andrew/akbasic
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Document structures: a chapter, the architecture, and the differences

Browse Source 
docs/16-structures.md is the feature: records, nesting, copy-on-assign, strict
pointers, lists, what is checked and what is not, and how a host shares its own
C structs. Every example in it is executed by docs_examples and byte-compared,
including the refusals -- so a message that changes fails the suite rather than
quietly making the chapter wrong.

The chapter makes one contrast explicitly, because it is the question a reader
will actually have: a misspelled *field* is refused and a misspelled *variable*
still prints zero. The rule underneath is that what the program declared gets
checked and what it did not gets shrugged at -- a variable's name is never
declared, a TYPE's field list is. Structures end up the strictest thing in the
language, not from a higher standard but because they are the only named thing
whose valid spellings are written down.

Chapter 14 gains the layout: an instance is a contiguous run of value slots with
a diagram of where the fields sit, the three-pass prescan and why each pass
exists, why the copy cannot live in akbasic_value_clone(), and why the render
depth bound is four rather than eight. Chapter 3 gains the @ suffix, chapter 13
records that all of this is an addition BASIC 7.0 has nothing like, and the verb
reference gains TYPE, POINT and DIM ... AS.

MAINTENANCE.md gains the two rules that are on a maintainer rather than on a
test: a structure copy must not go through clone, and a field chain gets its own
leaf field. TODO.md section 5 records what was invented and the three limits
that are ours, and section 8 records the two defects the work exposed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Andrew Kesterson
2026-08-01 12:03:02 -04:00
parent 631c70ce7d
commit 6bac929901
8 changed files with 484 additions and 6 deletions
Download Patch File Download Diff File Expand all files Collapse all files

21
MAINTENANCE.md
View File

@@ -497,6 +497,27 @@ So: still do it during single-threaded init, before the host game spawns anythin
because the call would race, but because there is one operation in the registry that cannot be
made safe and this is the discipline that avoids needing it.
### Structures reuse the array machinery, deliberately
`TYPE` declares a record, so an instance has a known slot count and takes one contiguous
run from the same value pool `DIM A#(10)` draws from. **There is no structure pool**, and
adding one would be the wrong instinct: the only new table holds *descriptors* — names and
slot offsets — and the data goes where array data already goes.
Two rules for changing any of it:
- **A structure copy must not go through `akbasic_value_clone()`.** Clone copies one slot,
and one slot holds a *reference* to an instance rather than the instance, so a structure
taking that path aliases instead of copying — which is the semantics the language
deliberately does not have. `akbasic_environment_assign()` intercepts first and calls
`akbasic_struct_copy()`. If you add a place a structure can be assigned, it goes through
that, not through clone.
- **A field chain gets its own leaf type and its own link field.** `include/akbasic/grammar.h`
records three separate defects that came from giving one link field two meanings;
`AKBASIC_LEAF_FIELD` keeps its base on `.left`, which nothing else on that leaf type uses.
`docs/14-architecture.md` has the layout diagram and the three-pass prescan.
### Nothing calls malloc
`libakgl`'s hard rule, and ours: obtain objects from `akgl_heap_next_*` and release them back,

76
TODO.md
View File

@@ -1021,6 +1021,52 @@ deviations from the reference's *program*: `main.go` and the SDL half of
### Deviations in the structure and error verbs (groups A and C)
61. **Structures are an addition, not a port.** BASIC 7.0 has no records at all, so
`TYPE`/`END TYPE`, `DIM X@ AS T`, `.` and `->`, `PTR TO` and `POINT ... AT` are
invented here. The `@` suffix was not: the Go reference reserved
`IDENTIFIER_STRUCT` and never used it, and `src/grammar.c` rendered such a leaf as
`"NOT IMPLEMENTED"` until this landed. `docs/16-structures.md` is the whole feature.
**Assignment copies and `POINT` shares**, which is the decision everything else rests
on. A structure is a value like every other value here, so a program that never writes
`POINT` can never be surprised by aliasing — and the two field operators are kept
apart so a reader always knows from the spelling which they are looking at. `.` on a
pointer and `->` on a value are both errors, each naming the other.
**A declared type is what buys the storage model.** An instance has a known slot count
and is laid out exactly as an array is, out of the same value pool, so nothing new
holds data — only a table of descriptors. It also bounds copy depth statically, since a
`TYPE` cannot contain itself by value.
**Three limits are ours and are stated rather than derived:** 16 types, 16 fields per
type, and four levels of nesting in `PRINT`. The last is not a safety bound on copying —
copy stops at pointers by construction — but on *rendering*, which must follow a
pointer and would not come back on a cycle. Four rather than eight because the bound
has to bite before the 256-byte render buffer does, or a cycle stops because it ran out
of room rather than because it was told to.
**A type name shares a namespace with verbs and labels**, since all three are bare
words. Refused at declaration with a message that says so, because the parser's own
answer was "Expected expression or literal" pointing at the line rather than the
problem. Field names follow the same reserved-word rule variable names already do,
enforced by the loader's scan — `TO@` is a bad field name for exactly the reason `TO#`
is a bad variable name.
62. **A host's C struct is the same thing with its bytes somewhere else.**
`akbasic_host_register_type()` puts it in the *same* table a `TYPE` fills, so copy,
`PTR TO`, `.` and `->` all work across the boundary with no second set of rules — and
the language's own copy-versus-`POINT` distinction turns out to be exactly the
snapshot-versus-share distinction a host needs, so there is one API rather than two.
A binding takes a **shadow run** of slots; a read refreshes from host memory and a
write converts back. Conversion **refuses rather than truncates** — 70000 into an
`int16_t` names the field — and a field name's suffix must agree with the C type it
describes, refused at registration.
**It is the only pointer this interpreter holds that it did not allocate**, and that
is the one thing a host has to think about. `akbasic_host_unbind()` exists for it.
`examples/hoststruct.c` is a working host, built and run by every build.
44. **A whole loop on one line does not loop.** `DO : PRINT 1 : LOOP` runs once, exactly as
`FOR I=1 TO 3 : PRINT I : NEXT I` does. Block skipping walks *source lines*, which is the
reference's `waitingForCommand` model (§1.6), so a `LOOP` on the same line as its `DO` is
@@ -1527,6 +1573,24 @@ the reference's semantics reproduced faithfully; the third is the port's own.
listings read it there. Same known-failing test as item 19; the fix is a scoping decision
about where a loop counter is created, not an ordering one.
### Found while building structures
23. ~~**A host could not run one script twice.**~~ **Fixed.**
`akbasic_runtime_start()` never rewound: `RUN` sets `nextline = 0` and clears
`stopped`, and `start()` set neither. That cost nothing while a host started a script
once, because `nextline` is already 0 the first time — and cost everything to a host
running one script per enemy, where the second `start()` began past the end and
silently did nothing at all. `start` means start now. Found by writing
`examples/hoststruct.c`, which is exactly that shape.
24. ~~**The prescan wiped host-registered types.**~~ **Fixed.** It runs again on every
`RUN` and used to reinitialise the whole table, so the first `RUN` unregistered
everything the host had registered before loading — with no way for the host to know
it had to register them again. It now drops what the *script* declared and keeps what
the *host* registered, which works because host types are always a prefix and so every
index a field already recorded keeps pointing at the same type.
`tests/hoststruct.c` pins it.
### Found while auditing `akbasic_value_clone()` for the structures work
22. ~~**The numeric operators' `else` was a catch-all, not a float branch.**~~ **Fixed.**
@@ -1648,14 +1712,14 @@ requirement; exactly one case has diverged on purpose since, and
| Gate | Result |
|---|---|
| `ctest` | 95/95 — 41 reference golden cases, 15 local ones, 36 unit tests, 2 embedding examples, and `docs_examples` |
| `ctest` with `-DAKBASIC_WITH_AKGL=ON` | 95/95 on a machine with a display; 94 passed + 1 skipped headless. The same set minus the three `no_device` cases the SDL driver contradicts, plus `akgl_backends`, `akgl_frontend`, `docs_screenshots` and `akgl_typing` — the last of which is the skip, and the `akgl_build` CI job is where it skips |
| `docs_examples` | Every fenced block in `README.md`, `MAINTENANCE.md` and `docs/` executed and byte-compared: 41 programs, 9 transcripts, 49 output comparisons, 2 excerpts, 2 shell blocks and 8 figures in the default build; 57 programs and 48 output comparisons in the AKGL one. The C-snippet count reads 0 when the harness is run by hand without `--cflags-file`; CTest passes it. `MAINTENANCE.md` documents the fence-tag convention |
| `ctest` | 104/104 — 41 reference golden cases, 20 local ones, 39 unit tests, 3 embedding examples, and `docs_examples` |
| `ctest` with `-DAKBASIC_WITH_AKGL=ON` | 104/104 on a machine with a display; 103 passed + 1 skipped headless. The same set minus the three `no_device` cases the SDL driver contradicts, plus `akgl_backends`, `akgl_frontend`, `docs_screenshots` and `akgl_typing` — the last of which is the skip, and the `akgl_build` CI job is where it skips |
| `docs_examples` | Every fenced block in `README.md`, `MAINTENANCE.md` and `docs/` executed and byte-compared: 49 programs, 9 transcripts, 57 output comparisons, 2 excerpts, 2 shell blocks and 8 figures in the default build; 65 programs and 56 output comparisons in the AKGL one. The C-snippet count reads 0 when the harness is run by hand without `--cflags-file`; CTest passes it. `MAINTENANCE.md` documents the fence-tag convention |
| `docs_screenshots` | 8/8 figures re-rendered and byte-identical to the checked-in PNGs. AKGL build only — rendering a picture needs the SDL half |
| Golden corpus | 41/41 byte-exact from `tests/reference/` — **and 41/41 again through the SDL binary**, which is most of what proves the frontend changes no output |
| ASan + UBSan | 95/95 |
| Line coverage | 95.1% (5916/6222) — above the 90% gate |
| Function coverage | 98.5% (403/409) |
| ASan + UBSan | 104/104 |
| Line coverage | 94.8% (6648/7013) — above the 90% gate |
| Function coverage | 98.4% (441/448) |
| Warnings | none under `-Wall -Wextra` |
| `doxygen Doxyfile` | clean |
| Mutation (`src/symtab.c`) | 74.1%, against a gate of 65 — see `.gitea/workflows/ci.yaml` |

6
docs/03-the-language.md
View File

@@ -10,6 +10,7 @@ a character that says what it holds:
| `#` | integer | `COUNT#`, `I#` |
| `%` | floating point | `RATE%`, `X%` |
| `$` | string | `NAME$` |
| `@` | structure | `ENEMY@` |
```basic
10 COUNT# = 42
@@ -20,6 +21,11 @@ a character that says what it holds:
There is no such thing as a variable with no suffix. A bare name is a **label** — see
Chapter 4 — so `GOTO DONE` and `LABEL DONE` are how the two meet.
`@` is the odd one out: it says "a structure" without saying *which*, so a structure
variable has to be declared with `DIM E@ AS ENEMY` before it can be used. That is
**[Chapter 16](16-structures.md)**, along with records, pointers and how a host shares its
own C structs with a script.
On a C128 the suffixes mean something different (`%` is integer, no suffix is float).
Here `%` is float and `#` is integer, following the Go implementation this was ported
from. Chapter 13 lists it with the other differences.

3
docs/11-verb-reference.md
View File

@@ -33,6 +33,7 @@ for the reasoning in each case.
| `DEF` | `DEF NAME(args) = expr` | Define a function. Multi-line definitions end in `RETURN`. |
| `DELETE` | `DELETE [n][-n]` | Delete lines, with the same range forms as `LIST`. |
| `DIM` | `DIM A#(n [,...])` | Make an array. Subscripts start at zero; `n` is the count. |
| `DIM` … `AS` | `DIM S@ AS T`, `DIM P@ AS PTR TO T` | Make a structure, or a strict pointer to one. See Chapter 16. |
| `DIRECTORY` | `DIRECTORY` | **Refused.** Needs a directory-reading wrapper that does not exist yet. |
| `DLOAD` | `DLOAD "name"` | Load a program from a file. |
| `DO` | `DO [WHILE c | UNTIL c]` | Start a loop. The condition may be here, on the `LOOP`, or neither. |
@@ -70,6 +71,7 @@ for the reasoning in each case.
| `ON` | `ON e GOTO|GOSUB t [,...]` | Branch to the `e`th target, counting from one. |
| `PAINT` | `PAINT src, x, y` | Flood-fill the region containing a point. |
| `PLAY` | `PLAY "notes"` | Queue notes. Does not block. |
| `POINT` | `POINT P@ AT s@` | Aim a strict pointer at a structure. See Chapter 16. |
| `POKE` | `POKE addr, byte` | Write a byte to a real address. |
| `PRINT` | `PRINT [expr]` | Print a value and a newline. |
| `PRINT#` | `PRINT #n, expr` | Write a line to a channel. |
@@ -101,6 +103,7 @@ for the reasoning in each case.
| `TRAP` | `TRAP [target]` | Send errors to a handler. No target disarms it. |
| `TROFF` | `TROFF` | Turn line tracing off. |
| `TRON` | `TRON` | Turn line tracing on; each line prints its number in brackets. |
| `TYPE` | `TYPE NAME` … `END TYPE` | Declare a record, its fields one per line. See Chapter 16. |
| `VERIFY` | `VERIFY "name"` | Compare the program in memory against a file. |
| `VOL` | `VOL n` | Set the overall volume, 0 to 15. |
| `WAIT` | `WAIT addr, mask [,xor]` | Poll a byte until it matches. Holds the program. |

11
docs/13-differences.md
View File

@@ -42,6 +42,17 @@ There is no escape character.
Integers are 64-bit and floats are IEEE doubles, so `PRINT 1.5` gives `1.500000`. A
leading zero is not octal; `0x` is hexadecimal.
### Structures are an addition
BASIC 7.0 has no records at all. `TYPE`/`END TYPE`, `DIM X@ AS T`, the `@` suffix, `.` and
`->`, `PTR TO` and `POINT ... AT` are all new here, and **[Chapter 16](16-structures.md)**
is the whole of it. Nothing about it changes an existing program.
Two consequences a C128 programmer should know. A structure assignment **copies**, like
every other assignment; sharing is spelled `POINT`. And a type name is a bare word, so it
shares a namespace with verbs and labels — `TYPE POINT` is refused because `POINT` is now a
verb.
## Block structure
**A whole loop on one line does not loop.**

77
docs/14-architecture.md
View File

@@ -421,6 +421,83 @@ normally true, and evaluating `A#` then yields a clone drawn from the per-line v
own evaluation. If you are debugging an assignment that wrote to the wrong place, that
flag is the first thing to check.
## Structures are laid out exactly as arrays are
The whole storage model falls out of one decision: **a `TYPE` is declared**, so an
instance has a known slot count before the program runs.
That means a structure needs no pool of its own. `DIM R@ AS RECT` calls the same
`akbasic_valuepool_take()` that `DIM A#(10)` calls, and the variable's `values` run *is*
the instance. A field access is offset arithmetic against an offset the type descriptor
already knows. A nested value field **flattens into its container's run** — a `SEGMENT`
holding two `COORD`s and a string is five slots, not three — which is why the nesting is
free rather than a second indirection.
```text
DIM S@ AS SHAPE SHAPE: NAME$, ORIGIN@ AS COORD, AREA%
COORD: X#, Y#
variable S@
structtype ──► type table entry SHAPE (slotcount 4)
values ──────► ┌────────┬────────┬────────┬────────┐
│ NAME$ │ X# │ Y# │ AREA% │
└────────┴────────┴────────┴────────┘
off 0 off 1 off 2 off 3
└── ORIGIN@ is offset 1, two slots ──┘
```
`src/structtype.c` fills the table, and it does so **in three passes**, each for a case
the pass before cannot handle. Names first, so a field can refer to a type declared
further down. Then field lists, now able to resolve every reference. Then sizes, by
repeated resolution — a type whose fields are all sized can be sized, and repeating that
settles any legal ordering. **Whatever never resolves is a cycle of by-value containment**,
which is how "a `TYPE` cannot contain itself by value" is a diagnosis rather than an
assumption.
It is a *prescan*, run from `akbasic_runtime_set_mode()` beside the label and `DATA`
scans, for the reason all three are: a declaration has to be in effect wherever control
goes, including when a branch skips the lines that made it. The `TYPE` verb's whole job at
run time is to **jump past its own `END TYPE`**, because the field lines are declarations
and executing `W#` would evaluate a bare identifier and quietly create a global.
### Copy, and where it has to happen
A `STRUCT` value and a `POINTER` value carry the same thing: a type index and a base. What
differs is what *assignment* does with it.
**The copy cannot live in `akbasic_value_clone()`**, and this is the trap to know about.
Clone copies one slot, and one slot holds a *reference* to an instance rather than the
instance — so a structure going through clone would alias, which is precisely the
semantics the language does not have. `akbasic_environment_assign()` intercepts a
structure before that path and calls `akbasic_struct_copy()`, which walks the descriptor
and copies slot by slot. Deliberately not one `memcpy` of the run: a pointer field must
copy its reference where a value field must copy its slots, and only the descriptor knows
which is which.
Copy is therefore **deep through values and stops at pointers**, as it is for a C struct
holding a pointer. Since a `TYPE` cannot contain itself by value, copy depth is fixed by
the type graph before the program starts and no copy can recurse away. Only *rendering*
needs a runtime bound, because a pointer can make the graph cyclic — that is
`AKBASIC_MAX_STRUCT_DEPTH`, four, chosen so the bound bites before the 256-byte render
buffer does.
### A host structure is the same thing with its bytes somewhere else
`akbasic_host_register_type()` puts a host's C struct in the *same* table, so `PTR TO`,
copy-on-assign, `.` and `->` all work across the boundary with no second set of rules. The
only difference is where a field's bytes live, and that is carried by three members on the
field descriptor — `hostkind`, `hostoffset`, `hostwidth`.
A binding takes a **shadow run** of slots from the ordinary pool. A field read refreshes
its slot from host memory first; a field write converts back and stores. So the script
always sees current values and its writes always land, while everything else in the
interpreter goes on seeing one storage model. If you are debugging a host binding that
reads stale data, `akbasic_host_refresh()` is where to look.
**This is the one pointer the interpreter holds that it did not allocate.** Everything
else is pool-bounded; a binding whose instance has been freed is the only way to get a
wild pointer in here, which is what `akbasic_host_unbind()` is for.
## Errors come in two kinds, and the distinction is the point
```text

295
docs/16-structures.md Normal file
View File

@@ -0,0 +1,295 @@
# 16. Structures
A structure groups values that belong together. Commodore BASIC 7.0 has nothing like
it — this is entirely an addition, and Chapter 13 lists it with the other differences.
A structure variable's name ends in **`@`**, the fourth type suffix:
| Suffix | Type |
|---|---|
| `#` | integer |
| `%` | floating point |
| `$` | string |
| `@` | structure |
## Declaring a type
`TYPE` … `END TYPE` names a record and lists its fields, one per line. **Each field takes
its type from its own suffix**, the same rule every other name in this language follows,
so a field list needs no type column:
```basic
10 TYPE RECT
20 W#
30 H#
40 END TYPE
50 DIM R@ AS RECT
60 R@.W# = 3
70 R@.H# = 4
80 PRINT R@.W# * R@.H#
90 PRINT R@
```
```output
12
RECT(W#=3, H#=4)
```
`DIM name@ AS TYPE` is how a variable gets storage, and it is required — unlike an
ordinary variable, a structure cannot spring into existence on first use, because
nothing would say which type it is.
**A type name is a bare word, and so is every verb**, so the two share a namespace.
`TYPE POINT` is refused, because `POINT` is a verb:
```basic
10 TYPE POINT
20 X#
30 END TYPE
40 PRINT 1
```
```output
? 40 : PARSE ERROR TYPE POINT: POINT is a reserved word and cannot name a type
```
The same applies to field names, for the same reason a variable cannot be called `TO#`.
## Nesting
A field may be another structure, named with `AS`:
```basic
10 TYPE COORD
20 X#
30 Y#
40 END TYPE
50 TYPE SHAPE
60 NAME$
70 ORIGIN@ AS COORD
80 END TYPE
90 DIM S@ AS SHAPE
100 S@.NAME$ = "BOX"
110 S@.ORIGIN@.X# = 10
120 PRINT S@
```
```output
SHAPE(NAME$=BOX, ORIGIN@=COORD(X#=10, Y#=0))
```
`@` on its own says "a structure" but not *which*, which is why a structure field has to
name its type where `W#` does not. Three primitive types fit in three suffix characters;
ten declared types do not fit in one.
## Assignment copies
**This is the rule to remember.** A structure behaves like every other value here:
```basic
10 TYPE RECT
20 W#
30 END TYPE
40 DIM A@ AS RECT
50 DIM B@ AS RECT
60 A@.W# = 1
70 B@ = A@
80 A@.W# = 99
90 PRINT B@.W#
```
```output
1
```
`B@` is its own record from line 70 onward. Nesting copies too — a whole record, however
deep, moves as a unit.
## Pointers
When you want two names for *one* record, say so. A pointer is a distinct declared kind:
```basic
10 TYPE RECT
20 W#
30 END TYPE
40 DIM A@ AS RECT
50 DIM P@ AS PTR TO RECT
60 A@.W# = 1
70 POINT P@ AT A@
80 P@->W# = 99
90 PRINT A@.W#
```
```output
99
```
Three things are deliberate:
- **`POINT` is the only way to share.** A program that never writes it can never be
surprised by aliasing.
- **`.` reaches a field of a structure and `->` reaches one through a pointer.** They do
not stand in for one another, and using the wrong one is an error that names the other.
So a reader always knows from the spelling whether the thing on the left is their own
copy or somebody else's data.
- **A pointer with nothing behind it is `NOTHING`**, and dereferencing it is refused
rather than being a crash.
```basic
10 TYPE RECT
20 W#
30 END TYPE
40 DIM P@ AS PTR TO RECT
50 PRINT P@
60 PRINT P@->W#
```
```output
NOTHING
? 60 : RUNTIME ERROR This pointer is not pointing at anything yet; POINT it AT a structure first
```
## Lists and trees
A `TYPE` may refer to **itself only through a pointer** — by value it would have no
finite size, and that is refused at declaration. Which is exactly what makes a list
possible:
```basic
10 TYPE NODE
20 COUNT#
30 TAIL@ AS PTR TO NODE
40 END TYPE
50 DIM N1@ AS NODE
60 DIM N2@ AS NODE
70 N1@.COUNT# = 10
80 N2@.COUNT# = 20
90 POINT N1@.TAIL@ AT N2@
100 DIM WALK@ AS PTR TO NODE
110 POINT WALK@ AT N1@
120 PRINT WALK@->COUNT#
130 WALK@ = WALK@->TAIL@
140 PRINT WALK@->COUNT#
150 PRINT N1@
```
```output
10
20
NODE(COUNT#=10, TAIL@=NODE(COUNT#=20, TAIL@=NOTHING))
```
Note line 130: assigning one *pointer* to another copies the reference, not the record.
That is the one place assignment does not deep-copy, and it is why pointers are declared
separately rather than being a mode a structure can be in.
**Walk a list with a loop, not with a recursive `DEF`.** A recursive multi-line `DEF`
does not return in this interpreter; it is a known defect recorded in `TODO.md`, and it
is not specific to structures.
`PRINT` follows pointers, so a cycle would not come back — it stops after four levels and
prints `(...)`.
## What is checked, and what is not
A field name is checked against the set the type declared, and the refusal lists the
fields that do exist:
```basic
10 TYPE RECT
20 W#
30 H#
40 END TYPE
50 DIM R@ AS RECT
60 PRINT TOTLA#
70 PRINT R@.NOPE#
```
```output
0
? 70 : RUNTIME ERROR RECT has no field NOPE# (W#, H#)
```
Line 60 is the contrast worth understanding. **A misspelled variable is still silent** —
`TOTLA#` prints zero, as it does in every BASIC ever written. A misspelled *field* is
not, because the set of fields is closed and the program wrote it down.
That is the rule underneath both: **what the program declared gets checked, and what it
did not gets shrugged at.** A variable's name is never declared, so it cannot be checked.
A `TYPE`'s field list is, so it can be. Structures end up the strictest thing in the
language, not because they are held to a higher standard but because they are the only
named thing whose valid spellings are written down.
## Sharing a structure with a host
If you are embedding the interpreter in a game, a script can read and write **the game's
own C structs** — not a copy of them. The host describes its struct once:
```c norun
typedef struct
{
char name[32];
int32_t hp;
float x;
bool hostile;
} game_Enemy;
static const akbasic_HostField ENEMY_FIELDS[] = {
/* struct member BASIC name C representation */
AKBASIC_HOST_FIELD( game_Enemy, name, "NAME$", AKBASIC_HOSTFIELD_CSTRING ),
AKBASIC_HOST_FIELD( game_Enemy, hp, "HP#", AKBASIC_HOSTFIELD_INT32 ),
AKBASIC_HOST_FIELD( game_Enemy, x, "X%", AKBASIC_HOSTFIELD_FLOAT ),
AKBASIC_HOST_FIELD( game_Enemy, hostile, "HOSTILE#", AKBASIC_HOSTFIELD_BOOL )
};
static const akbasic_HostType ENEMY_TYPE = {
"ENEMY", sizeof(game_Enemy), ENEMY_FIELDS, 4
};
akbasic_host_register_type(&SCRIPT, &ENEMY_TYPE);
akbasic_host_bind(&SCRIPT, "FOE@", "ENEMY", &GOBLIN);
```
and the script then works on `FOE@` like any other structure:
```basic norun
10 PRINT FOE@.NAME$ + " HAS " + FOE@.HP#
20 FOE@.HP# = FOE@.HP# - 10
```
Line 20 decrements `GOBLIN.hp` in place. There is no marshalling step.
`examples/hoststruct.c` is a complete working host, built and run by every build.
Chapter 10 covers the rest of the embedding API; the parts specific to structures:
| Call | Does |
|---|---|
| `akbasic_host_register_type` | Makes a C struct available as a BASIC type |
| `akbasic_host_bind` | Binds one instance to a script variable |
| `akbasic_host_rebind` | Points that name at a different instance — the per-frame call |
| `akbasic_host_unbind` | Breaks the binding before the storage goes away |
Three things a host should know:
- **Conversion refuses rather than truncates.** Assigning 70000 to an `int16_t` field, or
forty characters to a `char[32]`, is an error naming the field.
- **A host `float` round-trips lossily**, because BASIC floats are doubles. A `char[]` has
a width that BASIC strings do not.
- **A bound instance must outlive the binding.** It is the only pointer this interpreter
holds that it did not allocate; `akbasic_host_unbind()` exists for exactly that.
## Limits
| | |
|---|---|
| Types | 16 |
| Fields per type | 16 |
| Nesting shown by `PRINT` | 4 levels |
An instance's fields come out of the same value pool arrays use, so the 4096-element
budget in Chapter 13 covers both. **Nothing is reclaimed** — a structure lasts until
`CLR` or `NEW`, exactly as an array does.

1
docs/README.md
View File

@@ -32,6 +32,7 @@ embedding it, debugging it or changing it.
| **[13. Differences from BASIC 7.0](13-differences.md)** | What a C128 programmer needs to know |
| **[14. Architecture](14-architecture.md)** | How the interpreter is put together, how to debug it, how to change it |
| **[15. Error codes](15-error-codes.md)** | Appendix: every value `ER#` can hold and every error line the interpreter prints |
| **[16. Structures](16-structures.md)** | `TYPE`, records, strict pointers, and sharing a C struct with an embedding host |
## The shortest possible start