* The Sex language
Sex is a S-expressions language, which transpiles to C.

Sex is also Chicken, since all source processing and compile-time
computations are written in Chicken.

And Chicken is [[https://call-cc.org][R5RS Scheme]].

* Compilation and usage
First, get yourself a Chicken. Second, some Chicken deps.

** Install Chicken Eggs
By the way, there's a way to make Chicken install eggs non-globally. Refer to
the documentation for more info:
https://wiki.call-cc.org/man/5/Extension%20tools#changing-the-repository-location

~chicken-install fmt getopt-long brev-separate~

** Compilation
~make~

** Usage
You'll also need a C compiler, so pick any.
#+begin_src
cat hello-world.sex | sexc > hello_world.c
cc hello_world.c -o hello_world
#+end_src

* Example
Here is an example, demonstrating what Sex source looks like, and what
it compiles too. An avid reader also shall notice how we call Chicken
procedures in Sex source.

The Sex source:
#+begin_src
(include stdio.h)

(define (foo)
  "Hello from Chicken code!\n")

(pub fn int main ((int argc) (char **argv))
    (puts "Hello from Sex!")
    (var (array char 512) name)
    (puts "What is your name?")
    (scanf "%s" &name)
    (printf "Hello, %s!\n" name)
    (printf ,(foo))
    0)
#+end_src

The resulting C source:
#+begin_src
#include <stdio.h>

int main (int argc, char **argv) {
    puts("Hello from Sex!");
    char name[512];
    puts("What is your name?");
    scanf("%s", &name);
    printf("Hello, %s!\n", name);
    printf("Hello from Chicken code!\n");
    return 0;
}
#+end_src

* Features
** Full C interoperability
Just ~(include "Your/Favourite/Library.h")~ and use it as you would
have is C.

For hardcore fans of traditional Lisp naming convention,
Sex offers automatic unkebabification of all symbols, i.e. no more
ugly ~GL_ARRAY_BUFFER~ s in your code, they may be written in their
proper form: ~GL-ARRAY-BUFFER~.

** Auto typedef for structs
Probably harmless idk. Example:
#+begin_src
(struct foo
  ((float a)
   (int b)))
#+end_src
expands to
#+begin_src
typedef struct foo foo;

struct foo {
    float a;
    int b;
};
#+end_src

** Syntactic templates
Sex has support for template substitutions. Any piece of code can be
templated. Template declarations look like functions: they have a
name, an argument list and a body. When declared template is
encountered during reading of Sex code, its body will udergo syntactic
rewriting using the provided values by the following rules:
1. If the value is a symbol, all arguments in a body are replaced with
   the value, and also all /parts/ of any other symbol equal to the
   value also get replaced.
2. If the value is a non-symbolic form, all arguments in a body are
   replaced with it, but no symbolic substitution is performed.

Formally, template declaration has the following syntax:
#+begin_src
(template (name . substitute-args) . body)
#+end_src

*** Examples:
**** Structure with templated value type
#+begin_src
(template (foo ?T)
  (struct foo-?T
    ((?T value))))

(foo float)
#+end_src
->
#+begin_src
typedef struct foo_float foo_float;

struct foo_float {
    float value;
};
#+end_src

Note that ~?~ at the start of template argument is not syntax, just
convention.

**** Wrapper for checking return codes
#+begin_src
(template (check-sdl-return call message ret-code)
  (if (< 0 call)
      (begin
       (puts message)
       (return ret-code))))

(fn int init ()
  (check-sdl-return
    (SDL-Init SDL-INIT-VIDEO) "Failed to initialize SDL" 1)
...)
#+end_src
->
#+begin_src
static int init () {
    if (0 < SDL_Init(SDL_INIT_VIDEO)) {
        puts("Failed to initialize SDL");
        return 1;
    }
    return 0;
}
#+end_src

**** A bit of everything
#+begin_src
(template (list-T ?T)
  (struct list-?T
    ((?T value)
     ((* list-?T) next))))

(template (list-for-each type list-var elt-var body)
  (var type elt-var (-> list-var value))
  (while (!= (-> list-var next) NULL)
    body
    (= list-var (-> list-var next))
    (= elt-var (-> list-var value))))

; ... somewhere later
(list-T int)

(pub fn void print-list (((const list-int) *l))
     (list-for-each int l v (printf "%d " v))
     (printf "\n"))
#+end_src
Then will be expanded in the following code:
#+begin_src
(typedef struct list_int list_int)
(struct list_int ((int value) ((* list_int) next)))
(%fun void
      print_list
      (((const list_int) *l))
      (%var int v (-> l value))
      (while (!= (-> l next) NULL)
             (printf "%d " v)
             (= l (-> l next))
             (= v (-> l value)))
      (printf "\n"))
#+end_src

And then translated to:
#+begin_src
typedef struct list_int list_int;

struct list_int {
    int value;
    list_int *next;
};

void print_list (const list_int *l) {
    int v = l->value;
    while (l->next != NULL) {
        printf("%d ", v);
        l = l->next;
        v = l->value;
    }
    printf("\n");
}
#+end_src

** Use an established environment for development
As Sex is S-expressions, you always have Emacs with paredit as your
best option.

*** sex-mode.el
To harness the power of sex-mode, add the following lines to your
~$HOME/.config/emacs/init.el~:
#+begin_src
(use-package sex-mode
  :load-path "/path/to/sex"
  :mode ("\\.sex\\'" "\\.seh\\'"))
#+end_src

** COMING SOON?: Polymorphism