#+title: Alisp
#+author: Aryadev Chavali
#+date: 2025-08-20
#+filetags: :alisp:

* Notes
** Overview
~alisp.h~ is a single header for the entire runtime. We'll also have a
compiled shared library ~alisp.so~ which one may link against to get
implementation.  That's all that's necessary for one to write C code
that targets our Lisp machine.

We'll have a separate header + library for the compiler since that's
not strictly necessary for transpiled C code to consume.  This will
transpile Lisp code into C, which uses the aforementioned ~alisp~
header and library to compile into a native executable.

** WIP How does transpiled code operate?
My current idea is: we're transpiling into C for the actual Lisp code.
User made functions can be transpiled into C functions, which we can
mangle names for.  Macros... I don't know, maybe we could have two
function pointer tables so we know how to execute them?

Then, we'll have an associated "descriptor" file which describes the
functions we've transpiled.  Bare minimum, this file has to have a
"symbol name" to C mangled function name dictionary.  We can also add
other metadata as we need.

*** TODO Deliberate on whether we compile into a shared library or not
If we compile these C code objects into shared libraries, the
descriptor needs to concern itself with code locations.  This might be
easier in a sense, since the code will already be compiled.
** WIP How do we call native code?
When we're calling a natively compiled function, we can use this
metadata mapping to call the C function.  This native code will use
our Lisp runtime, same as any other code, so it should be pretty
seamless in that regard.  But we'll need to set a calling convention
in order to make calling into this seamless from a runtime
perspective.
* Tasks
** TODO Potentially optimise symbol table ingress :optimisation:design:
Should we capitalise symbols?  This way, we limit the symbol table's
possible options a bit (potentially we could design a better hashing
algorithm?) and it would be kinda like an actual Lisp.
** WIP Test containers constructors and destructors :test:
Test if ~make_vec~ works with ~as_vec~, ~cons~ with ~as_cons~ AND
~CAR~, ~CDR~.

We may need to think of effective ways to deal with NILs in ~car~ and
~cdr~.  Maybe make functions as well as the macros so I can choose
between them?
** TODO Test system registration of allocated units :test:
In particular, does clean up work as we expect?  Do we have situations
where we may double free or not clean up something we should've?
** TODO Design garbage collection scheme :design:gc:
Really, regardless of what I do, we need to have some kind of garbage
collection header on whatever we allocate e.g. references if we
reference count for GC.
*** TODO Mark stage
When some item is being used by another, we need a way to adjust the
metadata such that the system is aware of it being used.

For example, say I have X, Y as random allocated objects.  Then I
construct CONS(X, Y).  Then, ref(X) and ref(Y) need to be incremented
to say I'm using them.
*** TODO Sweep
Say I have an object that I construct, C.  If ref(C) = 0, then C is no
longer needed, and is free.

There are two components to this:
- we need a way of decrementing references if an object is no longer needed.
- we need a way of running through everything we've allocated so far
  to figure out what's free to take away.

Once we've filtered out what we don't need anymore, what should we do
with them?  Naive approach would be to just actually ~free~ the cells
in question.  But I think the next item may be a better idea.
*** TODO Use previous allocations if they're free to use
If we have no references to a cell, this cell is free to use.  In
other words, if I later allocate something of the same type, instead
of allocating a new object, why not just use the one I've already got?

This way, instead of deleting the memory or forgetting about it, we
can reuse it.  We need to be really careful to make sure our ref(X) is
actually precise, we don't want to trample on the user's hard work.

If we implement our "free cells" as a linked list, we'll essentially
need to take items out of it when we decide to set it back up in the
system.  Similarly, if we classify something as unused during the
sweep, we can add it to the free linked list.

Question: should this be separate linked lists for each container type
(i.e. one for conses, one for vectors) or just one big one?  The main
task for these free lists is just "can I get a cell or nah?".  We'll
analyse the time complexity of this task

Former approach time complexity:
- O(1) time to get a free cell since we just need to check the first
  item of the relevant free list (or if it's NIL, we know already)
- O(1) worst case time if there isn't a free cell

Latter approach time complexity:
- Since we have ~get_tag~ it's O(1) time to check the type of the
  container.
- Therefore, it would be worst case O(n) if the cell type we need is
  only at the end of the list, or if there isn't any cell of the type
  we need.

Former approach is better time complexity wise, but latter is way
better in terms of simplicity of code.  Must deliberate.
** DONE Test value constructors and destructors :test:
Test if ~make_int~ works with ~as_int,~ ~intern~ with ~as_sym~.
Latter will require a symbol table.