This BASIC is styled after [Commodore BASIC 7.0](http://www.jbrain.com/pub/cbm/manuals/128/C128PRG.pdf) and the [Dartmouth BASIC from 1964](https://www.dartmouth.edu/basicfifty/basic.html). It is a C rewrite of [basicinterpreter](https://source.starfort.tech/andrew/basicinterpreter), which was itself built from the instructions for the Java implementation of Lox in [craftinginterpreters.com](https://craftinginterpreters.com) before striking off on its own. The Go version is vendored at `deps/basicinterpret` and is the behavioural specification: when a question about semantics comes up, the answer lives in that code and in its `tests/`.
The interpreter is meant to end up *inside* [libakgl](https://source.starfort.tech/andrew/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 — `[MAX_SOURCE_LINES]BasicSourceLine`, 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. Rewriting it in the idiom of `libakerror` and `libakstdlib` was less work than it sounds.
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. See "What Isn't Implemented / Isn't Working", below.
# What Works?
Everything the Go version does. All 41 `.bas` files in `deps/basicinterpret/tests/` produce **byte-identical** output from both implementations, including error messages and the trailing blank line after one. Those files are driven in place as individual CTest cases rather than copied, so the corpus cannot drift from upstream.
## Case Sensitivity
The old computers BASIC was originally written on only had CAPITAL LETTER KEYS on their keyboards. Modern keyboards have the indescribable luxury of upper and lower case. In this basic, verbs and function names are case insensitive. Variable names are case sensitive.
## Variables
*`A#` Integer variables
*`A%` Float variables
*`A$` String variables. Strings support addition operations with other types.
*`LET` is supported but optional
* Variables are strongly typed
Note that `%` means *float* here, which inverts the Commodore convention where `%` is integer. That is what the Go version does and it is not being changed out from under anybody.
## Arrays
*`DIM IDENTIFIER(DIMENSION[, ...])` allows for provisioning of multiple dimensional arrays
*`DIM A$(3)` results in a single dimensional array of strings with 3 elements
*`PRINT A$(2)` accesses the last element in an array and returns it to the verb
*`LEN(A#)` on an array returns its total element count
* Arrays are strongly typed
## Expressions
*`+`
*`-`
*`^`
*`*` (also works on strings)
*`/`
*`< <= <> == >= >` less than, less than equal, not equal, equal, greater equal, greater than
Expressions can be grouped with `()` arbitrarily deeply. Currently the interpreter has a limit of 32 tokens and leaves per line. In effect this means about 16 operations in a single line.
## Commands (Verbs)
The following commands/verbs are implemented:
*`AUTO n` : Turn automatic line numbering on/off at increments of `n`
*`REM` : everything after this is a comment
*`DATA LITERAL[, ...]`: Define a series of literal values that can be read by a preceding `READ` verb
*`DEF FN(X, ...) = expression` : Define a function with arguments that performs a given expression. See also "Subroutines", below.
*`DELETE [n-n]`: Delete some portion of the lines in the current program
*`DELETE`: Delete ALL lines in the program
*`DELETE n-n`: Delete lines between `n` and `n` (inclusive)
*`DELETE -n`: Delete lines from 0 to `n`
*`DELETE n`: Delete lines from `n` to the end of the program
*`DLOAD FILENAME`: Load the BASIC program in the file FILENAME (string literal or string variable) into memory
*`DSAVE FILENAME`: Save the current BASIC program in memory to the file specified by FILENAME (string literal or string variable)
*`EXIT`: Exit a loop before it would normally finish
*`FOR` : Iterate over a range of values and perform (statement) or block each time.
```
10 FOR I# = 1 TO 5
20 REM Do some stuff in here
30 NEXT I#
10 FOR I# = 1 TO 5 STEP 2
20 REM Do some stuff here
30 NEXT I#
```
*`GOTO n`: Go to line n in the program
*`GOSUB n`: Go to line n in the program and return here when `RETURN` is found
*`IF (comparison) THEN (statement) [ELSE (statement)]` : Conditional branching
*`INPUT "PROMPT STRING" VARIABLE`: Read input from the user and store it in the named variable
*`LABEL IDENTIFIER`: Place a label at the current line number. Labels are constant integer identifiers that can be used in expressions like variables (including GOTO) but which cannot be assigned to. Labels do not have a type suffix (`$`, `#` or `%`).
*`LIST [n-n]`: List all or a portion of the lines in the current program, with the same range forms as `DELETE`
*`POKE ADDRESS, VALUE`: Poke the single byte VALUE into the ADDRESS
*`PRINT (expression)`
*`QUIT` : Exit the interpreter
*`READ IDENTIFIER[, ...]` : Fill the named variables with data from a subsequent DATA statement
*`RETURN` : return from `GOSUB` to the point where it was called
*`RUN [n]`: Run the program currently in memory, optionally starting at line `n`
*`STOP`: Stop program execution at the current point
## Functions
The following functions are implemented
*`ABS(x#|x%)`: Return the absolute value of the float or integer argument
*`ATN(x#|x%)`: Return the arctangent of the float or integer argument. Input and output are in radians.
*`CHR(x#)`: Return the character value of the UTF-8 unicode codepoint in x#. Returns as a string.
*`COS(x#|x%)`: Return the cosine of the float or integer argument. Input and output are in radians.
*`HEX(x#)`: Return the string representation of the integer number in x#
*`INSTR(X$, Y$)`: Return the index of `Y$` within `X$` (-1 if not present)
*`LEN(var$)`: Return the length of the object `var$` (either a string or an array)
*`LEFT(X$, Y#)`: Return the leftmost Y# characters of the string in X$. Y# is clamped to LEN(X$).
*`LOG(X#|X%)`: Return the natural logarithm of X#|X%
*`MID(var$, start, length)` : Return a substring from `var$`
*`MOD(x#, y#)`: Return the modulus of ( x / y). Only works on integers.
*`PEEK(X)`: Return the value of the BYTE at the memory location of integer X and return it as an integer
*`POINTER(X)`: Return the address in memory for the value of the variable identified in X. This is the direct integer, float or string value stored, it is not a reference to the internal variable structure.
*`POINTERVAR(X)` : Return the address in memory of the variable X. This is the address of the internal `akbasic_Variable` structure, which includes additional metadata about the variable, in addition to the value. For a pointer directly to the value, use `POINTER`.
*`RAD(X#|X%)`: Convert degrees to radians
*`RIGHT(X$, Y#)`: Return the rightmost Y# characters of the string in X$. Y# is clamped to LEN(X$).
*`SGN(X#)`: Returns the sign of X# (-1 for negative, 1 for positive, 0 if 0).
*`SHL(X#, Y#)`: Returns the value of X# shifted left Y# bits
*`SHR(X#, Y#)`: Returns the value of X# shifted right Y# bits
*`SIN(X#|X%)`: Returns the sine of the float or integer argument. Input and output are radians.
*`SPC(X#)`: Returns a string of X# spaces. This is included for compatibility, you can also use `(" " * X)` to multiply strings.
*`STR(X#)`: Returns the string representation of X.
*`TAN(X#|X%)`: Returns the tangent of the float or integer variable X. Input and output are in radians.
*`VAL(X$)`: Returns the float value of the number in X$
*`XOR(X#, Y#)`: Performs a bitwise exclusive OR on the two integer arguments
Unlike the Go version, none of these are bootstrapped by running a BASIC program of `DEF` statements through the interpreter at startup. A builtin's name, arity and handler are one row in the dispatch table in `src/verbs.c`, which means the interpreter no longer has to be running before the interpreter is ready.
## Subroutines
In addition to `DEF`, `GOTO` and `GOSUB`, this BASIC also implements subroutines that accept arguments, return a value, and can be called as functions. Example
```
10 DEF ADDTWO(A#, B#)
20 C# = A# + B#
30 RETURN C#
40 D# = ADDTWO(3, 5)
50 PRINT D#
```
Subroutines must be defined before they are called. Subroutines share the global variable scope with the rest of the program.
# Embedding the interpreter
The whole point of the rewrite. `libakbasic` is a static library with a thin `src/main.c` driver on top; the REPL, argv handling and `QUIT` belong to the driver, not to the library. A host program links the library and keeps control.
Four rules the library holds to, because a game engine cannot tolerate a scripting language that breaks any of them:
* **Nothing terminates the process.** No `exit()`, no `abort()`, no `panic`. Errors come back as `akerr_ErrorContext *`. `FINISH_NORETURN` never appears in the library at all — only in a `main()`, which today means the driver's and the embedding example's.
* **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` source lines and returns. A script with an infinite loop costs you `n` lines per frame and nothing else.
A complete, compiled, runnable example is in [`examples/embed.c`](examples/embed.c) — it is built by every build and registered as a test, so it cannot rot. The shape is:
```c
#include <akerror.h>
#include <akbasic/runtime.h>
#include <akbasic/sink.h>
/*
* The runtime carries every pool the interpreter owns -- a few megabytes -- so
* it goes in static storage or inside your own game state, never on the stack.
`akbasic_sink_init_stdio()` ships with the library and is what the driver and the golden-file suite use. A game supplies its own and draws into a text layer. The interpreter never owns a window, a renderer or an event loop, and `readline` is expected to set `*eof` rather than block — that is how `INPUT` behaves sanely inside a frame.
`akbasic_sink_init_tee()` also ships with the library and composes two sinks into one: writes go to both, and `readline` comes from whichever of the two you name as the reader. That is how the SDL build puts `PRINT` in a window *and* on stdout without the interpreter carrying a second write. It needs no SDL, so a game can use it to log a script's output to a file while it draws.
The library reserves status values **512–767** with `libakerror`'s registry, under the owner string `"akbasic"`. `akbasic_runtime_init()` claims the range for you and is idempotent, so calling it twice is harmless and calling it after your own initialization is fine. If something else in the process already owns part of that band, `init` fails loudly rather than silently aliasing your error codes onto somebody else's.
For reference, the coordinated map across this dependency stack is `libakerror` 0–255, `libakgl` 256–260, and `akbasic` 512–767.
`akbasic` links `akerror::akerror` and `akstdlib::akstdlib` publicly, so you inherit both. If your project already declares those targets, declare them *before* adding this one — the same rule that applies to `libakgl`.
`libakgl` itself is **not** a dependency of the core library. `-DAKBASIC_WITH_AKGL=ON` builds two additional targets, and the split between them is the one that matters if you are embedding this:
| Target | What it is | Link it? |
|---|---|---|
| `akbasic` | The interpreter. No SDL, no window, nothing that terminates the process. | Always. |
| `akbasic_akgl` | The akgl-backed text sink and the graphics, sound and input backends. Every one takes a renderer, a font or a device **you** already created. | If you want the interpreter drawing through your `libakgl` renderer. |
| `akbasic_frontend` | The standalone program's host: it creates the window, opens the font, pumps SDL events and owns the frame loop. | **No.** You are the host. This exists so `basic` can be one. |
Both are off by default, which is why the interpreter and its whole test suite build on a machine with no SDL.
The graphics, sound and console verbs reach hardware through records of function pointers on the runtime — `akbasic_GraphicsBackend`, `akbasic_AudioBackend` and `akbasic_InputBackend`, attached with `akbasic_runtime_set_devices()`. All three may be `NULL`, which is what a default build of the standalone driver gives them: a verb that needs a device it was not given raises rather than crashing, and a `PRINT`-only program never notices. An SDL build attaches all three. A host that renders some other way supplies its own records and never links `libakgl` at all.
Two things about those verbs that differ from a real C128, because both would otherwise surprise you. `PLAY` does not block — it queues its notes and `akbasic_runtime_step()` releases them against whatever time you last passed to `akbasic_runtime_settime()`, so the statement after a `PLAY` runs immediately. And `GETKEY` holds the program without blocking you: the step still returns, it simply does not advance until a key arrives.
Everything is bounded and pre-declared. The numbers are in `include/akbasic/types.h` and are the Go version's, plus three the Go version did not need because it called `make()`:
| Constant | Value | What it bounds |
|---|---|---|
| `AKBASIC_MAX_LEAVES` / `_TOKENS` | 32 | AST nodes and tokens per source line (~16 operations) |
| `AKBASIC_MAX_VALUES` | 64 | Intermediate values per line |
| `AKBASIC_MAX_VARIABLES` | 128 | Variables per scope |
| `AKBASIC_MAX_SOURCE_LINES` | 9999 | Program length |
| `AKBASIC_MAX_LINE_LENGTH` | 256 | Characters per line |
| `AKBASIC_MAX_STRING_LENGTH` | 256 | Characters in a string value |
| `AKBASIC_MAX_ENVIRONMENTS` | 32 | Nesting depth of `FOR` and `GOSUB` |
| `AKBASIC_MAX_ARRAY_ELEMENTS` | 1024 | Elements in one array |
| `AKBASIC_MAX_ARRAY_VALUES` | 4096 | Array elements across all variables |
Raising any of them costs BSS and nothing else. An `akbasic_Runtime` is presently **10.1MB**, and it is worth knowing where that goes before you reach for a knob: 4.1MB is the environment pool, 2.5MB the 9999-line source table, 2.3MB the function-definition pool and 1.2MB the array value pool. `AKBASIC_MAX_ENVIRONMENTS` is the expensive one — each environment carries its own token, leaf and value arrays — so halving it to 16 saves twice what halving the source table does.
# What Isn't Implemented / Isn't Working
## Defects inherited from the Go version
These are reproduced deliberately, not fixed. The Go version's test corpus is the acceptance suite, so a silent correction here is a behaviour change that would show up as a failing golden file. Each is catalogued in [`TODO.md`](TODO.md) section 6 with the file and line, and `tests/known_reference_defects.c` asserts the **correct** contract for six of them under `AKBASIC_KNOWN_FAILING_TESTS` — when one is fixed, CTest reports "unexpectedly passed", which is the cue to move it.
Five of them the port found; nobody knew about these before:
* **`1 - 2 - 3` computes `1 - 2`.** Subtraction stops after one operator where addition loops, so the rest of the line is abandoned in the token stream. This is the bad one — it is a *wrong answer*, not a refused one. `2 ^ 3 ^ 2` has the same shape.
* **A negative literal cannot be passed to a builtin.** `ABS(-9)` is rejected with "function ABS takes 1 arguments, received 2", because the arity counter walks the same `.right` pointer a unary-minus leaf keeps its operand on. Assign to a variable first, as the Go version's own `sgn.bas` test quietly does.
* **A comparison operator in a line's final column is dropped.** `A# =` produces one token, not two, because the scanner cannot peek past the end of the line and gives up without recording the operator.
* **Hex literals do not work.** `0xff` lexes as `0x` followed by an identifier `ff`. The base-16 branch in the literal parser is unreachable, so the hex support the Go README implies has never existed.
* **`PRINT$` is accepted as a variable name.** The "Reserved word in variable name" check compares the lexeme *including* its type suffix against the keyword table, so it never matches and never fires.
Plus six more the original already carried: a leading `0` selects base 8, so `PRINT 010` prints 8 and `PRINT 08` will not parse; `setBoolean` tags the value it builds `TYPE_STRING`; `stopWaiting` ignores its argument, so an inner block can clear an outer block's pending wait; `toString` on a variable has its emptiness test inverted; `EXIT` pops a loop without clearing that wait; and `mathPlus` mutates its left operand in place where every other operator clones. That last one is load-bearing rather than merely wrong — `NEXT` relies on it to advance the loop counter, so it cannot be fixed on its own.
## Not implemented
* Multiple statements on one line (e.g. `10 PRINT A$ : REM This prints the thing`). The `COLON` token exists and nothing consumes it.
* Using an array reference inside a parameter list (e.g. `READ A$(0), B#`) results in parsing errors
One of those is still blocked on a missing `libakgl` capability, and it is only one. Four were — text measurement, immediate-mode drawing, audio, and a non-blocking keystroke read — and rather than work around them here they were filed in [`deps/libakgl/TODO.md`](deps/libakgl/TODO.md) under "API gaps blocking akbasic". All four have since landed upstream (`akgl_text_measure`, the `akgl_draw_*` family, `akgl_audio_*`, and `akgl_controller_poll_key`), so what is left is akbasic-side work.
The exception is `FILTER`, which sets the SID's filter cutoff, band switches and resonance. `akgl_audio_*` synthesises raw waveforms and mixes them; there is no filter stage to configure, and SDL3 supplies no primitive to build one from. Until an `akgl_audio_filter()` exists, `FILTER` parses and then refuses at execution rather than being silently ignored — a program that asks for a low-pass and gets an unfiltered square wave has been lied to.
* [libakstdlib](https://source.starfort.tech/andrew/libakstdlib) 0.2.0 — libc wrappers that report through `libakerror`.
* [libakgl](https://source.starfort.tech/andrew/libakgl) 0.2.0 — **optional**, only for `-DAKBASIC_WITH_AKGL=ON`. Pulls in SDL3, and requires `libakstdlib` 0.2, which is why the two move together.
* [basicinterpret](https://source.starfort.tech/andrew/basicinterpret) — the Go original, vendored as the behavioural spec and the acceptance corpus. Not linked, not built.
Everything is a submodule; `git submodule update --init --recursive` gets all of it. There is nothing to install first.
String-to-number conversion goes straight to `libakstdlib`, which raises on no digits, on trailing junk and on overflow — so `VAL("garbage")` is an error rather than a silent `0`. This repository carried its own `src/convert.c` wrapper until `libakstdlib` 0.2.0 grew that contract; `tests/numeric_contract.c` now pins it from this side, because a regression in it would make four things quietly return zero.
