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