akbasic

A BASIC interpreter written in C, styled after Commodore BASIC 7.0 and the Dartmouth BASIC of 1964. It runs a .bas file, it gives you a prompt, and — the point of the exercise — it links into a C program as a scripting engine.

It is a rewrite of basicinterpreter, a Go implementation that started from the Java Lox instructions in craftinginterpreters.com and then struck off on its own. That project is deprecated. It is vendored here as the behavioural spec to read when a question about semantics comes up, and its acceptance corpus is checked in at tests/reference/ and runs on every build — so nothing about building or testing this project needs it.

Quickstart

git submodule update --init --recursive
cmake -S . -B build
cmake --build build --parallel
ctest --test-dir build --output-on-failure

./build/basic                                          # the REPL
./build/basic tests/reference/language/functions.bas   # run a program

10 FOR I# = 1 TO 3
20   PRINT "HELLO " + I#
30 NEXT I#

HELLO 1
HELLO 2
HELLO 3

Two things in that program are not Commodore BASIC and will catch you out immediately: variables carry a type suffix (I# is an integer), and + concatenates a string with a number. Chapter 3 explains both; Chapter 13 is the whole list of what differs from a C128.

Graphics, sound and sprites need the SDL build, which is off by default because the interpreter and its entire test suite build on a machine with no SDL installed at all:

cmake -S . -B build-akgl -DAKBASIC_WITH_AKGL=ON
cmake --build build-akgl --parallel

That basic is a different program: it opens a window, draws BASIC output into it in the Commodore font, and still puts every byte on stdout.

Why rewrite it in C?

Three reasons, in the order they matter.

The interpreter is meant to end up inside libakgl as a scripting engine for game authors, and libakgl is C. Embedding a Go runtime in a C game is not a thing anybody should do to themselves.

The Go version was already written against static pools and explicit state structs — a fixed source table, a fixed variable pool, a 32-leaf ceiling per line — so it ports across almost directly. It reads like C that happens to be spelled in Go.

And the port is a good excuse to find out what the original actually does, as opposed to what it looks like it does. It found five defects nobody knew about.

Design philosophy

A game engine cannot tolerate a scripting language that surprises it. Five rules follow from that, and between them they explain most of what looks unusual in this interpreter:

• Nothing in the library terminates the process. No exit(), no abort(), no panic. Errors come back as akerr_ErrorContext * for the host to handle. FINISH_NORETURN appears only in a main().
• Nothing calls malloc. Every object comes from a fixed pool inside akbasic_Runtime. Exhausting one is a diagnosable error, not a crash and not a slow leak.
• No file-scope mutable state. Interpreter state lives in an akbasic_Runtime you own. Two of them in one process do not interfere.
• The host owns the loop. akbasic_runtime_run(rt, n) executes at most n steps and returns. A script containing 10 GOTO 10 costs you n steps per frame and nothing else.
• Hardware is a record of function pointers. Graphics, audio, input and sprites attach as backends, and any of them may be NULL — that is how a host withholds a capability, and how a host that renders some other way never links libakgl at all.

Nothing is silently ignored, either. A verb that needs a device it was not given, or a capability nothing underneath can supply, refuses by name and says why.

Two ways to use it

As a program. basic is a REPL and a script runner, and the standalone driver owns the things a library has no business owning: argv, QUIT, and the window in the SDL build. The guide is written for this reader.

As a library. A host links akbasic::akbasic, hands the interpreter a script, and steps it a frame at a time:

add_subdirectory(deps/akbasic EXCLUDE_FROM_ALL)
target_link_libraries(YOUR_GAME PRIVATE akbasic::akbasic)

Host and script exchange variables through the same pool the script itself uses — no marshalling layer and no copy. PRINT goes through an akbasic_TextSink the host supplies, so a game draws BASIC output into its own text layer. examples/embed.c and examples/hostvars.c are complete and runnable, and both are built and run by every build, so they cannot rot. Chapter 10 walks through the API.

What state it is in

Everything the Go version does, and by now a good deal more. All 41 .bas files of the reference's corpus produce the expected output, including error messages, each as a separate CTest case so a failure names the file.

What is still missing, what is refused deliberately, and the eleven defects inherited from the Go version are catalogued in TODO.md and summarised for a BASIC programmer in Chapter 13.

Where everything else lives

docs/	The guide: eighteen chapters, the language then each hardware area then a reference section for every verb and function, Chapter 14 on the interpreter's own architecture, Chapter 15 listing every error code, and Chapters 17 and 18 building a whole game twice
MAINTENANCE.md	For contributors and maintainers: the documentation-example harness, the three test lists, mutation testing, error-code allocation, style
TODO.md	Outstanding defects, with file, line and consequence
tests/reference/README.md	Where the golden corpus came from, and the rule for changing it

API documentation builds with doxygen Doxyfile, into build/docs/html.

Dependencies

Everything is a submodule; git submodule update --init --recursive gets all of it. There is nothing to install first.

• libakerror 2.0.1 — TRY/CATCH-style error contexts. Every function that can fail returns one. 2.0.0 made it thread safe and broke the ABI; anything built against a 1.x header must be rebuilt rather than relinked.
• libakstdlib 0.2.0 — libc wrappers that report through libakerror. String-to-number conversion goes straight to it, which is why VAL("garbage") is an error rather than a silent 0.
• libakgl 0.9.0 — optional, only for -DAKBASIC_WITH_AKGL=ON. Pulls in SDL3. Its soname carries MAJOR.MINOR while the major is 0, so rebuild rather than relink.
• basicinterpret — the Go original. Not linked, not built, and safe to omit.