TEHSSL

I started writing a new programming language, TEHSSL, a few days ago. Starting from scratch (again!) wasn’t easy, and I’m nowhere near done yet. I have got two things working so far: the garbage collector, and the tokenizer. I have no idea how to handle the glue in between (parser, evaluator, macros, builtin functions), or even what the glue will do and how it will work.

Garbage collector

The garbage collector is just a simple mark-and-sweep garbage collector, and I took inspiration from Bob Nystrom’s mark-sweep implementation, which, conveniently, was written in C.

Adding some debug printf()‘s to the code to see what the collector was freeing and when, I hooked up a simple test that created objects and then deliberately did nothing with them, to create garbage, while pushing some to the stack to make them “not garbage.” Here is the code:

tehssl_vm_t vm = tehssl_new_vm();
// Make some garbage
for (int i = 0; i < 5; i++) {
    tehssl_make_number(vm, 123);
    tehssl_make_number(vm, 456.789123);
    tehssl_make_string(vm, "i am cow hear me moo");
    tehssl_make_symbol(vm, "Symbol!", SYMBOL_WORD);
    // This is not garbage, it is on the stack now
    tehssl_push(vm, &vm->stack, tehssl_make_number(vm, 1.7E+123));
    tehssl_push(vm, &vm->stack, tehssl_make_string(vm, "Foo123"));
}
printf("%lu objects\n", vm->num_objects);
tehssl_gc(vm);
printf("%lu objects after gc\n", vm->num_objects);
tehssl_destroy(vm);

And here is the output:

16 objects
Freeing a SYMBOL name-> "Symbol!"
Freeing a STRING name-> "i am cow hear me moo"
Freeing a NUMBER number-> 456.789
Freeing a NUMBER number-> 123
12 objects after gc
Freeing a LIST
Freeing a LIST
Freeing a LIST
Freeing a LIST
Freeing a LIST
Freeing a LIST
Freeing a LIST
Freeing a LIST
Freeing a LIST
Freeing a STRING name-> "Foo123"
Freeing a LIST
Freeing a NUMBER number-> 1.7e+123

Everything that comes after the 12 objects after gc mark is what happens when the entire VM is freed (i.e. the tehssl_destroy() call). tehssl_destroy() simply marks everything as garbage, and then garbage collects – that way all objects are freed, and all that’s left to do is free the VM itself.

This output also demonstrates TEHSSL’s string and number interning; since strings and numbers are immutable, it only has to store one of each different value in use and just point to it multiple times. I don’t know how this affects the speed of TEHSSL, but that’s not something I’m terribly concerned about: if any routine is terribly speed-critical, it can just be written in C!

Tokenizer

TEHSSL tokens fall into one of three categories: a paren (square, round, or curly), a semicolon, or a word. The tokenizer reads from a C FILE* pointer to allow for files and stdin and custom streams to be used as input irrespective of what; and returns the next token in the stream in a dynamic malloc()‘ed string. I hope Arduino uses GNU C extensions so I can use fopencookie() to create wrappers for common protocols such as I2C, SPI, Serial, etc.

Here’s the tokenizer test code. It outputs the tokens in between backticks (` `) so I could paste them into this Markdown document.

const char* str = "Def Myfunction as {\n    ~~Hello world! I am in a block!\n    DoSomething with: \\a-symbol and a number 45678;\n    Print \"Did something!!\";push a block{Foo bar Baz};\n    and then Do it\n}\n";
printf("%s", str);
FILE* s = fmemopen((void*)str, strlen(str), "r");
char* token = NULL;
while (!feof(s)) {
    token = tehssl_next_token(s);
    if (token == NULL) { printf("**ERR!**"); break; }
    if (strlen(token) == 0) { printf("**EOF.**"); free(token); break; }
    printf("`%s` ", token);
    free(token);
}
fclose(s);

For reference, here’s what the tokenizer is tokenizing, without the C backslash escapes:

Def Myfunction as {
    ~~Hello world! I am in a block!
    DoSomething with: \a-symbol and a number 45678;
    Print "Did something!!";push a block{Foo bar Baz};
    and then Do it
}

And it produced these tokens: Def Myfunction { DoSomething with: \a-symbol 45678 ; Print "Did something!! ; { Foo Baz } ; Do } EOF.

5 things are happenning here:

1. The comment on line 2 is discarded entirely.
2. The parens and semicolons are being separated out into their own tokens.
3. All of the lowercase words are being discareded, because they are “informal syntax” that are treated as comments.
4. The with: is not discarded, because it is a keyword property that actually means something.
5. The string token ("Did something!!) doesn’t have the closing quote. This is so the parser (which reads the tokens) can look at the first character to determine the type of symbol, and in the case of a string, it can just slice off the first character without having to blank out the last as well (which is considerably harder in C than in another lnaguage such as Python) to get the raw string contents.

TEHSSL is more than just a grabage colelctor and a tokenizer, but that’s all I have written at this moment. But apparently it looks promising, because David Kobalia starred TEHSSL on GitHub yesterday!

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