Andrew Kesterson
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0.6.0 and 0.7.0 break nothing here: 0.6.0 is three arcade-physics fixes and a physics.max_timestep property this port does not use, and 0.7.0 reports failures libakstdlib's wrappers were already catching and takes libakerror 2.0.1 so it can drop the exit-status trap its own suites needed. That is the same defect include/akbasic/error.h guards for this band, now fixed at the source rather than worked around in two places. The floor moves to 0.7.0 anyway. The soname carries MAJOR.MINOR while the major is 0, so deciding for ourselves which of libakgl's minor releases were really compatible is exactly the judgement it exists to take away. 0.7.0 is largely about a defect class worth checking for here rather than assuming past: ten unterminated strncpy calls into fixed-width name fields. Ours are clean -- every one is bounded to size - 1 and every buffer is either explicitly terminated afterwards or memset first, checked site by site. But two of them, both written last week in src/structtype.c, truncated an over-long name silently, and truncation is an error everywhere else in this interpreter. It matters more for a name than for a value: two long type names trimmed to the same 31 characters collide, and the second is then reported as redeclaring a type the program never wrote. Both are refused now, showing the name as written. The check was unreachable as first drafted, because the prescan read words into a buffer the size of the limit and so trimmed them before anything could look -- the scratch is twice the limit now, which is what makes the two cases distinguishable at all. tests/struct_types.c pins that it stays reachable. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
152 lines
7.2 KiB
Markdown
152 lines
7.2 KiB
Markdown
# akbasic
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A BASIC interpreter written in C, styled after [Commodore BASIC 7.0](http://www.jbrain.com/pub/cbm/manuals/128/C128PRG.pdf)
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and the [Dartmouth BASIC of 1964](https://www.dartmouth.edu/basicfifty/basic.html). It runs a
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`.bas` file, it gives you a prompt, and — the point of the exercise — it links into a C program
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as a scripting engine.
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It is a rewrite of [basicinterpreter](https://source.starfort.tech/andrew/basicinterpreter), a Go
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implementation that started from the Java Lox instructions in
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[craftinginterpreters.com](https://craftinginterpreters.com) and then struck off on its own. That
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project is deprecated. It is vendored here as the behavioural spec to read when a question about
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semantics comes up, and its acceptance corpus is checked in at
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[`tests/reference/`](tests/reference/README.md) and runs on every build — so nothing about
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building or testing this project needs it.
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## Quickstart
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```sh norun
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git submodule update --init --recursive
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cmake -S . -B build
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cmake --build build --parallel
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ctest --test-dir build --output-on-failure
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```
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```sh norun
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./build/basic # the REPL
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./build/basic tests/reference/language/functions.bas # run a program
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```
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```basic
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10 FOR I# = 1 TO 3
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20 PRINT "HELLO " + I#
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30 NEXT I#
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```
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```output
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HELLO 1
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HELLO 2
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HELLO 3
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```
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Two things in that program are not Commodore BASIC and will catch you out immediately: variables
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carry a **type suffix** (`I#` is an integer), and **`+` concatenates a string with a number**.
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[Chapter 3](docs/03-the-language.md) explains both; [Chapter 13](docs/13-differences.md) is the
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whole list of what differs from a C128.
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Graphics, sound and sprites need the SDL build, which is off by default because the interpreter
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and its entire test suite build on a machine with no SDL installed at all:
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```sh norun
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cmake -S . -B build-akgl -DAKBASIC_WITH_AKGL=ON
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cmake --build build-akgl --parallel
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```
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That `basic` is a different program: it opens a window, draws BASIC output into it in the
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Commodore font, and still puts every byte on stdout.
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## Why rewrite it in C?
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Three reasons, in the order they matter.
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The interpreter is meant to end up *inside*
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[libakgl](https://source.starfort.tech/andrew/libakgl) as a scripting engine for game authors,
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and libakgl is C. Embedding a Go runtime in a C game is not a thing anybody should do to
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themselves.
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The Go version was already written against static pools and explicit state structs — a fixed
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source table, a fixed variable pool, a 32-leaf ceiling per line — so it ports across almost
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directly. It reads like C that happens to be spelled in Go.
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And the port is a good excuse to find out what the original actually does, as opposed to what it
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looks like it does. It found five defects nobody knew about.
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## Design philosophy
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A game engine cannot tolerate a scripting language that surprises it. Five rules follow from
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that, and between them they explain most of what looks unusual in this interpreter:
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* **Nothing in the library terminates the process.** No `exit()`, no `abort()`, no `panic`.
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Errors come back as `akerr_ErrorContext *` for the host to handle. `FINISH_NORETURN` appears
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only in a `main()`.
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* **Nothing calls `malloc`.** Every object comes from a fixed pool inside `akbasic_Runtime`.
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Exhausting one is a diagnosable error, not a crash and not a slow leak.
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* **No file-scope mutable state.** Interpreter state lives in an `akbasic_Runtime` you own. Two
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of them in one process do not interfere.
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* **The host owns the loop.** `akbasic_runtime_run(rt, n)` executes at most `n` steps and
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returns. A script containing `10 GOTO 10` costs you `n` steps per frame and nothing else.
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* **Hardware is a record of function pointers.** Graphics, audio, input and sprites attach as
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backends, and any of them may be `NULL` — that is how a host withholds a capability, and how a
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host that renders some other way never links libakgl at all.
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Nothing is silently ignored, either. A verb that needs a device it was not given, or a capability
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nothing underneath can supply, refuses by name and says why.
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## Two ways to use it
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**As a program.** `basic` is a REPL and a script runner, and the standalone driver owns the
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things a library has no business owning: argv, `QUIT`, and the window in the SDL build.
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[The guide](docs/README.md) is written for this reader.
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**As a library.** A host links `akbasic::akbasic`, hands the interpreter a script, and steps it a
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frame at a time:
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```cmake
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add_subdirectory(deps/akbasic EXCLUDE_FROM_ALL)
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target_link_libraries(YOUR_GAME PRIVATE akbasic::akbasic)
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```
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Host and script exchange variables through the same pool the script itself uses — no marshalling
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layer and no copy. `PRINT` goes through an `akbasic_TextSink` the host supplies, so a game draws
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BASIC output into its own text layer. [`examples/embed.c`](examples/embed.c) and
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[`examples/hostvars.c`](examples/hostvars.c) are complete and runnable, and both are built and
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run by every build, so they cannot rot. [Chapter 10](docs/10-embedding.md) walks through the API.
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## What state it is in
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Everything the Go version does, and by now a good deal more. All 41 `.bas` files of the
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reference's corpus produce the expected output, including error messages, each as a separate
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CTest case so a failure names the file.
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What is still missing, what is refused deliberately, and the eleven defects inherited from the Go
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version are catalogued in [`TODO.md`](TODO.md) and summarised for a BASIC programmer in
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[Chapter 13](docs/13-differences.md).
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## Where everything else lives
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| [`docs/`](docs/README.md) | The guide: fourteen chapters, the language then each hardware area then a reference section for every verb and function, [Chapter 14](docs/14-architecture.md) on the interpreter's own architecture, and [Chapter 15](docs/15-error-codes.md) listing every error code |
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| [`MAINTENANCE.md`](MAINTENANCE.md) | For contributors and maintainers: the documentation-example harness, the three test lists, mutation testing, error-code allocation, style |
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| [`TODO.md`](TODO.md) | Outstanding defects, with file, line and consequence |
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| [`tests/reference/README.md`](tests/reference/README.md) | Where the golden corpus came from, and the rule for changing it |
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API documentation builds with `doxygen Doxyfile`, into `build/docs/html`.
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## Dependencies
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Everything is a submodule; `git submodule update --init --recursive` gets all of it. There is
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nothing to install first.
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* [libakerror](https://source.starfort.tech/andrew/libakerror) 2.0.1 — TRY/CATCH-style error
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contexts. Every function that can fail returns one. 2.0.0 made it thread safe and broke the
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ABI; anything built against a 1.x header must be rebuilt rather than relinked.
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* [libakstdlib](https://source.starfort.tech/andrew/libakstdlib) 0.2.0 — libc wrappers that
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report through `libakerror`. String-to-number conversion goes straight to it, which is why
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`VAL("garbage")` is an error rather than a silent `0`.
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* [libakgl](https://source.starfort.tech/andrew/libakgl) 0.7.0 — **optional**, only for
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`-DAKBASIC_WITH_AKGL=ON`. Pulls in SDL3. Its soname carries `MAJOR.MINOR` while the major is 0,
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so rebuild rather than relink.
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* [basicinterpret](https://source.starfort.tech/andrew/basicinterpret) — the Go original.
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Not linked, not built, and safe to omit.
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