C functions and Nock formulas are equivalent, but this is a research problem for the future.)

Two, we need to validate that the payload is appropriate for the battery. We should note that jets are a Nock feature and have no reference to Hoon. A driver which relies on the Hoon type system to only pair it with valid payloads is a broken driver, and breaks the Nock compliance of the system as a whole. So don't.

Now, a casual observer might look at [battery payload] and expect the simplest case of it to be [formula subject]. That is: to execute a simple core whose battery is a single formula, we compute

nock(+.a -.a)

Then, naturally, when we go from Hoon or a high-level language containing functions down to Nock, [function arguments] turns into [formula subject]. This seems like an obvious design, and we mention it only because it is completely wrong.

Rather, to execute a one-armed core like the above, we run

nock(a -.a)

and the normal structure of a gate, which is simply Urbitese for "function," is:

[formula [sample context]]

where sample is Urbitese for "arguments" - and context, any Lisper will at once recognize, is Urbitese for "environment."

To slam or call the gate, we simply replace the default sample with the caller's data, then nock the formula on the entire gate.

What's in the context? Unlike in most dynamic languages, it is not some secret system-level bag of tricks. Almost always it is another core. This onion continues until at the bottom, there is an atomic constant, conventionally is the kernel version number.

Thus a (highly desirable) static core is one of the form

[battery constant]
[battery static-core]

ie, a solid stack of nested libraries without any dynamic data. The typical gate will thus be, for example,

[formula [sample [battery battery battery constant]]]

but we would be most foolish to restrict the jet mechanism to cores of this particular structure. We cannot constrain a payload to be [sample static-core], or even [sample core]. Any such constraint would not be rich enough to handle Hoon, let alone other languages.

u3j: jet state

There are two fundamental rules of computer science: (1) every system is best understood through its state; (2) less state is better than more state. Sadly, a pier has three different jet state systems: cold, warm and hot. It needs all of them.

Hot state is associated with this particular Unix process. The persistent pier is portable not just between process and process, but machine and machine or OS and OS. The set of jets loaded into a pier may itself change (in theory, though not in the present implementation) during the lifetime of the process. Hot state is a pure C data structure.

Cold state is associated with the logical execution history of the pier. It consists entirely of nouns and ignores restarts.

Warm state contains all dependencies between cold and hot state. It consists of C structures allocated on the loom.

Warm state is purely a function of cold and hot states, and we can wipe and regenerate it at any time. On any restart where the hot state might have changed, we clear the warm state with u3j_ream().

There is only one hot state, the global jet dashboard u3j_Dash or u3D for short. In the present implementation, u3D is a static structure not modified at runtime, except for numbering itself on process initialization. This structure - which embeds function pointers to all the jets - is defined in j/tree.c. The data structures:

/* u3j_harm: driver arm.
*/
  typedef struct _u3j_harm {
    c3_c*               fcs_c;            //  `.axe` or name
    u3_noun           (*fun_f)(u3_noun);  //  compute or 0 / semitransfer
    c3_o                ice;              //  perfect (don't test)
    c3_o                tot;              //  total (never punts)
    c3_o                liv;              //  live (enabled)
  } u3j_harm;

/* u3j_core: C core driver.
*/
  typedef struct _u3j_core {
    c3_c*             cos_c;              //  control string
    struct _u3j_harm* arm_u;              //  blank-terminated static list
    struct _u3j_core* dev_u;              //  blank-terminated static list
    struct _u3j_core* par_u;              //  dynamic parent pointer
    c3_l              jax_l;              //  dynamic jet index
  } u3j_core;

/* u3e_dash, u3_Dash, u3D: jet dashboard singleton
*/
  typedef struct _u3e_dash {
    u3j_core* dev_u;                      //  null-terminated static list
    c3_l      len_l;                      //  ray_u filled length
    c3_l      all_l;                      //  ray_u allocated length
    u3j_core* ray_u;                      //  dynamic driver array
  } u3j_dash;

Warm and cold state is per road. In other words, as we nest roads, we also nest jet state. The jet state in the road is:

  struct {                                //  jet dashboard
    u3p(u3h_root) har_p;                  //  warm state
    u3_noun       das;                    //  cold state
  } jed;

In case you understand Hoon, das (cold state) is a ++dash, and har_p (warm state) is a map from battery to ++calx:

++  bane  ,@tas                                 ::  battery name
++  bash  ,@uvH                                 ::  label hash
++  bosh  ,@uvH                                 ::  local battery hash
++  batt  ,*                                    ::  battery
++  calf                                        ::
  $:  jax=,@ud                                  ::  hot core index
      hap=(map ,@ud ,@ud)                       ::  axis/hot arm index
      lab=path                                  ::  label as path
      jit=*                                     ::  arbitrary data
  ==                                            ::
++  calx  (trel calf (pair bash cope) club)     ::  cached by battery
++  clog  (pair cope (map batt club))           ::  identity record
++  club  (pair corp (map term nock))           ::  battery pattern
++  cope  (trel bane axis (each bash noun))     ::  core pattern
++  core  ,*                                    ::  core
++  corp  (each core batt)                      ::  parent or static
++  dash  (map bash clog)                       ::  jet system

The driver index jax in a ++calx is an index into ray_u in the dashboard - ie, a pointer into hot state. This is why the warm state has to be reset when we reload the pier in a new process.

Why is jet state nested? Nock of course is a functional system, so as we compute we don't explicitly create state. Jet state is an exception to this principle (which works only because it can't be semantically detected from Nock/Hoon) - but it can't violate the fundamental rules of the allocation system.

For instance, when we're on an inner road, we can't allocate on an outer road, or point from an outer road to an inner. So if we learn something - like a mapping from battery to jet - in the inner road, we have to keep it in the inner road.

Mitigating this problem, when we leave an inner road (with u3m_love()), we call u3j_reap() to promote jet information in the dying road. Reaping promotes anything we've learned about any battery that either (a) already existed in the outer road, or (b) is being saved to the outer road.

u3j: jet binding

Jet binding starts with a %fast hint. (In Hoon, this is produced by the runes ~%, for the general case, or ~/ for simple functions.) To bind a jet, execute a formula of the form:

[10 [%fast clue-formula] core-formula]

core-formula assembles the core to be jet-propelled. clue-formula produces the hint information, or ++clue above, which we want to annotate it with.

A clue is a triple of name, parent, and hooks:

++  clue  (trel chum nock (list (pair term nock)))

The name, or ++chum, has a bunch of historical structure which we don't need (cleaning these things up is tricky), but just gets flattened into a term.

The parent axis is a nock formula, but always reduces to a simple axis, which is the address of this core's parent. Consider again an ordinary gate

[formula [sample context]]

Typically the context is itself a library core, which itself has a jet binding. If so, the parent axis of this gate is 7.

If the parent is already bound - and the parent must be already bound, in this road or a road containing it - we can hook this core bottom-up into a tree hierarchy. Normally the child core is produced by an arm of the parent core, so this is not a problem - we wouldn't have the child if we hadn't already made the parent.

The clue also contains a list of hooks, named nock formulas on the core. Usually these are arms, but they need not be. The point is that we often want to call a core from C, in a situation where we have no type or other source information. A common case of this is a complex system in which we're mixing functions which are jet-propelled with functions that aren't.

In any case, all the information in the %fast hint goes to u3j_mine(), which registers the battery in cold state (das in jed in u3R), then warm state (har_p in jed).

It's essential to understand that the %fast hint has to be, well, fast - because we apply it whenever we build a core. For instance, if the core is a Hoon gate - a function - we will call u3j_mine every time the function is called.

u3j: the cold jet dashboard

For even more fun, the jet tree is not actually a tree of batteries. It's a tree of battery labels, where a label is an [axis term] path from the root of the tree. (At the root, if the core pattern is always followed properly, is a core whose payload is an atomic constant, conventionally the Hoon version.)

Under each of these labels, it's normal to have an arbitrary number of different Nock batteries (not just multiple copies of the same noun, a situation we do strive to avoid). For instance, one might be compiled with debugging hints, one not.

We might even have changed the semantics of the battery without changing the label - so long as those semantics don't invalidate any attached driver.

et tree. For instance, it's normal to have two equivalent Nock batteries at the same time in one pier: one battery compiled with debugging hints, one not.

Rather, the jet tree is a semantic hierarchy. The root of the hierarchy is a constant, by convention the Hoon kernel version because any normal jet-propelled core has, at the bottom of its onion of libraries, the standard kernel. Thus if the core is

[foo-battery [bar-battery [moo-battery 164]]]

we can reverse the nesting to construct a hierarchical core path. The static core

164/moo/bar/foo

extends the static core 164/moo/bar by wrapping the foo battery (ie, in Hoon, |%) around it. With the core above, you can compute foo stuff, bar stuff, and moo stuff. Rocket science, not.

Not all cores are static, of course - they may contain live data, like the sample in a gate (ie, argument to a function). Once again, it's important to remember that we track jet bindings not by the core, which may not be static, but by the battery, which is always static.

(And if you're wondering how we can use a deep noun like a Nock formula or battery as a key in a key-value table, remember mug_w, the lazily computed short hash, in all boxed nouns.)

In any case, das, the dashboard, is a map from bash to jet location record (++clog). A clog in turn contains two kinds of information: the ++cope, or per-location noun; and a map of batteries to a per-battery ++club.

The cope is a triple of ++bane (battery name, right now just a term); ++axis, the axis, within this core, of the parent; and (each bash noun), which is either [0 bash] if the parent is another core, or [1 noun], for the constant noun (like 164) if there is no parent core.

A bash is just the noun hash (++sham) of a cope, which uniquely expresses the battery's hierarchical location without depending on the actual formulas.

The club contains a ++corp, which we use to actually validate the core. Obviously jet execution has to be perfectly compatible with Nock. We search on the battery, but getting the battery right is not enough - a typical battery is dependent on its context. For example, your jet-propelled library function is very likely to call ++dec or other advanced kernel technology. If you've replaced the kernel in your context with something else, we need to detect this and not run the jet.

There are two cases for a jet-propelled core - either the entire core is a static constant, or it isn't. Hence the definition of corp:

++  corp  (each core batt)                ::  parent or static

Ie, a corp is [0 core] or [1 batt]. If it's static - meaning that the jet only works with one specific core, ie, the parent axis of each location in the hierarchy is 3 - we can validate with a single comparison. Otherwise, we have to recurse upward by checking the parent.

Note that there is at present no way to force a jet to depend on static data.

u3j: the warm jet dashboard

We don't use the cold state to match jets as we call them. We use the cold state to register jets as we find them, and also to rebuild the warm state after the hot state is reset.

What we actually use at runtime is the warm state, jed->har_p, which is a u3h (built-in hashtable), allocated on the loom, from battery to ++calx.

A calx is a triple of a ++calf, a [bash cope] cell, and a club. The latter two are all straight from cold state.

The calf contains warm data dependent on hot state. It's a quadruple: of jax, the hot driver index (in ray_u in u3j_dash); hap, a table from arm axis (ie, the axis of each formula within the battery) to driver arm index (into arm_u in u3j_core); lab, the complete label path; and jit, any other dynamic data that may speed up execution.

We construct hap, when we create the calx, by iterating through the arms registered in the u3j_core. Note the way a u3j_harm declares itself, with the string fcs_c which can contain either an axis or a name. Most jetted cores are of course gates, which have one formula at one axis within the core: fcs_c is ".3".

But we do often have fast cores with more complex arm structure, and it would be sad to have to manage their axes by hand. To use an fcs_c with a named arm, it's sufficient to make sure the name is bound to a formula [0 axis] in the hook table.

jit, as its name suggests, is a stub where any sort of optimization data computed on battery registration might go. To use it, fill in the _cj_jit() function.

u3j: the hot dashboard

Now it should be easy to see how we actually invoke jets. Every time we run a nock 9 instruction (pretty often, obviously), we have a core and an axis. We pass these to u3j_kick(), which will try to execute them.

Because nouns with a reference count of 1 are precious, u3j_kick() has a tricky reference control definition. It reserves the right to return u3_none in the case where there is no driver, or the driver does not apply for this case; in this case, it retains argument cor. If it succeeds, though, it transfers cor.

u3j_kick() searches for the battery (always the head of the core, of course) in the hot dashboard. If the battery is registered, it searches for the axis in hap in the calx. If it exists, the core matches a driver and the driver jets this arm. If not, we return u3_none.

Otherwise, we call fun_f in our u3j_harm. This obeys the same protocol as u3j_kick(); it can refuse to function by returning u3_none, or consume the noun.

Besides the actual function pointer fun_f, we have some flags in the u3j_harm which tell us how to call the arm function.

If ice is yes (&, 0), the jet is known to be perfect and we can just trust the product of fun_f. Otherwise, we need to run both the Nock arm and fun_f, and compare their results.

(Note that while executing the C side of this test, we have to set ice to yes; on the Nock side, we have to set liv to no. Otherwise, many non-exponential functions become exponential. When auto-testing jets in this way, the principle is that the test is on the outermost layer of recursion.)

(Note also that anyone who multi-threads this execution environment has a slight locking problem with these flags if arm testing is multi-threaded.)

If tot is yes, (&, 0), the arm function is total and has to return properly (though it can still return u3_none). Otherwise, it is partial and can u3_cm_bail() out with c3__punt. This feature has a cost: the jet runs in a subroad.

Finally, if liv is no (|, 1), the jet is off and doesn't run.

It should be easy to see how the tree of cores gets declared - precisely, in j/dash.c. We declare the hierarchy as a tree of u3j_core structures, each of which comes with a static list of arms arm_u and sub-cores dev_u.

In u3j_boot(), we traverse the hierarchy, fill in parent pointers par_u, and enumerate all u3j_core structures into a single flat array u3j_dash.ray_u. Our hot state then appears ready for action.

u3j: jet functions

At present, all drivers are compiled statically into u3. This is not a long-term permanent solution or anything. However, it will always be the case with a certain amount of core functionality.

For instance, there are some jet functions that we need to call as part of loading the Arvo kernel - like ++cue to unpack a noun from an atom. And obviously it makes sense, when jets are significant enough to compile into u3, to export their symbols in headers and the linker.

There are three interface prefixes for standard jet functions: u3k, u3q, and u3w. All jets have u3w interfaces; most have u3q; some have u3k. Of course the actual logic is shared.

u3w interfaces use the same protocol as fun_f above: the caller passes the entire core, which is retained if the function returns u3_none, transferred otherwise. Why? Again, use counts of 1 are special and precious for performance hackers.

u3q interfaces break the core into C arguments, retain noun arguments, and transfer noun returns. u3k interfaces are the same, except with more use of u3_none and other simple C variations on the Hoon original, but transfer both arguments and returns. Generally, u3k are most convenient for new code.

Following u3k/q/w is [a-f], corresponding to the 6 logical tiers of the kernel, or g for user-level jets. Another letter is added for functions within subcores. The filename, under j/, follows the tier and the function name.

For instance, ++add is u3wa_add(cor), u3qa_add(a, b), or u3ka_add(a, b), in j/a/add.c. ++get in ++by is u3wdb_get(cor), u3kdb_get(a, b), etc, in j/d/by_get.c.

For historical reasons, all internal jet code in j/[a-f] retains noun arguments, and transfers noun results. Please do not do this in new g jets! The new standard protocol is to transfer both arguments and results.

u3a: allocation functions

u3a allocates on the current road (u3R). Its internal structures are uninteresting and typical of a naive allocator.

The two most-used u3a functions are u3a_gain() to add a reference count, and u3a_lose() to release one (and free the noun, if the use count is zero). For convenience, u3a_gain() returns its argument. The pair are generally abbreviated with the macros u3k() and u3z() respectively.

Normally we create nouns through u3i functions, and don't call the u3a allocators directly. But if you do:

One, there are two sets of allocators: the word-aligned allocators and the fully-aligned (ie, malloc compatible) allocators. For instance, on a typical OS X setup, malloc produces 16-byte aligned results - needed for some SSE instructions.

These allocators are not compatible. For 32-bit alignment as used in nouns, call

/* u3a_walloc(): allocate storage measured in words.
*/
  void*
  u3a_walloc(c3_w len_w);

/* u3a_wfree(): free storage.
*/
  void
  u3a_wfree(void* lag_v);

/* u3a_wealloc(): word realloc.
*/
  void*
  u3a_wealloc(void* lag_v, c3_w len_w);

For full alignment, call:

/* u3a_malloc(): aligned storage measured in bytes.
*/
  void*
  u3a_malloc(size_t len_i);

/* u3a_realloc(): aligned realloc in bytes.
*/
  void*
  u3a_realloc(void* lag_v, size_t len_i);

/* u3a_realloc2(): gmp-shaped realloc.
*/
  void*
  u3a_realloc2(void* lag_v, size_t old_i, size_t new_i);

/* u3a_free(): free for aligned malloc.
*/
  void
  u3a_free(void* tox_v);

/* u3a_free2(): gmp-shaped free.
*/
  void
  u3a_free2(void* tox_v, size_t siz_i);

There are also a set of special-purpose allocators for building atoms. When building atoms, please remember that it's incorrect to have a high 0 word - the word length in the atom structure must be strictly correct.

Of course, we don't always know how large our atom will be. Therefore, the standard way of building large atoms is to allocate a block of raw space with u3a_slab(), then chop off the end with u3a_malt() (which does the measuring itself) or u3a_mint() in case you've measured it yourself.

Once again, do not call malloc() (or C++ new) within any code that may be run within a jet. This will cause rare sporadic corruption when we interrupt execution within a malloc(). We'd just override the symbol, but libuv uses malloc() across threads within its own synchronization primitives - for this to work with u3a_malloc(), we'd have to introduce our own locks on the surface-level road (which might be a viable solution).

u3n: nock execution

The u3n routines execute Nock itself. On the inside, they have a surprising resemblance to the spec proper (the only interesting detail is how we handle tail-call elimination) and are, as one would expect, quite slow. (There is no such thing as a fast tree interpreter.)

There is only one Nock, but there are lots of ways to call it. (Remember that all u3n functions transfer C arguments and returns.)

The simplest interpreter, u3n_nock_on(u3_noun bus, u3_noun fol) invokes Nock on bus (the subject) and fol (the formula). (Why is it[subject formula], not [formula subject]? The same reason 0 is true and 1 is false.)

A close relative is u3n_slam_on(u3_noun gat, u3_noun sam), which slams a gate (gat) on a sample (sam). (In a normal programming language which didn't talk funny and was retarded, u3n_slam_on() would call a function on an argument.) We could write it most simply as:

u3_noun
u3n_slam_on(u3_noun gat, u3_noun sam)
{
  u3_noun pro = u3n_nock_on
                  (u3nc(u3k(u3h(gat)),
                        u3nc(sam, u3k(u3t(u3t(gat))))),
                   u3k(u3h(gat)));
  u3z(gat);
  return pro;
}

Simpler is u3n_kick_on(u3_noun gat), which slams a gate (or, more generally, a trap - because sample structure is not even needed here) without changing its sample:

u3_noun
u3n_kick_on(u3_noun gat, u3_noun sam)
{
  return u3n_nock_on(gat, u3k(u3h(gat)));
}

The _on functions in u3n are all defined as pure Nock. But actually, even though we say we don't extend Nock, we do. But we don't. But we do.

Note that u3 has a well-developed error handling system - u3m_bail() to throw an exception, u3m_soft_* to catch one. But Nock has no exception model at all. That's okay - all it means if that if an _on function bails, the exception is an exception in the caller.

However, u3's exception handling happens to match a convenient virtual super-Nock in hoon.hoon, the infamous ++mock. Of course, Nock is slow, and mock is Nock in Nock, so it is (logically) super-slow. Then again, so is decrement.

With the power of u3, we nest arbitrary layers of mock without any particular performance cost. Moreover, we simply treat Nock proper as a special case of mock. (More precisely, the internal VM loop is ++mink and the error compiler is ++mook. But we call the whole sandbox system mock.)

The nice thing about mock functions is that (by executing within u3m_soft_run(), which as you may recall uses a nested road) they provide both exceptions and the namespace operator - .^ in Hoon, which becomes operator 11 in mock.

11 requires a namespace function, or fly, which produces a ++unit - ~ (0) for no binding, or [0 value]. The sample to a fly is a ++path, just a list of text span.

mock functions produce a ++toon. Fully elaborated:

++  noun  ,*                                      ::  any noun
++  path  (list ,@ta)                             ::  namespace path
++  span  ,@ta                                    ::  text-atom (ASCII)
++  toon  $%  [%0 p=noun]                         ::  success
              [%1 p=(list path)]                  ::  blocking paths
              [%2 p=(list tank)]                  ::  stack trace
          ==                                      ::
++  tank                                          ::  printable
          $%  [%leaf p=tape]                      ::  flat text
              $:  %palm                           ::  backstep list
                  p=[p=tape q=tape r=tape s=tape] ::  mid cap open close
                  q=(list tank)                   ::  contents
              ==                                  ::
              $:  %rose                           ::  straight list
                  p=[p=tape q=tape r=tape]        ::  mid open close
                  q=(list tank)                   ::  contents
              ==                                  ::
          ==

(Note that tank is overdesigned and due for replacement.)

What does a toon mean? Either your computation succeded ([0 noun], or could not finish because it blocked on one or more global paths ([1 (list path)]), or it exited with a stack trace ([2 (list tank)]).

Note that of all the u3 exceptions, only %exit is produced deterministically by the Nock definition. Therefore, only %exit produces a 2 result. Any other argument to u3m_bail() will unwind the virtualization stack all the way to the top - or to be more exact, to u3m_soft_top().

In any case, the simplest mock functions are u3n_nock_un() and u3n_slam_un(). These provide exception control without any namespace change, as you can see by the code:

/* u3n_nock_un(): produce .*(bus fol), as ++toon.
*/
u3_noun
u3n_nock_un(u3_noun bus, u3_noun fol)
{
  u3_noun fly = u3nt(u3nt(11, 0, 6), 0, 0);  //  |=(a=* .^(a))

  return u3n_nock_in(fly, bus, fol);
}

/* u3n_slam_un(): produce (gat sam), as ++toon.
*/
u3_noun
u3n_slam_un(u3_noun gat, u3_noun sam)
{
  u3_noun fly = u3nt(u3nt(11, 0, 6), 0, 0);  //  |=(a=* .^(a))

  return u3n_slam_in(fly, gat, sam);
}

The fly is added as the first argument to u3n_nock_in() and u3n_slam_in(). Of course, logically, fly executes in the caller's exception layer. (Maintaining this illusion is slightly nontrivial.) Finally, u3n_nock_an() is a sandbox with a null namespace.

u3e: persistence

The only u3e function you should need to call is u3e_save(), which saves the loom. As it can be restored on any platform, please make sure you don't have any state in the loom that is bound to your process or architecture - except for exceptions like the warm jet state, which is actively purged on reboot.

u3r: reading nouns (weak)

As befits accessors they don't make anything, u3r noun reading functions always retain their arguments and their returns. They never bail; rather, when they don't work, they return a u3_weak result.

Most of these functions are straightforward and do only what their comments say. A few are interesting enough to discuss.

u3r_at() is the familiar tree fragment function, / from the Nock spec. For taking complex nouns apart, u3r_mean() is a relatively funky way of deconstructing nouns with a varargs list of axis, u3_noun *. For cells, triples, etc, decompose with u3r_cell(), u3r_trel(), etc. For the tagged equivalents, use u3r_pq() and friends.

u3r_sing(u3_noun a, u3_noun b) (true if a and b are a single noun) are interesting because it uses mugs to help it out. Clearly, different nouns may have the same mug, but the same nouns cannot have a different mug. It's important to understand the performance characteristics of u3r_sing(): the worst possible case is a comparison of duplicate nouns, which have the same value but were created separately. In this case, the tree is traversed

u3r_sung() is a deeply funky and frightening version of u3r_sing() that unifies pointers to the duplicate nouns it finds, freeing the second copy. Obviously, do not use u3r_sung() when you have live, but not reference counted, noun references from C - if they match a noun with a refcount of 1 that gets freed, bad things happen.

It's important to remember that u3r_mug(), which produces a 31-bit, nonzero insecure hash, uses the mug_w slot in any boxed noun as a lazy cache. There are a number of variants of u3r_mug() that can get you out of building unneeded nouns.

u3x: reading nouns (bail)

u3x functions are like u3r functions, but instead of returning u3_none when (for instance) we try to take the head of an atom, they bail with %exit. In other words, they do what the same operation would do in Nock.

u3h: hash tables.

We can of course use the Hoon map structure as an associative array. This is a balanced treap and reasonably fast. However, it's considerably inferior to a custom structure like an HAMT (hash array-mapped trie). We use u3_post to allocate HAMT structures on the loom.

(Our HAMT implements the classic Bagwell algorithm which depends on the gcc standard directive __builtin_popcount(). On a CPU which doesn't support popcount or an equivalent instruction, some other design would probably be preferable.)

There's no particular rocket science in the API. u3h_new() creates a hashtable; u3h_free() destroys one; u3h_put() inserts, u3h_get() retrieves. You can transform values in a hashtable with u3h_walk().

The only funky function is u3h_gut(), which unifies keys with u3r_sung(). As with all cases of u3r_sung(), this must be used with extreme caution.

u3z: memoization

Connected to the ~+ rune in Hoon, via the Nock %memo hint, the memoization facility is a general-purpose cache.

(It's also used for partial memoization - a feature that'll probably be removed, in which conservative worklist algorithms (which would otherwise be exponential) memoize everything in the subject except the worklist. This is used heavily in the Hoon compiler jets (j/f/*.c). Unfortunately, it's probably not possible to make this work perfectly in that it can't be abused to violate Nock, so we'll probably remove it at a later date, instead making ++ut keep its own monadic cache.)

Each u3z function comes with a c3_m mote which disambiguates the function mapping key to value. For Nock itself, use 0. For extra speed, small tuples are split out in C; thus, find with

u3_weak u3z_find(c3_m, u3_noun);
u3_weak u3z_find_2(c3_m, u3_noun, u3_noun);
u3_weak u3z_find_3(c3_m, u3_noun, u3_noun, u3_noun);
u3_weak u3z_find_4(c3_m, u3_noun, u3_noun, u3_noun, u3_noun);

and save with

u3_noun u3z_save(c3_m, u3_noun, u3_noun);
u3_noun u3z_save_2(c3_m, u3_noun, u3_noun, u3_noun);
u3_noun u3z_save_3(c3_m, u3_noun, u3_noun, u3_noun, u3_noun);
u3_noun u3z_save_4(c3_m, u3_noun, u3_noun, u3_noun, u3_noun, u3_noun);

where the value is the last argument. To eliminate duplicate nouns, there is also

u3_noun
u3z_uniq(u3_noun);

u3z functions retain keys and transfer values.

The u3z cache, built on u3h hashes, is part of the current road, and goes away when it goes away. (In future, we may wish to promote keys/values which outlive the road, as we do with jet state.) There is no cache reclamation at present, so be careful.

u3t: tracing and profiling.

TBD.

u3v: the Arvo kernel

An Arvo kernel - or at least, a core that compiles with the Arvo interface - is part of the global u3 state. What is an Arvo core? Slightly pseudocoded:

++  arvo
  |%
  ++  come  |=  [yen=@ ova=(list ovum) nyf=pone]  ::  11
            ^-  [(list ovum) _+>]
            !!
  ++  keep  |=  [now=@da hap=path]                ::  4
            ^-  (unit ,@da)
            !!
  ++  load  |=  [yen=@ ova=(list ovum) nyf=pane]  ::  86
            ^-  [(list ovum) _+>]
            !!
  ++  peek  |=  [now=@da path]                    ::  87
            ^-  (unit)
            !!
  ++  poke  |=  [now=@da ovo=ovum]                ::  42
            ^-  [(list ovum) _+>]
            !!
  ++  wish  |=  txt=@ta                           ::  20
            ^-  *
            !!
  --
++  card  ,[p=@tas q=*]                           ::  typeless card
++  ovum  ,[p=wire q=card]                        ::  Arvo event
++  wire  path                                    ::  event cause

This is the Arvo ABI in a very real sense. Arvo is a core with these six arms. To use these arms, we hardcode the axis of the formula (11, 4, 86, etc) into the C code that calls Arvo, because otherwise we'd need type metadata - which we can get, by calling Arvo.

It's important to understand the Arvo event/action structure, or ++ovum. An ovum is a card, which is any [term noun] cell, and a ++wire, a path which indicates the location of the event. At the Unix level, the wire corresponds to a system module or context. For input events, this is the module that caused the event; for output actions, it's the module that performs the action.

++poke sends Arvo an event ovum, producing a cell of action ova and a new Arvo core.

++peek dereferences the Arvo namespace. It takes a date and a key, and produces ~ (0) or [~ value].

++keep asks Arvo the next time it wants to be woken up, for the given wire. (This input will probably be eliminated in favor of a single global timer.)

++wish compiles a string of Hoon source. While just a convenience, it's a very convenient convenience.

++come and ++load are used by Arvo to reset itself (more precisely, to shift the Arvo state from an old kernel to a new one); there is no need to call them from C.

Now that we understand the Arvo kernel interface, let's look at the u3v API. As usual, all the functions in u3v are commented, but unfortunately it's hard to describe this API as clean at present. The problem is that u3v remains design coupled to the old vere event handling code written for u2. But let's describe the functions you should be calling, assuming you're not writing the next event system. There are only two.

u3v_wish(str_c) wraps the ++wish functionality in a cache (which is read-only unless you're on the surface road).

u3v_do() uses wish to provide a convenient interface for calling Hoon kernel functions by name. Even more conveniently, we tend to call u3v_do() with these convenient aliases:

#define  u3do(txt_c, arg)         u3v_do(txt_c, arg)
#define  u3dc(txt_c, a, b)        u3v_do(txt_c, u3nc(a, b))
#define  u3dt(txt_c, a, b, c)     u3v_do(txt_c, u3nt(a, b, c))
#define  u3dq(txt_c, a, b, c, d)  u3v_do(txt_c, u3nt(a, b, c, d))