## Aocla: the Advent of Code toy language

Aocla (Advent of Code inspired Language) is a toy stack-based programming
language written as an extension of [day 13 Advent of Code 2022 puzzle](https://adventofcode.com/2022/day/13).

This story starts with me doing Advent of Code for the first time in my life. I hadn't written a single line of code for two years, busy, as I was, writing my [sci-fi novel](https://www.amazon.com/Wohpe-English-Rimmel-Salvatore-Sanfilippo-ebook/dp/B0BQ3HRDPF/). I wanted to start coding again, but without a project in my hands, what to do? The AoC puzzles helped quite a lot, at first, but they become repetitive and a bit futile quite soon. After completing day 13, a puzzle about comparing nested lists, I saw many other solutions resorting to `eval`. They are missing the point, I thought. To me, the puzzle seemed a hint at writing parsers for nested objects.

Now, a nice fact about parsers of lists with integers and nested
lists is that they are dangerously near to become interpreters of Lisp-alike or FORTH-alike programming languages.

The gentle reader should be aware that I've a soft spot for [little languages](http://oldblog.antirez.com/page/picol.html). However, Picol was too much of a toy, while [Jim](http://jim.tcl.tk/index.html/doc/www/www/index.html) was too big as a coding example. Other than interpreters, I like writing small programs that serve as [examples](https://github.com/antirez/kilo) of how you could design bigger programs, while retaining a manageable size. Don't take me wrong: it's not like I believe my code should be taken as an example, it's just that I learned a lot from such small programs, so, from time to time, I like writing new ones, and while I'm at it I share them in the hope somebody could be interested. This time I wanted to obtain something of roughly the size of the Kilo editor, that is around ~1000 lines of code, showing the real world challenges arising when writing an actual interpreter for a programming language more complex than Picol. That's the result, and as a side effect I really started programming again: after Aocla I wrote more and more code, and now [I've a new project, too](https://github.com/antirez/protoview).

## Let's start

This README will first explain the language briefly. Later we will talk extensively about the implementation and its design. Without counting comments, the Aocla implementation is shorter than 1000 lines of code, and the core itself is around 500 lines (the rest of the code is the library implementation, the REPL, and other accessory parts). I hope you will find the code easy to follow, even if you are not used to C and to writing interpreters. I tried to keep all simple, as I always do when I write code, for myself and the others having the misfortune of reading or modifying it in the future.

Not every feature I desired to have is implemented, and certain data types, like the string type, lack any useful procedure to work with them. This choice was made in order to avoid making the source code more complex than needed, and also, on my side, to avoid writing too much useless code, given that this language will never be used in the real world. Besides, implementing some of the missing parts is a good exercise for the willing reader, assuming she or he are new to this kind of stuff. Even with all this limitations, it is possible to write small working programs with Aocla, and that's all we need for our goals.

## Aocla overview

Aocla is a very simple language, more similar to Joy than to FORTH (higher level). It has a total of six datatypes:

* Lists: `[1 2 3 "foo"]`
* Symbols: `mysymbol`, `==` or `$x`
* Integers: `500`
* Booleans: `#t` or `#f`
* Tuples: `(x y z)`
* Strings: `"Hello World!\n"`

Floating point numbers are not provided for simplicity (writing an implementation should not be too hard, and is a good exercise). Aocla programs are valid Aocla lists, so the language is [homoiconic](https://en.wikipedia.org/wiki/Homoiconicity). While Aocla is a stack-based language, like FORTH, Joy and Factor, it introduces the idea of *local variables capturing*. Because of this construct, Aocla programs look a bit different (and simpler to write and understand in my opinion) compared to other stack-based languages. Locals capturing is optional: any program using locals can be rewritten to avoid using them, yet the existence of this feature deeply affects the language in many ways.

## Our first program

The following is a valid Aocla program, taking 5 and squaring it, to obtain 25.

    [5 dup *]

Since all the programs must be valid lists, and thus are enclosed between `[` and `]`, both the Aocla CLI (Command Line Interface) and the execution of programs from files are designed to avoid needing the brackets. Aocla will put the program inside `[]` for you, so the above program should be written like that:

    5 dup *

Programs are executed from left to right, *word by word*. If a word is not a symbol nor a tuple, its execution results into pushing its value on the stack. Symbols will produce a procedure call: the symbol name will be looked up in the table of procedures, and if a procedure with a matching name is found, it gets called. So the above program will perform the following steps:

* `5`: the value 5 is pushed on the stack. The stack will contain `(5)`.
* `dup`: is a symbol. A procedure called `dup` is looked up and executed. What `dup` does is to take the top value on the stack and duplicate it, so now the stack will contain `(5 5)`.
* `*`: is another symbol. The procedure is called. It will take the last two elements on the stack, check if they are integers, multiply them together and push the result on the stack. Now the stack will contain `(25)`.

If an Aocla word is a tuple, like `(x y)`, its execution has the effect of removing a corresponding number of elements from the stack and binding them to the local variables having the specified names:

    10 20 (x y)

After the above program is executed, the stack will be empty and the local variables x and y will contain 10 and 20.

Finally, if an Aocla word is a symbol starting with the `$` character and a single additional character, the object stored at the specified variable is pushed on the stack. So the program to square 5 we wrote earlier can be rewritten as:

    5 (x) $x $x *

The ability to capture stack values into locals allow to make complex stack manipulations in a simple way, and makes programs more explicit to read and easier to write. Still locals have the remarkable quality of not making the language semantically more complex (if not for a small thing we will cover later -- search `upeval` inside this document if you want to know ASAP, but if you know the Tcl programming language, you already understood from the name, that is similar to Tcl's `uplevel`). In general, while locals help the handling of the stack in the local context of the procedure, words communicate via the stack, so the main advantages of stack-based languages are untouched.

*Note: why locals must have just single letter names? The only reason is to make the implementation of the Aocla interpreter simpler to understand. This way, we don't need to make use of any dictionary data structure. If I would design Aocla to be a real language, I would remove this limitation.*

We said that symbols normally trigger a procedure call. But symbols can also be pushed on the stack like any other value. To do so, symbols must be quoted, with the `'` character at the start.

    'Hello printnl

The `printnl` procedure prints the last element in the stack and also prints a newline character, so the above program will just print `Hello` on the screen. You may wonder what's the point of quoting symbols. After all, you could just use strings, but later we'll see how this is important in order to write Aocla programs that write Aocla programs.

Quoting also works with tuples, so if you want to push the tuple `(a b c)` on the stack, instead of capturing the variables a, b and c, you can write:

    '(a b c) printnl

## Inspecting the stack content

When you start the Aocla interpreter without a file name, it gets executed
in REPL mode (Read Eval Print Loop). You write a code fragment, press enter, the code gets executed and the current state of the stack is shown:

    aocla> 1
    1
    aocla> 2
    1 2
    aocla> [a b "foo"]
    1 2 [a b "foo"]

This way you always know the stack content.
When you execute programs from files, in order to debug their executions you can print the stack content using the `showstack` procedure.

## User defined procedures

Aocla programs are just lists, and Aocla functions are lists bound to a
name. The name is given as a symbol, and the way to bind a list with a
symbol is an Aocla procedure itself. Not special syntax is required.

    [dup *] 'square def

The `def` procedure will bind the list `[dup *]` to the `square` symbol,
so later we can use the `square` symbol and it will call our procedure:

    aocla> 5 square
    25

Calling a symbol that is not bound to any list will produce an error:

    aocla> foobar
    Symbol not bound to procedure: 'foobar' in unknown:0

## Working with lists

Lists are the central data structure of the language: they are used to represent programs and are useful as a general purpose data structure to represent data. So most of the very few built-in procedures that Aocla offers are lists manipulation procedures.

The more direct way to show how to write Aocla programs is probably showing examples via its REPL, so I'll procede in this way.

To push an empty list on the stack, you can use:

    aocla> []
    []

Then it is possible to add elements to the tail or the head of the list using the `<-` and `->` procedures:

aocla> 1 swap ->
[1] 
aocla> 2 swap ->
[1 2] 

Note that these procedures are designed to insert the last element in the
stack into the list that is the penultimate element in the stack, so,
in this specific case, we have to swap the order of the last two elements
on the stack before calling `->`. It is possible to design these procedures
in a different way, that is: to the expect `list, element` on the stack instead
of `element, list`. There is no clear winner: one or the other approach is
better or worse depending on the use case. In Aocla, local variables make
all this less important compared to other stack based languages. It is always
possible to make things more explicit, like in the following example:

    aocla> [1 2 3] 
    [1 2 3] 
    aocla> (l) 4 $l ->
    [1 2 3 4] 
    aocla> (l) 5 $l ->
    [1 2 3 4 5] 

Then, to know how many elements there are in the list, we can use the
`len` procedure, that also works for other data types:

    aocla> ['a 'b 1 2] 
    [a b 1 2] 
    aocla> len
    4 
    aocla> "foo"
    4 "foo" 
    aocla> len
    4 3 

Other useful list operations are the following:

    aocla> [1 2 3] [4 5 6] cat
    [1 2 3 4 5 6]
    aocla> [1 2 3] first
    1
    aocla> [1 2 3] rest
    [2 3]

*Note: cat also works with strings, tuples, symbols.*

There is, of course, map:

    aocla> [1 2 3] [dup *] map
    [1 4 9]

If you want to do something with list elements, in an imperative way, you can use foreach:

    aocla> [1 2 3] [printnl] foreach
    1
    2
    3

There are a few more list procedures. There is `get@` to get a specific element in a given position, `sort`, to sort a list, and if I remember correctly nothing
more about lists. Many of the above procedures are implemented inside the
C source code of Aocla, in Aocla language itself. Others are implemented
in C because of performance concerns or because it was simpler to do so.
For instance, this is the implementation of `foreach`:

    [(l f) // list and function to call with each element.
        $l len (e)  // Get list len in "e"
        0 (j)       // j is our current index
        [$j $e <] [
            $l $j get@  // Get list[j]
            $f upeval   // We want to evaluate in the context of the caller
            $j 1 + (j)  // Go to the next index
        ] while
    ] 'foreach def

As you can see from the above code, Aocla syntax also supports comments.
Anything starting from `//` to the end of the line is ignored.

## Conditionals

Aocla conditionals are just `if` and `ifelse`. There is also a
quite imperative looping construct, that is `while`. You could loop
in the Scheme way, using recursion, but I wanted to give the language
a Common Lisp vibe, where you can write imperative code, too.

The words `if` and `ifelse` do what you could imagine:

    aocla> 5 (a)
    5
    aocla> [$a 2 >] ["a is > 2" printnl] if
    a is > 2

So `if` takes two programs (two lists), one is evaluated to see if it is
true or false. The other is executed only if the first program is true.

The `ifelse` procedure works similarly, but it takes three programs: condition, true-program, false-program:

    aocla> 9 (a)
    aocla> [$a 11 ==] ["11 reached" printnl] [$a 1 + (a)] ifelse
    aocla> [$a 11 ==] ["11 reached" printnl] [$a 1 + (a)] ifelse
    aocla> [$a 11 ==] ["11 reached" printnl] [$a 1 + (a)] ifelse
    11 reached

And finally, an example of while:

    aocla> 10 [dup 0 >] [dup printnl 1 -] while
    10
    9
    8
    7
    6
    5
    4
    3
    2
    1

Or, for a longer but more recognizable program making use of Aocla locals:

    aocla> 10 (x) [$x 0 >] [$x printnl $x 1 - (x)] while
    10
    9
    8
    7
    6
    5
    4
    3
    2
    1

In some way, two programming styles are possible: one that uses the stack
mainly in order to pass state from different procedures, and otherwise
uses locals a lot for local state, and another one where almost everything
will use the stack, like in FORTH. Even in the second case, locals can be
used from time to time when stack manipulation is more clear using them.
For instance Imagine I've three values on the stack:

    aocla> 1 2 3
    1 2 3

If I want to sum the first and the third, and leave the second one
on the stack, even in a programming style where the code mainly uses
the stack to hold state, one could write:

    aocla> (a _ b) $_ $a $b +
    2 4 

## Evaluating lists

Words like `map` or `foreach` are written in Aocla itself. They are not 
implemented in C, even if they could and probably should for performance
reasons (and this is why `while` is implemented in C).

In order to implement procedures that execute code, Aocla provides the
`eval` built-in word. It just consumes the list at the top of the
stack and evaluates it.

    aocla> 5 [dup dup dup] eval
    5 5 5 5

In the above example, we executed the list containing the program that calls
`dup` three times. Let's write a better example, a procedure that executes
the same code a specified number of times:

    [(n l)
        [$n 0 >]
        [$l eval $n 1 - (n)]
        while
    ] 'repeat def

Example usage:

    aocla> 3 ["Hello!" printnl] repeat
    Hello!
    Hello!
    Hello!

## Eval and local variables

There is a problem with the above implementation of `repeat`, it does
not mix well with local variables. The following program will not have the expected behavior:

    aocla> 10 (x) 3 [$x printnl] repeat
    Unbound local var: '$x' in eval:0  in unknown:0

Here the problem is that once we call a new procedure, that is `repeat`,
the local variable `x` no longer exists in the context of the called
procedure. It belongs to the previous procedure, that is, in this specific
case, the *top level* execution stack frame. So when `repeat` evaluates our
program we get an error.

This is the only case where Aocla local variables make the semantics of
Aocla more complex than other stack based languages without this feature.
In order to solve the problem above, Aocla has a special form of
`eval` called `upeval`: it executes a program in the context
(again, stack frame, in low level terms) of the caller. Let's rewrite
the `repeat` procedure using `upeval`:

    [(n l)
        [$n 0 >]
        [$l upeval $n 1 - (n)]
        while
    ] 'repeat def

After the change, it works as expected:

    aocla> 10 (x) 3 [$x printnl] repeat
    10
    10
    10

Now, out of the blue, without even knowing how Aocla is implemented,
let's check the C implementation of `uplevel`:

    /* Like eval, but the code is evaluated in the stack frame of the calling
     * procedure, if any. */
    int procUpeval(aoclactx *ctx) {
        if (checkStackType(ctx,1,OBJ_TYPE_LIST)) return 1;
        obj *l = stackPop(ctx);
        stackframe *saved = NULL;
        if (ctx->frame->prev) {
            saved = ctx->frame;
            ctx->frame = ctx->frame->prev;
        }
        int retval = eval(ctx,l);
        if (saved) ctx->frame = saved;
        release(l);
        return retval;
    }

What happens here is quite clear: we check to see if the stack contains
a list, as top level element. If so, we capture that value in the variable
`l`, then save the current stack frame, that contains our local variables
for the current procedure, and substitute it with the *previous procedure*
stack frame. Now we can call `eval()` and finally restore the original
stack frame.

## Creating programs at runtime

Aocla is homoiconic, as we already said earlier. Programs are
represented with the same data structures that Aocla code can manipulate.
Because of that, we can write programs writing programs. For instance let's
create a program that creates a procedure incrementing a variable of
the specified name.

The procedure expects two objects on the stack: the name of the procedure
we want to create, and the variable name that the procedure will increment. Two symbols, basically:

    proc-name, var-name

This is the listing of the procedure. Even if each line is commented, being
written in a language that you didn't know until ten minutes ago, and even
a strange enough language, you may want to carefully read each word it is
composed of.

    [ (p v) // Procedure, var.
        []                      // Accumulate our program into an empty list
        '$ $v cat swap ->       // Push $<varname> into the stack
        1 swap ->               // Push 1
        '+ swap ->              // Call +
        $v [] -> make-tuple swap -> // Capture back value into <varname>
        [] ->                       // Put all into a nested list
        'upeval swap ->             // Call upeval against the program
        $p def // Create the procedure  // Bind to the specified proc name
    ] 'create-incrementing-proc def

Basically calling `create-incrementing-proc` will end generating
a list like that (you can check the intermediate results by adding
`showstack` calls in your programs):

    [[$x 1 + (x)] upeval]

And finally the list is bound to the specified symbol using `def`.

*Note: programs like the above show that, after all, maybe the `->` and `<-` operators should expect the arguments in reverse order. Maybe I'll change my mind.*

Certain times, programs that write programs can be quite useful. They are a
central feature in many Lisp dialects. However in the specific case of
Aocla, different procedures can be composed via the stack, and we also
have `uplevel`, so I feel their usefulness is greatly reduced. Also note
that if Aocla was a serious language, it would have a lot more constructs
to make writing programs that write programs a lot simpler than the above. Anyway, as you saw earlier, when we implemented the `repeat` procedure, in Aocla
it is possible to do interesting stuff without using this programming paradigm.

Ok, I think that's enough. We saw the basic of stack languages, the specific
stuff Aocla adds and how the language feels like. This isn't a course
on stack languages, nor I would be the best person to talk about the
argument. This is a course on how to write a small interpreter in C, so
let's dive into the Aocla interpreter internals.

# From puzzle 13 to Aocla

At the start of this README I told you Aocla started from an Advent of
Code puzzle. The Puzzle could be solved by parsing the literal representation
of lists like the one below, and then writing a comparison function for
the the list internal representation (well, actually this is how I solved it,
but one could even take the approach of comparing *while* parsing,
probably). This is an example of such lists:

        [1,[2,[3,[4,[5,6,7]]]],8,9]

Parsing flat lists is not particularly hard, however this is
not a single-level object. It has elements that are sub-lists. So
a recursive parser was the most obvious solution. This is what I wrote
back then, the 13th of December:

    /* This describes our elf object type. It can be used to represent
     * nested lists of lists and/or integers. */
    #define ELFOBJ_TYPE_INT  0
    #define ELFOBJ_TYPE_LIST 1
    typedef struct elfobj {
        int type;       /* ELFOBJ_TYPE_... */
        union {
            int i;      /* Integer value. */
            struct {    /* List value. */
                struct elfobj **ele;
                size_t len;
            } l;
        } val;
    } elfobj;

Why `elfobj`? Well, because it was Christmas and AoC is about elves.
The structure above is quite trivial, just two types and a union in order
to represent both types.

Let's see the parser, that is surely more interesting.

    /* Given the string 's' return the elfobj representing the list or
     * NULL on syntax error. '*next' is set to the next byte to parse, after
     * the current value was completely parsed. */
    elfobj *parseList(const char *s, const char **next) {
        elfobj *obj = elfalloc(sizeof(*obj));
        while(isspace(s[0])) s++;
        if (s[0] == '-' || isdigit(s[0])) {
            char buf[64];
            size_t len = 0;
            while((*s == '-' || isdigit(*s)) && len < sizeof(buf)-1)
                buf[len++] = *s++;
            buf[len] = 0;
            obj->type = ELFOBJ_TYPE_INT;
            obj->val.i = atoi(buf);
            if (next) *next = s;
            return obj;
        } else if (s[0] == '[') {
            obj->type = ELFOBJ_TYPE_LIST;
            obj->val.l.len = 0;
            obj->val.l.ele = NULL;
            s++;
            /* Parse comma separated elements. */
            while(1) {
                /* The list may be empty, so we need to parse for "]"
                 * ASAP. */
                while(isspace(s[0])) s++;
                if (s[0] == ']') {
                    if (next) *next = s+1;
                    return obj;
                }

                /* Parse the current sub-element recursively. */
                const char *nextptr;
                elfobj *element = parseList(s,&nextptr);
                if (element == NULL) {
                    freeElfObj(obj);
                    return NULL;
                }
                obj->val.l.ele = elfrealloc(obj->val.l.ele,
                                            sizeof(elfobj*)*(obj->val.l.len+1));
                obj->val.l.ele[obj->val.l.len++] = element;
                s = nextptr; /* Continue from first byte not parsed. */

                while(isspace(s[0])) s++;
                if (s[0] == ']') continue; /* Will be handled by the loop. */
                if (s[0] == ',') {
                    s++;
                    continue; /* Parse next element. */
                }

                /* Syntax error. */
                freeElfObj(obj);
                return NULL;
            }
            /* Syntax error (list not closed). */
            freeElfObj(obj);
            return NULL;
        } else {
            /* In a serious program you don't printf() in the middle of
             * a function. Just return NULL. */
            fprintf(stderr,"Syntax error parsing '%s'\n", s);
            return NULL;
        }
        return obj;
    }

OK, what are the important parts of the above code? First: the parser is,
as I already said, recursive. To parse each element of the list we call
the same function again and again. This will make the magic of handling
any complex nested list without having to do anything special. I know, I know.
This is quite obvious for experienced enough programmers, but I claim it
is still kinda of a revelation, like a Mandelbrot set, like standing with a
mirror in front of another mirror admiring the infinite repeating images one
inside the other. Recursion remains magic even after it is understood.

The second point to note: the function gets a pointer to a string, and returns
the object parsed and also, by referene, the pointer to the start of the *next*
object to parse, that is just at some offset inside the same string.
This is a very comfortable way to write such a parser: we can call the same
function again to get the next object in a loop to parse all the tokens and
sub-tokens. And I'm saying tokens for a reason, because the same exact
structure can be used also when writing tokenizers that just return tokens
one after the other, without any conversion to object.

Now, what I did was to take this program and make it the programming language
you just learned about in the first part of this README. How? Well, to
start I upgraded the object structure for more complex object types:

    /* Type are defined so that each type ID is a different set bit, this way
     * in checkStackType() we may ask the function to check if some argument
     * is one among a list of types just bitwise-oring the type IDs together. */
    #define OBJ_TYPE_INT    (1<<0)
    #define OBJ_TYPE_LIST   (1<<1)
    #define OBJ_TYPE_TUPLE  (1<<2)
    #define OBJ_TYPE_STRING (1<<3)
    #define OBJ_TYPE_SYMBOL (1<<4)
    #define OBJ_TYPE_BOOL   (1<<5)
    #define OBJ_TYPE_ANY    INT_MAX /* All bits set. For checkStackType(). */
    typedef struct obj {
        int type;       /* OBJ_TYPE_... */
        int refcount;   /* Reference count. */
        int line;       /* Source code line number where this was defined, or 0. */
        union {
            int i;      /* Integer. Literal: 1234 */
            int istrue; /* Boolean. Literal: #t or #f */
            struct {    /* List or Tuple: Literal: [1 2 3 4] or (a b c) */
                struct obj **ele;
                size_t len;
                int quoted; /* Used for quoted tuples. Don't capture vars if true.
                               Just push the tuple on stack. */
            } l;
            struct {    /* Mutable string & unmutable symbol. */
                char *ptr;
                size_t len;
                int quoted; /* Used for quoted symbols: when quoted they are
                               not executed, but just pushed on the stack by
                               eval(). */
            } str;
        };
    } obj;

A few important things to note, since this may look like just a trivial extension of the original puzzle structure, but it's not:

1. We now use reference counting. When the object is allocated, it gets a *refcount* of 1. Then the functions `retain()` and `release()` are used in order to increment the reference count when we store the same object elsewhere, or when we want to remove a reference. Finally, when the references drop to zero, the object gets freed.
2. The object types now are all powers of two: single bits, in binary representation. This means we can store or pass multiple types at once in a single integer, just performing the *bitwise or*. It is useful in practice. No need for functions with a variable number of arguments just to pass many times at once.
3. There is information about the line number where a given object was defined in the source code. Aocla can be a toy, but a toy that will try to give you some stack trace if there is a runtime error.

This is the release() function.

    /* Recursively free an Aocla object, if the refcount just dropped to zero. */
    void release(obj *o) {
        if (o == NULL) return;
        assert(o->refcount >= 0);
        if (--o->refcount == 0) {
            switch(o->type) {
            case OBJ_TYPE_LIST:
            case OBJ_TYPE_TUPLE:
                for (size_t j = 0; j < o->l.len; j++)
                    release(o->l.ele[j]);
                free(o->l.ele);
                break;
            case OBJ_TYPE_SYMBOL:
            case OBJ_TYPE_STRING:
                free(o->str.ptr);
                break;
            default:
                break;
                /* Nothing special to free. */
            }
            free(o);
        }
    }

Note that in this implementation, deeply nested data structures will produce many recursive calls. This can be avoided using *lazy freeing*, but that's not needed for something like Aocla. However some reader may want to search *lazy freeing* on the web.

Thanks to our parser (that is just a more complex version of the initial day 13 puzzle parser, and is not worth showing here), we can take an Aocla program, in the form of a string, parse it and get an Aocla object (`obj*` type) back. Now, in order to run an Aocla program, we have to *execute* this object. Stack based languages are particularly simple to execute: we just go form left to right, and depending on the object type, we do different actions:

* If the object is a symbol (and is not quoted, see the `quoted` field in the object structure), we try to lookup a procedure with that name, and if it exists we execute the procedure. How? By recursively executing the list bound to the symbol.
* If the object is a tuple with single characters elements, we capture the variables on the stack.
* If it's a symbol starting with `$` we push the variable on the stack, and if the variable is not bound, we raise an error.
* For any other type of object, we just push it on the stack.

The function responsible to execute the program is called `eval()`, and is so short we can put it fully here, but I'll present the function split in different parts, to explain each one carefully. I will start showing just the first three lines, as they already tell us something.

    int eval(aoclactx *ctx, obj *l) {
        assert (l->type == OBJ_TYPE_LIST);

        for (size_t j = 0; j < l->l.len; j++) {

Here there are three things going on. Eval() takes a context and a list. The list is our program, and it is scanned left-to-right, as Aocla programs are executed left to right, word by word. All should be clear but the context. What is an execution context for our program?

    /* Interpreter state. */
    #define ERRSTR_LEN 256
    typedef struct aoclactx {
        size_t stacklen;        /* Stack current len. */
        obj **stack;
        aproc *proc;            /* Defined procedures. */
        stackframe *frame;      /* Stack frame with locals. */
        /* Syntax error context. */
        char errstr[ERRSTR_LEN]; /* Syntax error or execution error string. */
    } aoclactx;

It contains the following elements:
1. The stack. Aocla is a stack based language, so we need a stack where to push and pop Aocla objects.
2. A list of procedures: lists bound to symbols, via the `def` word.
3. A stack frame, that is just what contains our local variables:

    /* We have local vars, so we need a stack frame. We start with a top level
     * stack frame. Each time a procedure is called, we create a new stack frame
     * and free it once the procedure returns. */
    #define AOCLA_NUMVARS 256
    typedef struct stackframe {
        obj *locals[AOCLA_NUMVARS];/* Local var names are limited to a,b,c,...,z. */
        aproc *curproc;            /* Current procedure executing or NULL.  */
        int curline;               /* Current line number during execution. */
        struct stackframe *prev;   /* Upper level stack frame or NULL. */
    } stackframe;

The stack frame has a pointer to the previous stack frame. This is useful both in order to implement `upeval` and to show a stack trace when an exception happens and the program is halted.

We can continue looking at the remaining parts of eval() now. We stopped at the `for` loop, so now we are inside the iteration doing something with each element of the list:

    obj *o = l->l.ele[j];
    aproc *proc;
    ctx->frame->curline = o->line;

    switch(o->type) {
    case OBJ_TYPE_TUPLE:                /* Capture variables. */
        /* Quoted tuples just get pushed on the stack, losing
         * their quoted status. */
        if (o->l.quoted) {
            obj *notq = deepCopy(o);
            notq->l.quoted = 0;
            stackPush(ctx,notq);
            break;
        }

        if (ctx->stacklen < o->l.len) {
            setError(ctx,o->l.ele[ctx->stacklen]->str.ptr,
                "Out of stack while capturing local");
            return 1;
        }

        /* Bind each variable to the corresponding locals array,
         * removing it from the stack. */
        ctx->stacklen -= o->l.len;
        for (size_t i = 0; i < o->l.len; i++) {
            int idx = o->l.ele[i]->str.ptr[0];
            release(ctx->frame->locals[idx]);
            ctx->frame->locals[idx] =
                ctx->stack[ctx->stacklen+i];
        }
        break;

The essence of the loop is a `switch` statement doing something different depending on the object type. The object is just the current element of the list. The first case is the tuple. Tuples capture local variables, unless they are quoted like this:

    (a b c)  // Normal tuple -- This will capture variables
    `(a b c) // Quoted tuple -- This will be pushed on the stack

So if the tuple is not quoted, we check if there are enough stack elements
according to the tuple length. Then, element after element, we move objects
from the Aocla stack to the stack frame, into the array representing the locals. Note that there could be already an object bound to a given local, so we `release()` it before the new assignment.

    case OBJ_TYPE_SYMBOL:
        /* Quoted symbols don't generate a procedure call, but like
         * any other object they get pushed on the stack. */
        if (o->str.quoted) {
            obj *notq = deepCopy(o);
            notq->str.quoted = 0;
            stackPush(ctx,notq);
            break;
        }

        /* Not quoted symbols get looked up and executed if they
         * don't start with "$". Otherwise are handled as locals
         * push on the stack. */
        if (o->str.ptr[0] == '$') {     /* Push local var. */
            int idx = o->str.ptr[1];
            if (ctx->frame->locals[idx] == NULL) {
                setError(ctx,o->str.ptr, "Unbound local var");
                return 1;
            }
            stackPush(ctx,ctx->frame->locals[idx]);
            retain(ctx->frame->locals[idx]);

For symbols, as we did for tuples, we check if the symbol is quoted, an in such case we just push it on the stack. Otherwise, we handle two different cases. The above is the one where symbol names start with a `$`. It is, basically, the reverse operation of what we saw earlier in tuples capturing local vars. This time the local variable is transferred to the stack. However **we still take the reference** in the local variable array, as the program may want to push the same variable again and again, so, after pushing the object on the stack, we have to call `retain()` to increment the reference count of the object.

If the symbol does not start with `$`, then it's a procedure call:

    } else {                        /* Call procedure. */
        proc = lookupProc(ctx,o->str.ptr);
        if (proc == NULL) {
            setError(ctx,o->str.ptr,
                "Symbol not bound to procedure");
            return 1;
        }
        if (proc->cproc) {
            /* Call a procedure implemented in C. */
            aproc *prev = ctx->frame->curproc;
            ctx->frame->curproc = proc;
            int err = proc->cproc(ctx);
            ctx->frame->curproc = prev;
            if (err) return err;
        } else {
            /* Call a procedure implemented in Aocla. */
            stackframe *oldsf = ctx->frame;
            ctx->frame = newStackFrame(ctx);
            ctx->frame->curproc = proc;
            int err = eval(ctx,proc->proc);
            freeStackFrame(ctx->frame);
            ctx->frame = oldsf;
            if (err) return err;
        }
    }

The `lookupProc()` function just scans a linked list of procedures
and returns a list object or, if there is no such procedure defined, NULL.
Now what happens immediately after is much more interesting. Aocla procedures
are just list objects, but it is possible to implement Aocla procedures
directly in C. If the `cproc` is not NULL, then it is a C function pointer
implementing a procedure, otherwise the procedure is *user defined*, that menas it is written in Aocla, and we need to evaluate it. We do this with a nested `eval()` call. As you can see, recursion is crucial in writing interpreters.

*A little digression: if we would like to speedup procedure call, we could cache the procedure lookup directly inside the symbol object. However in Aocla procedures can be redefined, so the next time the same procedure name may be bound to a different procedure. To still cache lookedup procedures, a simple way is to use the concept of "epoch". The context has a 64 bit integer called epoch, that is incremented every time a procedure is redefined. So, when we cache the procedure lookup into the object, we also store the current value of the epoch. Then, before using the cached value, we check if the epoch much. If there is no match, we perform the lookup again, and update the cached procedure and the epoch.*

Sorry, let's go back to our `eval` function. Another important thing that's worth noting is that each new Aocla procedure call has its own set of local variables. The scope of local variables, in Aocla, is the lifetime of the procedure call, like in many other languages. So, in the code above, before calling an Aocla procedure we allocate a new stack frame using `newStackFrame()`, then we can finally call `eval()`, free the stack frame and store the old stack frame back in the context structure. Procedures implemented in C don't need a stack frame, as they will not make any use of Aocla local variables. The following is the last part of the `eval()` function implementation:

    default:
        stackPush(ctx,o);
        retain(o);
        break;

This is the default behavior for all the other object types. They get pushed on the stack, and that's it.

Let's see how Aocla C-coded procedures are implemented, by observing the
C function implementing basic mathematical operations such as +, -, ...

    /* Implements +, -, *, %, ... */
    int procBasicMath(aoclactx *ctx) {
        if (checkStackType(ctx,2,OBJ_TYPE_INT,OBJ_TYPE_INT)) return 1;
        obj *b = stackPop(ctx);
        obj *a = stackPop(ctx);

        int res;
        const char *fname = ctx->frame->curproc->name;
        if (fname[0] == '+' && fname[1] == 0) res = a->i + b->i;
        if (fname[0] == '-' && fname[1] == 0) res = a->i - b->i;
        if (fname[0] == '*' && fname[1] == 0) res = a->i * b->i;
        if (fname[0] == '/' && fname[1] == 0) res = a->i / b->i;
        stackPush(ctx,newInt(res));
        release(a);
        release(b);
        return 0;
    }

Here I cheated: the code required to implement each math procedure separately would be almost the same. So we bind all the operators to the same C function, and check the name of the procedure called inside a single implementation (see the above function). Here is where we register many procedures to the same C function.

    void loadLibrary(aoclactx *ctx) {
        addProc(ctx,"+",procBasicMath,NULL);
        addProc(ctx,"-",procBasicMath,NULL);
        addProc(ctx,"*",procBasicMath,NULL);
        addProc(ctx,"/",procBasicMath,NULL);
        ...

The `procBasicMath()` is quite self-documenting, I guess. The proof of that
is that I didn't add any comment inside the function. When comments are needed, I add them automatically, I can't help myself. Anyway, what it does is
the following: it checks the type of the top objects on the stack, as they
must be integers. Get them with `stackPop()`, perform the math, push a new integer object, release the two old ones. That's it.

## Deep copy of objects

Believe it or not, that's it: you already saw all the most important
parts of the Aocla interpreter. But there are a few corner cases that
are forth a few more paragraphs of this README.

Imagine the execution of the following Aocla program:

    [1 2 3] (x)	// The varialbe x contains the list now
    4 $x ->		// Now the stack contains the list [1 2 3 4]
    $x		// What will be x now? [1 2 3] or [1 2 3 4]?

Aocla is designed to be kinda of a *pure* language: words manipulate
objects by taking them from the stack and pushing new objects to the
stack, that result from certain operations. We don't want to expose the
idea of references in such a language, I feel like that would be a mess,
a design error, and a programming nightmare. So if the variable `x` is
bound to the list `[1 2 3]`, pushing it to the stack and adding new
elements to the list should **not produce changes** to the list stored
at `x`.

At the same time, we don't want to write an inefficient crap where each
value is copied again and again. When we push our variable content on
the stack, we just push the pointer to the object and increment the reference
count. In order to have the best of both worlds, we want to implement something
called *copy on write*. So normally our objects can be shared, and thanks
to the count of references we know if and object is shared or not.
However, as soon as some operation is going to alter an object whose
reference count is greater than one, it gets copied first, only later modified.

In the above program, the list reference count is 2, because the same list
is stored in the array of local variables and in the stack. Let's
give a look at the implementation of the `->` operator:

    /* Implements -> and <-, appending element x in list with stack
     *
     * (x [1 2 3]) => ([1 2 3 x]) | ([x 1 2 3])
     *
     * <- is very inefficient as it memmoves all N elements. */
    int procListAppend(aoclactx *ctx) {
        int tail = ctx->frame->curproc->name[0] == '-';     /* Append on tail? */
        if (checkStackType(ctx,2,OBJ_TYPE_ANY,OBJ_TYPE_LIST)) return 1;
        obj *l = getUnsharedObject(stackPop(ctx));
        obj *ele = stackPop(ctx);
        l->l.ele = myrealloc(l->l.ele,sizeof(obj*)*(l->l.len+1));
        if (tail) {
            l->l.ele[l->l.len] = ele;
        } else {
            memmove(l->l.ele+1,l->l.ele,sizeof(obj*)*l->l.len);
            l->l.ele[0] = ele;
        }
        l->l.len++;
        stackPush(ctx,l);
        return 0;
    }

The interesting line here is the following one:

    obj *l = getUnsharedObject(stackPop(ctx));

We want an object that is not shared, right? This function will abstract
the work for us. Let's check, in turn, its implementation:

    obj *getUnsharedObject(obj *o) {
        if (o->refcount > 1) {
            release(o);
            return deepCopy(o);
        } else {
            return o;
        }
    }

So if the object is already not shared (its *refcount* is one), just return it as it is. Otherwise create a copy and remove a reference from the original object. Why, on copy, we need to remove a reference from the passed object? This may look odd at a first glance, but think at it: the invariant here should be that the caller of this function is the only owner of the object. We want the caller to be able to abstract totally what happens inside the `getUnsharedObject()` function. If the object was shared and we returned the caller a copy, the reference the caller had for the old object should be gone. Let's look at the following example:

    obj *o = stackPop(ctx);
    o = getUnsharedObject(o);
    doSomethingThatChanges(o);
    stackPush(ctx,o);

Stack pop and push functions don't change the reference count of the object,
so if the object is not shared we get it with a single reference, change it,
push it on the stack and the object has still a single reference.

Now imagine that, instead, the object is shared and also lives in a
variable. In this case we pop an object that has two references, call
`getUnsharedObject()` that will return us a copy with a *recount* of one. We
change the object and push it to the stack. The new object will have a
single reference on the stack, and has a reference count of one: all is
fine. What about the old object stored in the local variable? It should
have a reference count of one as well, but if we don't `release()` it
in `getUnsharedObject()` it would have two, causing a memory leak.

I'll not show the `deepCopy()` function, it just allocates a new object of the specified type and copy the content. But guess what? It's a recursive function, too. That's why it is a *deep* copy.

# The end

That's it, and thanks for reading that far. To know more about interpreters you have only one thing to do: write your own, or radically modify Aocla in some crazy way. Get your hands dirty, it's super fun and rewarding. I can only promise that what you will learn will be worthwhile, even if you'll never write an interpreter again.