fe.c

/*
** Copyright (c) 2020 rxi
**
** Permission is hereby granted, free of charge, to any person obtaining a copy
** of this software and associated documentation files (the "Software"), to
** deal in the Software without restriction, including without limitation the
** rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
** sell copies of the Software, and to permit persons to whom the Software is
** furnished to do so, subject to the following conditions:
**
** The above copyright notice and this permission notice shall be included in
** all copies or substantial portions of the Software.
**
** THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
** IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
** FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
** AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
** LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
** FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
** IN THE SOFTWARE.
*/

#include <string.h>
#include "fe.h"

#define unused(x)     ( (void) (x) )
#define car(x)        ( (x)->car.o )
#define cdr(x)        ( (x)->cdr.o )
#define tag(x)        ( (x)->car.c )
#define isnil(x)      ( (x) == &nil )
#define type(x)       ( tag(x) & 0x1 ? tag(x) >> 2 : FE_TPAIR )
#define settype(x,t)  ( tag(x) = (t) << 2 | 1 )
#define number(x)     ( (x)->cdr.n )
#define prim(x)       ( (x)->cdr.c )
#define cfunc(x)      ( (x)->cdr.f )
#define strbuf(x)     ( &(x)->car.c + 1 )

#define STRBUFSIZE    ( (int) sizeof(fe_Object*) - 1 )
#define GCMARKBIT     ( 0x2 )
#define GCSTACKSIZE   ( 256 )


enum {
 P_LET, P_SET, P_IF, P_FN, P_MAC, P_WHILE, P_QUOTE, P_AND, P_OR, P_DO, P_CONS,
 P_CAR, P_CDR, P_SETCAR, P_SETCDR, P_LIST, P_NOT, P_IS, P_ATOM, P_PRINT, P_LT,
 P_LTE, P_ADD, P_SUB, P_MUL, P_DIV, P_MAX
};

static const char *primnames[] = {
  "let", "=", "if", "fn", "mac", "while", "quote", "and", "or", "do", "cons",
  "car", "cdr", "setcar", "setcdr", "list", "not", "is", "atom", "print", "<",
  "<=", "+", "-", "*", "/"
};

static const char *typenames[] = {
  "pair", "free", "nil", "number", "symbol", "string",
  "func", "macro", "prim", "cfunc", "ptr"
};

typedef union { fe_Object *o; fe_CFunc f; fe_Number n; char c; } Value;

struct fe_Object { Value car, cdr; };

struct fe_Context {
  fe_Handlers handlers;
  fe_Object *gcstack[GCSTACKSIZE];
  int gcstack_idx;
  fe_Object *objects;
  int object_count;
  fe_Object *calllist;
  fe_Object *freelist;
  fe_Object *symlist;
  fe_Object *t;
  int nextchr;
};

static fe_Object nil = {{ (void*) (FE_TNIL << 2 | 1) }, { NULL }};


fe_Handlers* fe_handlers(fe_Context *ctx) {
  return &ctx->handlers;
}


void fe_error(fe_Context *ctx, const char *msg) {
  fe_Object *cl = ctx->calllist;
  /* reset context state */
  ctx->calllist = &nil;
  /* do error handler */
  if (ctx->handlers.error) { ctx->handlers.error(ctx, msg, cl); }
  /* error handler returned -- print error and traceback, exit */
  fprintf(stderr, "error: %s\n", msg);
  for (; !isnil(cl); cl = cdr(cl)) {
    char buf[64];
    fe_tostring(ctx, car(cl), buf, sizeof(buf));
    fprintf(stderr, "=> %s\n", buf);
  }
  exit(EXIT_FAILURE);
}


fe_Object* fe_nextarg(fe_Context *ctx, fe_Object **arg) {
  fe_Object *a = *arg;
  if (type(a) != FE_TPAIR) {
    if (isnil(a)) { fe_error(ctx, "too few arguments"); }
    fe_error(ctx, "dotted pair in argument list");
  }
  *arg = cdr(a);
  return car(a);
}


static fe_Object* checktype(fe_Context *ctx, fe_Object *obj, int type) {
  char buf[64];
  if (type(obj) != type) {
    sprintf(buf, "expected %s, got %s", typenames[type], typenames[type(obj)]);
    fe_error(ctx, buf);
  }
  return obj;
}


int fe_type(fe_Context *ctx, fe_Object *obj) {
  unused(ctx);
  return type(obj);
}


int fe_isnil(fe_Context *ctx, fe_Object *obj) {
  unused(ctx);
  return isnil(obj);
}


void fe_pushgc(fe_Context *ctx, fe_Object *obj) {
  if (ctx->gcstack_idx == GCSTACKSIZE) {
    fe_error(ctx, "gc stack overflow");
  }
  ctx->gcstack[ctx->gcstack_idx++] = obj;
}


void fe_restoregc(fe_Context *ctx, int idx) {
  ctx->gcstack_idx = idx;
}


int fe_savegc(fe_Context *ctx) {
  return ctx->gcstack_idx;
}


void fe_mark(fe_Context *ctx, fe_Object *obj) {
  fe_Object *car;
begin:
  if (tag(obj) & GCMARKBIT) { return; }
  car = car(obj); /* store car before modifying it with GCMARKBIT */
  tag(obj) |= GCMARKBIT;

  switch (type(obj)) {
    case FE_TPAIR:
      fe_mark(ctx, car);
      /* fall through */
    case FE_TFUNC: case FE_TMACRO: case FE_TSYMBOL: case FE_TSTRING:
      obj = cdr(obj);
      goto begin;

    case FE_TPTR:
      if (ctx->handlers.mark) { ctx->handlers.mark(ctx, obj); }
      break;
  }
}


static void collectgarbage(fe_Context *ctx) {
  int i;
  /* mark */
  for (i = 0; i < ctx->gcstack_idx; i++) {
    fe_mark(ctx, ctx->gcstack[i]);
  }
  fe_mark(ctx, ctx->symlist);
  /* sweep and unmark */
  for (i = 0; i < ctx->object_count; i++) {
    fe_Object *obj = &ctx->objects[i];
    if (type(obj) == FE_TFREE) { continue; }
    if (~tag(obj) & GCMARKBIT) {
      if (type(obj) == FE_TPTR && ctx->handlers.gc) {
        ctx->handlers.gc(ctx, obj);
      }
      settype(obj, FE_TFREE);
      cdr(obj) = ctx->freelist;
      ctx->freelist = obj;
    } else {
      tag(obj) &= ~GCMARKBIT;
    }
  }
}


static int equal(fe_Object *a, fe_Object *b) {
  if (a == b) { return 1; }
  if (type(a) != type(b)) { return 0; }
  if (type(a) == FE_TNUMBER) { return number(a) == number(b); }
  if (type(a) == FE_TSTRING) {
    for (; !isnil(a); a = cdr(a), b = cdr(b)) {
      if (car(a) != car(b)) { return 0; }
    }
    return a == b;
  }
  return 0;
}


static int streq(fe_Object *obj, const char *str) {
  while (!isnil(obj)) {
    int i;
    for (i = 0; i < STRBUFSIZE; i++) {
      if (strbuf(obj)[i] != *str) { return 0; }
      if (*str) { str++; }
    }
    obj = cdr(obj);
  }
  return *str == '\0';
}


static fe_Object* object(fe_Context *ctx) {
  fe_Object *obj;
  /* do gc if freelist has no more objects */
  if (isnil(ctx->freelist)) {
    collectgarbage(ctx);
    if (isnil(ctx->freelist)) { fe_error(ctx, "out of memory"); }
  }
  /* get object from freelist and push to the gcstack */
  obj = ctx->freelist;
  ctx->freelist = cdr(obj);
  fe_pushgc(ctx, obj);
  return obj;
}


fe_Object* fe_cons(fe_Context *ctx, fe_Object *car, fe_Object *cdr) {
  fe_Object *obj = object(ctx);
  car(obj) = car;
  cdr(obj) = cdr;
  return obj;
}


fe_Object* fe_bool(fe_Context *ctx, int b) {
  return b ? ctx->t : &nil;
}


fe_Object* fe_number(fe_Context *ctx, fe_Number n) {
  fe_Object *obj = object(ctx);
  settype(obj, FE_TNUMBER);
  number(obj) = n;
  return obj;
}


static fe_Object* buildstring(fe_Context *ctx, fe_Object *tail, int chr) {
  if (!tail || strbuf(tail)[STRBUFSIZE - 1] != '\0') {
    fe_Object *obj = fe_cons(ctx, NULL, &nil);
    settype(obj, FE_TSTRING);
    if (tail) {
      cdr(tail) = obj;
      ctx->gcstack_idx--;
    }
    tail = obj;
  }
  strbuf(tail)[strlen(strbuf(tail))] = chr;
  return tail;
}


fe_Object* fe_string(fe_Context *ctx, const char *str) {
  fe_Object *obj = buildstring(ctx, NULL, '\0');
  fe_Object *tail = obj;
  while (*str) {
    tail = buildstring(ctx, tail, *str++);
  }
  return obj;
}


fe_Object* fe_symbol(fe_Context *ctx, const char *name) {
  fe_Object *obj;
  /* try to find in symlist */
  for (obj = ctx->symlist; !isnil(obj); obj = cdr(obj)) {
    if (streq(car(cdr(car(obj))), name)) {
      return car(obj);
    }
  }
  /* create new object, push to symlist and return */
  obj = object(ctx);
  settype(obj, FE_TSYMBOL);
  cdr(obj) = fe_cons(ctx, fe_string(ctx, name), &nil);
  ctx->symlist = fe_cons(ctx, obj, ctx->symlist);
  return obj;
}


fe_Object* fe_cfunc(fe_Context *ctx, fe_CFunc fn) {
  fe_Object *obj = object(ctx);
  settype(obj, FE_TCFUNC);
  cfunc(obj) = fn;
  return obj;
}


fe_Object* fe_ptr(fe_Context *ctx, void *ptr) {
  fe_Object *obj = object(ctx);
  settype(obj, FE_TPTR);
  cdr(obj) = ptr;
  return obj;
}


fe_Object* fe_list(fe_Context *ctx, fe_Object **objs, int n) {
  fe_Object *res = &nil;
  while (n--) {
    res = fe_cons(ctx, objs[n], res);
  }
  return res;
}


fe_Object* fe_car(fe_Context *ctx, fe_Object *obj) {
  if (isnil(obj)) { return obj; }
  return car(checktype(ctx, obj, FE_TPAIR));
}


fe_Object* fe_cdr(fe_Context *ctx, fe_Object *obj) {
  if (isnil(obj)) { return obj; }
  return cdr(checktype(ctx, obj, FE_TPAIR));
}


static void writestr(fe_Context *ctx, fe_WriteFn fn, void *udata, const char *s) {
  while (*s) { fn(ctx, udata, *s++); }
}

void fe_write(fe_Context *ctx, fe_Object *obj, fe_WriteFn fn, void *udata, int qt) {
  char buf[32];

  switch (type(obj)) {
    case FE_TNIL:
      writestr(ctx, fn, udata, "nil");
      break;

    case FE_TNUMBER:
      sprintf(buf, "%.7g", number(obj));
      writestr(ctx, fn, udata, buf);
      break;

    case FE_TPAIR:
      fn(ctx, udata, '(');
      for (;;) {
        fe_write(ctx, car(obj), fn, udata, 1);
        obj = cdr(obj);
        if (type(obj) != FE_TPAIR) { break; }
        fn(ctx, udata, ' ');
      }
      if (!isnil(obj)) {
        writestr(ctx, fn, udata, " . ");
        fe_write(ctx, obj, fn, udata, 1);
      }
      fn(ctx, udata, ')');
      break;

    case FE_TSYMBOL:
      fe_write(ctx, car(cdr(obj)), fn, udata, 0);
      break;

    case FE_TSTRING:
      if (qt) { fn(ctx, udata, '"'); }
      while (!isnil(obj)) {
        int i;
        for (i = 0; i < STRBUFSIZE && strbuf(obj)[i]; i++) {
          if (qt && strbuf(obj)[i] == '"') { fn(ctx, udata, '\\'); }
          fn(ctx, udata, strbuf(obj)[i]);
        }
        obj = cdr(obj);
      }
      if (qt) { fn(ctx, udata, '"'); }
      break;

    default:
      sprintf(buf, "[%s %p]", typenames[type(obj)], (void*) obj);
      writestr(ctx, fn, udata, buf);
      break;
  }
}


static void writefp(fe_Context *ctx, void *udata, char chr) {
  unused(ctx);
  fputc(chr, udata);
}

void fe_writefp(fe_Context *ctx, fe_Object *obj, FILE *fp) {
  fe_write(ctx, obj, writefp, fp, 0);
}


typedef struct { char *p; int n; } CharPtrInt;

static void writebuf(fe_Context *ctx, void *udata, char chr) {
  CharPtrInt *x = udata;
  unused(ctx);
  if (x->n) { *x->p++ = chr; x->n--; }
}

int fe_tostring(fe_Context *ctx, fe_Object *obj, char *dst, int size) {
  CharPtrInt x;
  x.p = dst;
  x.n = size - 1;
  fe_write(ctx, obj, writebuf, &x, 0);
  *x.p = '\0';
  return size - x.n - 1;
}


fe_Number fe_tonumber(fe_Context *ctx, fe_Object *obj) {
  return number(checktype(ctx, obj, FE_TNUMBER));
}


void* fe_toptr(fe_Context *ctx, fe_Object *obj) {
  return cdr(checktype(ctx, obj, FE_TPTR));
}


static fe_Object* getbound(fe_Object *sym, fe_Object *env) {
  /* try to find in environment */
  for (; !isnil(env); env = cdr(env)) {
    fe_Object *x = car(env);
    if (car(x) == sym) { return x; }
  }
  /* return global */
  return cdr(sym);
}


void fe_set(fe_Context *ctx, fe_Object *sym, fe_Object *v) {
  unused(ctx);
  cdr(getbound(sym, &nil)) = v;
}


static fe_Object rparen;

static fe_Object* read_(fe_Context *ctx, fe_ReadFn fn, void *udata) {
  const char *delimiter = " \n\t\r();";
  fe_Object *v, *res, **tail;
  fe_Number n;
  int chr, gc;
  char buf[64], *p;

  /* get next character */
  chr = ctx->nextchr ? ctx->nextchr : fn(ctx, udata);
  ctx->nextchr = '\0';

  /* skip whitespace */
  while (chr && strchr(" \n\t\r", chr)) {
    chr = fn(ctx, udata);
  }

  switch (chr) {
    case '\0':
      return NULL;

    case ';':
      while (chr && chr != '\n') { chr = fn(ctx, udata); }
      return read_(ctx, fn, udata);

    case ')':
      return &rparen;

    case '(':
      res = &nil;
      tail = &res;
      gc = fe_savegc(ctx);
      fe_pushgc(ctx, res); /* to cause error on too-deep nesting */
      while ( (v = read_(ctx, fn, udata)) != &rparen ) {
        if (v == NULL) { fe_error(ctx, "unclosed list"); }
        if (type(v) == FE_TSYMBOL && streq(car(cdr(v)), ".")) {
          /* dotted pair */
          *tail = fe_read(ctx, fn, udata);
        } else {
          /* proper pair */
          *tail = fe_cons(ctx, v, &nil);
          tail = &cdr(*tail);
        }
        fe_restoregc(ctx, gc);
        fe_pushgc(ctx, res);
      }
      return res;

    case '\'':
      v = fe_read(ctx, fn, udata);
      if (!v) { fe_error(ctx, "stray '''"); }
      return fe_cons(ctx, fe_symbol(ctx, "quote"), fe_cons(ctx, v, &nil));

    case '"':
      res = buildstring(ctx, NULL, '\0');
      v = res;
      chr = fn(ctx, udata);
      while (chr != '"') {
        if (chr == '\0') { fe_error(ctx, "unclosed string"); }
        if (chr == '\\') {
          chr = fn(ctx, udata);
          if (strchr("nrt", chr)) { chr = strchr("n\nr\rt\t", chr)[1]; }
        }
        v = buildstring(ctx, v, chr);
        chr = fn(ctx, udata);
      }
      return res;

    default:
      p = buf;
      do {
        if (p == buf + sizeof(buf) - 1) { fe_error(ctx, "symbol too long"); }
        *p++ = chr;
        chr = fn(ctx, udata);
      } while (chr && !strchr(delimiter, chr));
      *p = '\0';
      ctx->nextchr = chr;
      n = strtod(buf, &p);  /* try to read as number */
      if (p != buf && strchr(delimiter, *p)) { return fe_number(ctx, n); }
      if (!strcmp(buf, "nil")) { return &nil; }
      return fe_symbol(ctx, buf);
  }
}


fe_Object* fe_read(fe_Context *ctx, fe_ReadFn fn, void *udata) {
  fe_Object* obj = read_(ctx, fn, udata);
  if (obj == &rparen) { fe_error(ctx, "stray ')'"); }
  return obj;
}


static char readfp(fe_Context *ctx, void *udata) {
  int chr;
  unused(ctx);
  return (chr = fgetc(udata)) == EOF ? '\0' : chr;
}

fe_Object* fe_readfp(fe_Context *ctx, FILE *fp) {
  return fe_read(ctx, readfp, fp);
}


static fe_Object* eval(fe_Context *ctx, fe_Object *obj, fe_Object *env, fe_Object **bind);

static fe_Object* evallist(fe_Context *ctx, fe_Object *lst, fe_Object *env) {
  fe_Object *res = &nil;
  fe_Object **tail = &res;
  while (!isnil(lst)) {
    *tail = fe_cons(ctx, eval(ctx, fe_nextarg(ctx, &lst), env, NULL), &nil);
    tail = &cdr(*tail);
  }
  return res;
}


static fe_Object* dolist(fe_Context *ctx, fe_Object *lst, fe_Object *env) {
  fe_Object *res = &nil;
  int save = fe_savegc(ctx);
  while (!isnil(lst)) {
    fe_restoregc(ctx, save);
    fe_pushgc(ctx, lst);
    fe_pushgc(ctx, env);
    res = eval(ctx, fe_nextarg(ctx, &lst), env, &env);
  }
  return res;
}


static fe_Object* argstoenv(fe_Context *ctx, fe_Object *prm, fe_Object *arg, fe_Object *env) {
  while (!isnil(prm)) {
    if (type(prm) != FE_TPAIR) {
      env = fe_cons(ctx, fe_cons(ctx, prm, arg), env);
      break;
    }
    env = fe_cons(ctx, fe_cons(ctx, car(prm), fe_car(ctx, arg)), env);
    prm = cdr(prm);
    arg = fe_cdr(ctx, arg);
  }
  return env;
}


#define evalarg() eval(ctx, fe_nextarg(ctx, &arg), env, NULL)

#define arithop(op) {                             \
    fe_Number x = fe_tonumber(ctx, evalarg());    \
    while (!isnil(arg)) {                         \
      x = x op fe_tonumber(ctx, evalarg());       \
    }                                             \
    res = fe_number(ctx, x);                      \
  }

#define numcmpop(op) {                            \
    va = checktype(ctx, evalarg(), FE_TNUMBER);   \
    vb = checktype(ctx, evalarg(), FE_TNUMBER);   \
    res = fe_bool(ctx, number(va) op number(vb)); \
  }


static fe_Object* eval(fe_Context *ctx, fe_Object *obj, fe_Object *env, fe_Object **newenv) {
  fe_Object *fn, *arg, *res;
  fe_Object cl, *va, *vb;
  int n, gc;

  if (type(obj) == FE_TSYMBOL) { return cdr(getbound(obj, env)); }
  if (type(obj) != FE_TPAIR) { return obj; }

  car(&cl) = obj, cdr(&cl) = ctx->calllist;
  ctx->calllist = &cl;

  gc = fe_savegc(ctx);
  fn = eval(ctx, car(obj), env, NULL);
  arg = cdr(obj);
  res = &nil;

  switch (type(fn)) {
    case FE_TPRIM:
      switch (prim(fn)) {
        case P_LET:
          va = checktype(ctx, fe_nextarg(ctx, &arg), FE_TSYMBOL);
          if (newenv) {
            *newenv = fe_cons(ctx, fe_cons(ctx, va, evalarg()), env);
          }
          break;

        case P_SET:
          va = checktype(ctx, fe_nextarg(ctx, &arg), FE_TSYMBOL);
          cdr(getbound(va, env)) = evalarg();
          break;

        case P_IF:
          while (!isnil(arg)) {
            va = evalarg();
            if (!isnil(va)) {
              res = isnil(arg) ? va : evalarg();
              break;
            }
            if (isnil(arg)) { break; }
            arg = cdr(arg);
          }
          break;

        case P_FN: case P_MAC:
          va = fe_cons(ctx, env, arg);
          fe_nextarg(ctx, &arg);
          res = object(ctx);
          settype(res, prim(fn) == P_FN ? FE_TFUNC : FE_TMACRO);
          cdr(res) = va;
          break;

        case P_WHILE:
          va = fe_nextarg(ctx, &arg);
          n = fe_savegc(ctx);
          while (!isnil(eval(ctx, va, env, NULL))) {
            dolist(ctx, arg, env);
            fe_restoregc(ctx, n);
          }
          break;

        case P_QUOTE:
          res = fe_nextarg(ctx, &arg);
          break;

        case P_AND:
          while (!isnil(arg) && !isnil(res = evalarg()));
          break;

        case P_OR:
          while (!isnil(arg) && isnil(res = evalarg()));
          break;

        case P_DO:
          res = dolist(ctx, arg, env);
          break;

        case P_CONS:
          va = evalarg();
          res = fe_cons(ctx, va, evalarg());
          break;

        case P_CAR:
          res = fe_car(ctx, evalarg());
          break;

        case P_CDR:
          res = fe_cdr(ctx, evalarg());
          break;

        case P_SETCAR:
          va = checktype(ctx, evalarg(), FE_TPAIR);
          car(va) = evalarg();
          break;

        case P_SETCDR:
          va = checktype(ctx, evalarg(), FE_TPAIR);
          cdr(va) = evalarg();
          break;

        case P_LIST:
          res = evallist(ctx, arg, env);
          break;

        case P_NOT:
          res = fe_bool(ctx, isnil(evalarg()));
          break;

        case P_IS:
          va = evalarg();
          res = fe_bool(ctx, equal(va, evalarg()));
          break;

        case P_ATOM:
          res = fe_bool(ctx, fe_type(ctx, evalarg()) != FE_TPAIR);
          break;

        case P_PRINT:
          while (!isnil(arg)) {
            fe_writefp(ctx, evalarg(), stdout);
            if (!isnil(arg)) { printf(" "); }
          }
          printf("\n");
          break;

        case P_LT: numcmpop(<); break;
        case P_LTE: numcmpop(<=); break;
        case P_ADD: arithop(+); break;
        case P_SUB: arithop(-); break;
        case P_MUL: arithop(*); break;
        case P_DIV: arithop(/); break;
      }
      break;

    case FE_TCFUNC:
      res = cfunc(fn)(ctx, evallist(ctx, arg, env));
      break;

    case FE_TFUNC:
      arg = evallist(ctx, arg, env);
      va = cdr(fn); /* (env params ...) */
      vb = cdr(va); /* (params ...) */
      res = dolist(ctx, cdr(vb), argstoenv(ctx, car(vb), arg, car(va)));
      break;

    case FE_TMACRO:
      va = cdr(fn); /* (env params ...) */
      vb = cdr(va); /* (params ...) */
      /* replace caller object with code generated by macro and re-eval */
      *obj = *dolist(ctx, cdr(vb), argstoenv(ctx, car(vb), arg, car(va)));
      fe_restoregc(ctx, gc);
      ctx->calllist = cdr(&cl);
      return eval(ctx, obj, env, NULL);

    default:
      fe_error(ctx, "tried to call non-callable value");
  }

  fe_restoregc(ctx, gc);
  fe_pushgc(ctx, res);
  ctx->calllist = cdr(&cl);
  return res;
}


fe_Object* fe_eval(fe_Context *ctx, fe_Object *obj) {
  return eval(ctx, obj, &nil, NULL);
}


fe_Context* fe_open(void *ptr, int size) {
  int i, save;
  fe_Context *ctx;

  /* init context struct */
  ctx = ptr;
  memset(ctx, 0, sizeof(fe_Context));
  ptr = (char*) ptr + sizeof(fe_Context);
  size -= sizeof(fe_Context);

  /* init objects memory region */
  ctx->objects = (fe_Object*) ptr;
  ctx->object_count = size / sizeof(fe_Object);

  /* init lists */
  ctx->calllist = &nil;
  ctx->freelist = &nil;
  ctx->symlist = &nil;

  /* populate freelist */
  for (i = 0; i < ctx->object_count; i++) {
    fe_Object *obj = &ctx->objects[i];
    settype(obj, FE_TFREE);
    cdr(obj) = ctx->freelist;
    ctx->freelist = obj;
  }

  /* init objects */
  ctx->t = fe_symbol(ctx, "t");
  fe_set(ctx, ctx->t, ctx->t);

  /* register built in primitives */
  save = fe_savegc(ctx);
  for (i = 0; i < P_MAX; i++) {
    fe_Object *v = object(ctx);
    settype(v, FE_TPRIM);
    prim(v) = i;
    fe_set(ctx, fe_symbol(ctx, primnames[i]), v);
    fe_restoregc(ctx, save);
  }

  return ctx;
}


void fe_close(fe_Context *ctx) {
  /* clear gcstack and symlist; makes all objects unreachable */
  ctx->gcstack_idx = 0;
  ctx->symlist = &nil;
  collectgarbage(ctx);
}


#ifdef FE_STANDALONE

#include <setjmp.h>

static jmp_buf toplevel;
static char buf[64000];

static void onerror(fe_Context *ctx, const char *msg, fe_Object *cl) {
  unused(ctx), unused(cl);
  fprintf(stderr, "error: %s\n", msg);
  longjmp(toplevel, -1);
}


int main(int argc, char **argv) {
  int gc;
  fe_Object *obj;
  FILE *volatile fp = stdin;
  fe_Context *ctx = fe_open(buf, sizeof(buf));

  /* init input file */
  if (argc > 1) {
    fp = fopen(argv[1], "rb");
    if (!fp) { fe_error(ctx, "could not open input file"); }
  }

  if (fp == stdin) { fe_handlers(ctx)->error = onerror; }
  gc = fe_savegc(ctx);
  setjmp(toplevel);

  /* re(p)l */
  for (;;) {
    fe_restoregc(ctx, gc);
    if (fp == stdin) { printf("> "); }
    if (!(obj = fe_readfp(ctx, fp))) { break; }
    obj = fe_eval(ctx, obj);
    if (fp == stdin) { fe_writefp(ctx, obj, stdout); printf("\n"); }
  }

  return EXIT_SUCCESS;
}

#endif