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   1 /*
   2  * Copyright © 2016 Keith Packard <keithp@keithp.com>
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License as published by
   6  * the Free Software Foundation, either version 2 of the License, or
   7  * (at your option) any later version.
   8  *
   9  * This program is distributed in the hope that it will be useful, but
  10  * WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12  * General Public License for more details.
  13  */
  14
  15 #include "ao_scheme.h"
  16 #include <assert.h>
  17
  18 struct ao_scheme_stack          *ao_scheme_stack;
  19 ao_poly                         ao_scheme_v;
  20 uint8_t                         ao_scheme_skip_cons_free;
  21
  22 ao_poly
  23 ao_scheme_set_cond(struct ao_scheme_cons *c)
  24 {
  25         ao_scheme_stack->state = eval_cond;
  26         ao_scheme_stack->sexprs = ao_scheme_cons_poly(c);
  27         return AO_SCHEME_NIL;
  28 }
  29
  30 static int
  31 func_type(ao_poly func)
  32 {
  33         if (func == AO_SCHEME_NIL)
  34                 return ao_scheme_error(AO_SCHEME_INVALID, "func is nil");
  35         switch (ao_scheme_poly_type(func)) {
  36         case AO_SCHEME_BUILTIN:
  37                 return ao_scheme_poly_builtin(func)->args & AO_SCHEME_FUNC_MASK;
  38         case AO_SCHEME_LAMBDA:
  39                 return ao_scheme_poly_lambda(func)->args;
  40         case AO_SCHEME_STACK:
  41                 return AO_SCHEME_FUNC_LAMBDA;
  42         default:
  43                 ao_scheme_error(AO_SCHEME_INVALID, "not a func");
  44                 return -1;
  45         }
  46 }
  47
  48 /*
  49  * Flattened eval to avoid stack issues
  50  */
  51
  52 /*
  53  * Evaluate an s-expression
  54  *
  55  * For a list, evaluate all of the elements and
  56  * then execute the resulting function call.
  57  *
  58  * Each element of the list is evaluated in
  59  * a clean stack context.
  60  *
  61  * The current stack state is set to 'formal' so that
  62  * when the evaluation is complete, the value
  63  * will get appended to the values list.
  64  *
  65  * For other types, compute the value directly.
  66  */
  67
  68 static int
  69 ao_scheme_eval_sexpr(void)
  70 {
  71         DBGI("sexpr: %v\n", ao_scheme_v);
  72         switch (ao_scheme_poly_type(ao_scheme_v)) {
  73         case AO_SCHEME_CONS:
  74                 if (ao_scheme_v == AO_SCHEME_NIL) {
  75                         if (!ao_scheme_stack->values) {
  76                                 /*
  77                                  * empty list evaluates to empty list
  78                                  */
  79                                 ao_scheme_v = AO_SCHEME_NIL;
  80                                 ao_scheme_stack->state = eval_val;
  81                         } else {
  82                                 /*
  83                                  * done with arguments, go execute it
  84                                  */
  85                                 ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->values)->car;
  86                                 ao_scheme_stack->state = eval_exec;
  87                         }
  88                 } else {
  89                         if (!ao_scheme_stack->values)
  90                                 ao_scheme_stack->list = ao_scheme_v;
  91                         /*
  92                          * Evaluate another argument and then switch
  93                          * to 'formal' to add the value to the values
  94                          * list
  95                          */
  96                         ao_scheme_stack->sexprs = ao_scheme_v;
  97                         ao_scheme_stack->state = eval_formal;
  98                         if (!ao_scheme_stack_push())
  99                                 return 0;
 100                         /*
 101                          * push will reset the state to 'sexpr', which
 102                          * will evaluate the expression
 103                          */
 104                         ao_scheme_v = ao_scheme_poly_cons(ao_scheme_v)->car;
 105                 }
 106                 break;
 107         case AO_SCHEME_ATOM:
 108                 DBGI("..frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 109                 ao_scheme_v = ao_scheme_atom_get(ao_scheme_v);
 110                 /* fall through */
 111         case AO_SCHEME_BOOL:
 112         case AO_SCHEME_INT:
 113         case AO_SCHEME_BIGINT:
 114         case AO_SCHEME_FLOAT:
 115         case AO_SCHEME_STRING:
 116         case AO_SCHEME_BUILTIN:
 117         case AO_SCHEME_LAMBDA:
 118                 ao_scheme_stack->state = eval_val;
 119                 break;
 120         }
 121         DBGI(".. result "); DBG_POLY(ao_scheme_v); DBG("\n");
 122         return 1;
 123 }
 124
 125 /*
 126  * A value has been computed.
 127  *
 128  * If the value was computed from a macro,
 129  * then we want to reset the current context
 130  * to evaluate the macro result again.
 131  *
 132  * If not a macro, then pop the stack.
 133  * If the stack is empty, we're done.
 134  * Otherwise, the stack will contain
 135  * the next state.
 136  */
 137
 138 static int
 139 ao_scheme_eval_val(void)
 140 {
 141         DBGI("val: "); DBG_POLY(ao_scheme_v); DBG("\n");
 142         /*
 143          * Value computed, pop the stack
 144          * to figure out what to do with the value
 145          */
 146         ao_scheme_stack_pop();
 147         DBGI("..state %d\n", ao_scheme_stack ? ao_scheme_stack->state : -1);
 148         return 1;
 149 }
 150
 151 /*
 152  * A formal has been computed.
 153  *
 154  * If this is the first formal, then check to see if we've got a
 155  * lamda, macro or nlambda.
 156  *
 157  * For lambda, go compute another formal.  This will terminate
 158  * when the sexpr state sees nil.
 159  *
 160  * For macro/nlambda, we're done, so move the sexprs into the values
 161  * and go execute it.
 162  *
 163  * Macros have an additional step of saving a stack frame holding the
 164  * macro value execution context, which then gets the result of the
 165  * macro to run
 166  */
 167
 168 static int
 169 ao_scheme_eval_formal(void)
 170 {
 171         ao_poly                 formal;
 172         struct ao_scheme_stack  *prev;
 173
 174         DBGI("formal: "); DBG_POLY(ao_scheme_v); DBG("\n");
 175
 176         /* Check what kind of function we've got */
 177         if (!ao_scheme_stack->values) {
 178                 switch (func_type(ao_scheme_v)) {
 179                 case AO_SCHEME_FUNC_LAMBDA:
 180                         DBGI(".. lambda\n");
 181                         break;
 182                 case AO_SCHEME_FUNC_MACRO:
 183                         /* Evaluate the result once more */
 184                         ao_scheme_stack->state = eval_macro;
 185                         if (!ao_scheme_stack_push())
 186                                 return 0;
 187
 188                         /* After the function returns, take that
 189                          * value and re-evaluate it
 190                          */
 191                         prev = ao_scheme_poly_stack(ao_scheme_stack->prev);
 192                         ao_scheme_stack->sexprs = prev->sexprs;
 193
 194                         DBGI(".. start macro\n");
 195                         DBGI("\t.. sexprs       "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 196                         DBGI("\t.. values       "); DBG_POLY(ao_scheme_stack->values); DBG("\n");
 197                         DBG_FRAMES();
 198
 199                         /* fall through ... */
 200                 case AO_SCHEME_FUNC_NLAMBDA:
 201                         DBGI(".. nlambda or macro\n");
 202
 203                         /* use the raw sexprs as values */
 204                         ao_scheme_stack->values = ao_scheme_stack->sexprs;
 205                         ao_scheme_stack->values_tail = AO_SCHEME_NIL;
 206                         ao_scheme_stack->state = eval_exec;
 207
 208                         /* ready to execute now */
 209                         return 1;
 210                 case -1:
 211                         return 0;
 212                 }
 213         }
 214
 215         /* Append formal to list of values */
 216         formal = ao_scheme__cons(ao_scheme_v, AO_SCHEME_NIL);
 217         if (!formal)
 218                 return 0;
 219
 220         if (ao_scheme_stack->values_tail)
 221                 ao_scheme_poly_cons(ao_scheme_stack->values_tail)->cdr = formal;
 222         else
 223                 ao_scheme_stack->values = formal;
 224         ao_scheme_stack->values_tail = formal;
 225
 226         DBGI(".. values "); DBG_POLY(ao_scheme_stack->values); DBG("\n");
 227
 228         /*
 229          * Step to the next argument, if this is last, then
 230          * 'sexpr' will end up switching to 'exec'
 231          */
 232         ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->cdr;
 233
 234         ao_scheme_stack->state = eval_sexpr;
 235
 236         DBGI(".. "); DBG_POLY(ao_scheme_v); DBG("\n");
 237         return 1;
 238 }
 239
 240 /*
 241  * Start executing a function call
 242  *
 243  * Most builtins are easy, just call the function.
 244  * 'cond' is magic; it sticks the list of clauses
 245  * in 'sexprs' and switches to 'cond' state. That
 246  * bit of magic is done in ao_scheme_set_cond.
 247  *
 248  * Lambdas build a new frame to hold the locals and
 249  * then re-use the current stack context to evaluate
 250  * the s-expression from the lambda.
 251  */
 252
 253 static int
 254 ao_scheme_eval_exec(void)
 255 {
 256         ao_poly v;
 257         struct ao_scheme_builtin        *builtin;
 258
 259         DBGI("exec: "); DBG_POLY(ao_scheme_v); DBG(" values "); DBG_POLY(ao_scheme_stack->values); DBG ("\n");
 260         ao_scheme_stack->sexprs = AO_SCHEME_NIL;
 261         switch (ao_scheme_poly_type(ao_scheme_v)) {
 262         case AO_SCHEME_BUILTIN:
 263                 ao_scheme_stack->state = eval_val;
 264                 builtin = ao_scheme_poly_builtin(ao_scheme_v);
 265                 v = ao_scheme_func(builtin) (
 266                         ao_scheme_poly_cons(ao_scheme_poly_cons(ao_scheme_stack->values)->cdr));
 267                 DBG_DO(if (!ao_scheme_exception && ao_scheme_poly_builtin(ao_scheme_v)->func == builtin_set) {
 268                                 struct ao_scheme_cons *cons = ao_scheme_poly_cons(ao_scheme_stack->values);
 269                                 ao_poly atom = ao_scheme_arg(cons, 1);
 270                                 ao_poly val = ao_scheme_arg(cons, 2);
 271                                 DBGI("set "); DBG_POLY(atom); DBG(" = "); DBG_POLY(val); DBG("\n");
 272                         });
 273                 builtin = ao_scheme_poly_builtin(ao_scheme_v);
 274                 if (builtin && (builtin->args & AO_SCHEME_FUNC_FREE_ARGS) && !ao_scheme_stack_marked(ao_scheme_stack) && !ao_scheme_skip_cons_free) {
 275                         struct ao_scheme_cons *cons = ao_scheme_poly_cons(ao_scheme_stack->values);
 276                         ao_scheme_stack->values = AO_SCHEME_NIL;
 277                         ao_scheme_cons_free(cons);
 278                 }
 279
 280                 ao_scheme_v = v;
 281                 ao_scheme_stack->values = AO_SCHEME_NIL;
 282                 ao_scheme_stack->values_tail = AO_SCHEME_NIL;
 283                 DBGI(".. result "); DBG_POLY(ao_scheme_v); DBG ("\n");
 284                 DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 285                 break;
 286         case AO_SCHEME_LAMBDA:
 287                 DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 288                 ao_scheme_stack->state = eval_begin;
 289                 v = ao_scheme_lambda_eval();
 290                 ao_scheme_stack->sexprs = v;
 291                 ao_scheme_stack->values = AO_SCHEME_NIL;
 292                 ao_scheme_stack->values_tail = AO_SCHEME_NIL;
 293                 DBGI(".. sexprs "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 294                 DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 295                 break;
 296         case AO_SCHEME_STACK:
 297                 DBGI(".. stack "); DBG_POLY(ao_scheme_v); DBG("\n");
 298                 ao_scheme_v = ao_scheme_stack_eval();
 299                 DBGI(".. value "); DBG_POLY(ao_scheme_v); DBG("\n");
 300                 DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 301                 break;
 302         }
 303         ao_scheme_skip_cons_free = 0;
 304         return 1;
 305 }
 306
 307 /*
 308  * Finish setting up the apply evaluation
 309  *
 310  * The value is the list to execute
 311  */
 312 static int
 313 ao_scheme_eval_apply(void)
 314 {
 315         struct ao_scheme_cons   *cons = ao_scheme_poly_cons(ao_scheme_v);
 316         struct ao_scheme_cons   *cdr, *prev;
 317
 318         /* Glue the arguments into the right shape. That's all but the last
 319          * concatenated onto the last
 320          */
 321         cdr = cons;
 322         for (;;) {
 323                 prev = cdr;
 324                 cdr = ao_scheme_poly_cons(prev->cdr);
 325                 if (cdr->cdr == AO_SCHEME_NIL)
 326                         break;
 327         }
 328         DBGI("before mangling: "); DBG_POLY(ao_scheme_v); DBG("\n");
 329         prev->cdr = cdr->car;
 330         ao_scheme_stack->values = ao_scheme_v;
 331         ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->values)->car;
 332         DBGI("apply: "); DBG_POLY(ao_scheme_stack->values); DBG ("\n");
 333         ao_scheme_stack->state = eval_exec;
 334         ao_scheme_skip_cons_free = 1;
 335         return 1;
 336 }
 337
 338 /*
 339  * Start evaluating the next cond clause
 340  *
 341  * If the list of clauses is empty, then
 342  * the result of the cond is nil.
 343  *
 344  * Otherwise, set the current stack state to 'cond_test' and create a
 345  * new stack context to evaluate the test s-expression. Once that's
 346  * complete, we'll land in 'cond_test' to finish the clause.
 347  */
 348 static int
 349 ao_scheme_eval_cond(void)
 350 {
 351         DBGI("cond: "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 352         DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 353         DBGI(".. saved frame "); DBG_POLY(ao_scheme_stack->frame); DBG("\n");
 354         if (!ao_scheme_stack->sexprs) {
 355                 ao_scheme_v = _ao_scheme_bool_false;
 356                 ao_scheme_stack->state = eval_val;
 357         } else {
 358                 ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->car;
 359                 if (!ao_scheme_v || ao_scheme_poly_type(ao_scheme_v) != AO_SCHEME_CONS) {
 360                         ao_scheme_error(AO_SCHEME_INVALID, "invalid cond clause");
 361                         return 0;
 362                 }
 363                 ao_scheme_v = ao_scheme_poly_cons(ao_scheme_v)->car;
 364                 if (ao_scheme_v == _ao_scheme_atom_else)
 365                         ao_scheme_v = _ao_scheme_bool_true;
 366                 ao_scheme_stack->state = eval_cond_test;
 367                 if (!ao_scheme_stack_push())
 368                         return 0;
 369         }
 370         return 1;
 371 }
 372
 373 /*
 374  * Finish a cond clause.
 375  *
 376  * Check the value from the test expression, if
 377  * non-nil, then set up to evaluate the value expression.
 378  *
 379  * Otherwise, step to the next clause and go back to the 'cond'
 380  * state
 381  */
 382 static int
 383 ao_scheme_eval_cond_test(void)
 384 {
 385         DBGI("cond_test: "); DBG_POLY(ao_scheme_v); DBG(" sexprs "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 386         DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 387         DBGI(".. saved frame "); DBG_POLY(ao_scheme_stack->frame); DBG("\n");
 388         if (ao_scheme_v != _ao_scheme_bool_false) {
 389                 struct ao_scheme_cons *car = ao_scheme_poly_cons(ao_scheme_poly_cons(ao_scheme_stack->sexprs)->car);
 390                 ao_poly c = car->cdr;
 391
 392                 if (c) {
 393                         ao_scheme_stack->state = eval_begin;
 394                         ao_scheme_stack->sexprs = c;
 395                 } else
 396                         ao_scheme_stack->state = eval_val;
 397         } else {
 398                 ao_scheme_stack->sexprs = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->cdr;
 399                 DBGI("next cond: "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 400                 ao_scheme_stack->state = eval_cond;
 401         }
 402         return 1;
 403 }
 404
 405 /*
 406  * Evaluate a list of sexprs, returning the value from the last one.
 407  *
 408  * ao_scheme_begin records the list in stack->sexprs, so we just need to
 409  * walk that list. Set ao_scheme_v to the car of the list and jump to
 410  * eval_sexpr. When that's done, it will land in eval_val. For all but
 411  * the last, leave a stack frame with eval_begin set so that we come
 412  * back here. For the last, don't add a stack frame so that we can
 413  * just continue on.
 414  */
 415 static int
 416 ao_scheme_eval_begin(void)
 417 {
 418         DBGI("begin: "); DBG_POLY(ao_scheme_v); DBG(" sexprs "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 419         DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 420         DBGI(".. saved frame "); DBG_POLY(ao_scheme_stack->frame); DBG("\n");
 421
 422         if (!ao_scheme_stack->sexprs) {
 423                 ao_scheme_v = AO_SCHEME_NIL;
 424                 ao_scheme_stack->state = eval_val;
 425         } else {
 426                 ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->car;
 427                 ao_scheme_stack->sexprs = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->cdr;
 428
 429                 /* If there are more sexprs to do, then come back here, otherwise
 430                  * return the value of the last one by just landing in eval_sexpr
 431                  */
 432                 if (ao_scheme_stack->sexprs) {
 433                         ao_scheme_stack->state = eval_begin;
 434                         if (!ao_scheme_stack_push())
 435                                 return 0;
 436                 }
 437                 ao_scheme_stack->state = eval_sexpr;
 438         }
 439         return 1;
 440 }
 441
 442 /*
 443  * Conditionally execute a list of sexprs while the first is true
 444  */
 445 static int
 446 ao_scheme_eval_while(void)
 447 {
 448         DBGI("while: "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 449         DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 450         DBGI(".. saved frame "); DBG_POLY(ao_scheme_stack->frame); DBG("\n");
 451
 452         ao_scheme_stack->values = ao_scheme_v;
 453         if (!ao_scheme_stack->sexprs) {
 454                 ao_scheme_v = AO_SCHEME_NIL;
 455                 ao_scheme_stack->state = eval_val;
 456         } else {
 457                 ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->car;
 458                 ao_scheme_stack->state = eval_while_test;
 459                 if (!ao_scheme_stack_push())
 460                         return 0;
 461         }
 462         return 1;
 463 }
 464
 465 /*
 466  * Check the while condition, terminate the loop if nil. Otherwise keep going
 467  */
 468 static int
 469 ao_scheme_eval_while_test(void)
 470 {
 471         DBGI("while_test: "); DBG_POLY(ao_scheme_v); DBG(" sexprs "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 472         DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 473         DBGI(".. saved frame "); DBG_POLY(ao_scheme_stack->frame); DBG("\n");
 474
 475         if (ao_scheme_v != _ao_scheme_bool_false) {
 476                 ao_scheme_stack->values = ao_scheme_v;
 477                 ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->cdr;
 478                 ao_scheme_stack->state = eval_while;
 479                 if (!ao_scheme_stack_push())
 480                         return 0;
 481                 ao_scheme_stack->state = eval_begin;
 482                 ao_scheme_stack->sexprs = ao_scheme_v;
 483         }
 484         else
 485         {
 486                 ao_scheme_stack->state = eval_val;
 487                 ao_scheme_v = ao_scheme_stack->values;
 488         }
 489         return 1;
 490 }
 491
 492 /*
 493  * Replace the original sexpr with the macro expansion, then
 494  * execute that
 495  */
 496 static int
 497 ao_scheme_eval_macro(void)
 498 {
 499         DBGI("macro: "); DBG_POLY(ao_scheme_v); DBG(" sexprs "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 500
 501         if (ao_scheme_v == AO_SCHEME_NIL)
 502                 ao_scheme_abort();
 503         if (ao_scheme_poly_type(ao_scheme_v) == AO_SCHEME_CONS) {
 504                 *ao_scheme_poly_cons(ao_scheme_stack->sexprs) = *ao_scheme_poly_cons(ao_scheme_v);
 505                 ao_scheme_v = ao_scheme_stack->sexprs;
 506                 DBGI("sexprs rewritten to: "); DBG_POLY(ao_scheme_v); DBG("\n");
 507         }
 508         ao_scheme_stack->sexprs = AO_SCHEME_NIL;
 509         ao_scheme_stack->state = eval_sexpr;
 510         return 1;
 511 }
 512
 513 static int (*const evals[])(void) = {
 514         [eval_sexpr] = ao_scheme_eval_sexpr,
 515         [eval_val] = ao_scheme_eval_val,
 516         [eval_formal] = ao_scheme_eval_formal,
 517         [eval_exec] = ao_scheme_eval_exec,
 518         [eval_apply] = ao_scheme_eval_apply,
 519         [eval_cond] = ao_scheme_eval_cond,
 520         [eval_cond_test] = ao_scheme_eval_cond_test,
 521         [eval_begin] = ao_scheme_eval_begin,
 522         [eval_while] = ao_scheme_eval_while,
 523         [eval_while_test] = ao_scheme_eval_while_test,
 524         [eval_macro] = ao_scheme_eval_macro,
 525 };
 526
 527 const char * const ao_scheme_state_names[] = {
 528         [eval_sexpr] = "sexpr",
 529         [eval_val] = "val",
 530         [eval_formal] = "formal",
 531         [eval_exec] = "exec",
 532         [eval_apply] = "apply",
 533         [eval_cond] = "cond",
 534         [eval_cond_test] = "cond_test",
 535         [eval_begin] = "begin",
 536         [eval_while] = "while",
 537         [eval_while_test] = "while_test",
 538         [eval_macro] = "macro",
 539 };
 540
 541 /*
 542  * Called at restore time to reset all execution state
 543  */
 544
 545 void
 546 ao_scheme_eval_clear_globals(void)
 547 {
 548         ao_scheme_stack = NULL;
 549         ao_scheme_frame_current = NULL;
 550         ao_scheme_v = AO_SCHEME_NIL;
 551 }
 552
 553 int
 554 ao_scheme_eval_restart(void)
 555 {
 556         return ao_scheme_stack_push();
 557 }
 558
 559 ao_poly
 560 ao_scheme_eval(ao_poly _v)
 561 {
 562         ao_scheme_v = _v;
 563
 564         ao_scheme_frame_init();
 565
 566         if (!ao_scheme_stack_push())
 567                 return AO_SCHEME_NIL;
 568
 569         while (ao_scheme_stack) {
 570                 if (!(*evals[ao_scheme_stack->state])() || ao_scheme_exception) {
 571                         ao_scheme_stack_clear();
 572                         return AO_SCHEME_NIL;
 573                 }
 574         }
 575         DBG_DO(if (ao_scheme_frame_current) {DBGI("frame left as "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");});
 576         ao_scheme_frame_current = NULL;
 577         return ao_scheme_v;
 578 }