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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         default:
 112                 ao_scheme_stack->state = eval_val;
 113                 break;
 114         }
 115         DBGI(".. result "); DBG_POLY(ao_scheme_v); DBG("\n");
 116         return 1;
 117 }
 118
 119 /*
 120  * A value has been computed.
 121  *
 122  * If the value was computed from a macro,
 123  * then we want to reset the current context
 124  * to evaluate the macro result again.
 125  *
 126  * If not a macro, then pop the stack.
 127  * If the stack is empty, we're done.
 128  * Otherwise, the stack will contain
 129  * the next state.
 130  */
 131
 132 static int
 133 ao_scheme_eval_val(void)
 134 {
 135         DBGI("val: "); DBG_POLY(ao_scheme_v); DBG("\n");
 136         /*
 137          * Value computed, pop the stack
 138          * to figure out what to do with the value
 139          */
 140         ao_scheme_stack_pop();
 141         DBGI("..state %d\n", ao_scheme_stack ? ao_scheme_stack->state : -1);
 142         return 1;
 143 }
 144
 145 /*
 146  * A formal has been computed.
 147  *
 148  * If this is the first formal, then check to see if we've got a
 149  * lamda, macro or nlambda.
 150  *
 151  * For lambda, go compute another formal.  This will terminate
 152  * when the sexpr state sees nil.
 153  *
 154  * For macro/nlambda, we're done, so move the sexprs into the values
 155  * and go execute it.
 156  *
 157  * Macros have an additional step of saving a stack frame holding the
 158  * macro value execution context, which then gets the result of the
 159  * macro to run
 160  */
 161
 162 static int
 163 ao_scheme_eval_formal(void)
 164 {
 165         ao_poly                 formal;
 166         struct ao_scheme_stack  *prev;
 167
 168         DBGI("formal: "); DBG_POLY(ao_scheme_v); DBG("\n");
 169
 170         /* Check what kind of function we've got */
 171         if (!ao_scheme_stack->values) {
 172                 switch (func_type(ao_scheme_v)) {
 173                 case AO_SCHEME_FUNC_LAMBDA:
 174                         DBGI(".. lambda\n");
 175                         break;
 176                 case AO_SCHEME_FUNC_MACRO:
 177                         /* Evaluate the result once more */
 178                         ao_scheme_stack->state = eval_macro;
 179                         if (!ao_scheme_stack_push())
 180                                 return 0;
 181
 182                         /* After the function returns, take that
 183                          * value and re-evaluate it
 184                          */
 185                         prev = ao_scheme_poly_stack(ao_scheme_stack->prev);
 186                         ao_scheme_stack->sexprs = prev->sexprs;
 187
 188                         DBGI(".. start macro\n");
 189                         DBGI("\t.. sexprs       "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 190                         DBGI("\t.. values       "); DBG_POLY(ao_scheme_stack->values); DBG("\n");
 191                         DBG_FRAMES();
 192
 193                         /* fall through ... */
 194                 case AO_SCHEME_FUNC_NLAMBDA:
 195                         DBGI(".. nlambda or macro\n");
 196
 197                         /* use the raw sexprs as values */
 198                         ao_scheme_stack->values = ao_scheme_stack->sexprs;
 199                         ao_scheme_stack->values_tail = AO_SCHEME_NIL;
 200                         ao_scheme_stack->state = eval_exec;
 201
 202                         /* ready to execute now */
 203                         return 1;
 204                 case -1:
 205                         return 0;
 206                 }
 207         }
 208
 209         /* Append formal to list of values */
 210         formal = ao_scheme__cons(ao_scheme_v, AO_SCHEME_NIL);
 211         if (!formal)
 212                 return 0;
 213
 214         if (ao_scheme_stack->values_tail)
 215                 ao_scheme_poly_cons(ao_scheme_stack->values_tail)->cdr = formal;
 216         else
 217                 ao_scheme_stack->values = formal;
 218         ao_scheme_stack->values_tail = formal;
 219
 220         DBGI(".. values "); DBG_POLY(ao_scheme_stack->values); DBG("\n");
 221
 222         /*
 223          * Step to the next argument, if this is last, then
 224          * 'sexpr' will end up switching to 'exec'
 225          */
 226         ao_scheme_v = ao_scheme_poly_cons(ao_scheme_stack->sexprs)->cdr;
 227
 228         ao_scheme_stack->state = eval_sexpr;
 229
 230         DBGI(".. "); DBG_POLY(ao_scheme_v); DBG("\n");
 231         return 1;
 232 }
 233
 234 /*
 235  * Start executing a function call
 236  *
 237  * Most builtins are easy, just call the function.
 238  * 'cond' is magic; it sticks the list of clauses
 239  * in 'sexprs' and switches to 'cond' state. That
 240  * bit of magic is done in ao_scheme_set_cond.
 241  *
 242  * Lambdas build a new frame to hold the locals and
 243  * then re-use the current stack context to evaluate
 244  * the s-expression from the lambda.
 245  */
 246
 247 static int
 248 ao_scheme_eval_exec(void)
 249 {
 250         ao_poly v;
 251         struct ao_scheme_builtin        *builtin;
 252
 253         DBGI("exec: "); DBG_POLY(ao_scheme_v); DBG(" values "); DBG_POLY(ao_scheme_stack->values); DBG ("\n");
 254         ao_scheme_stack->sexprs = AO_SCHEME_NIL;
 255         switch (ao_scheme_poly_type(ao_scheme_v)) {
 256         case AO_SCHEME_BUILTIN:
 257                 ao_scheme_stack->state = eval_val;
 258                 builtin = ao_scheme_poly_builtin(ao_scheme_v);
 259                 v = ao_scheme_func(builtin) (
 260                         ao_scheme_poly_cons(ao_scheme_poly_cons(ao_scheme_stack->values)->cdr));
 261                 DBG_DO(if (!ao_scheme_exception && ao_scheme_poly_builtin(ao_scheme_v)->func == builtin_set) {
 262                                 struct ao_scheme_cons *cons = ao_scheme_poly_cons(ao_scheme_stack->values);
 263                                 ao_poly atom = ao_scheme_arg(cons, 1);
 264                                 ao_poly val = ao_scheme_arg(cons, 2);
 265                                 DBGI("set "); DBG_POLY(atom); DBG(" = "); DBG_POLY(val); DBG("\n");
 266                         });
 267                 builtin = ao_scheme_poly_builtin(ao_scheme_v);
 268                 if (builtin && (builtin->args & AO_SCHEME_FUNC_FREE_ARGS) && !ao_scheme_stack_marked(ao_scheme_stack) && !ao_scheme_skip_cons_free) {
 269                         struct ao_scheme_cons *cons = ao_scheme_poly_cons(ao_scheme_stack->values);
 270                         ao_scheme_stack->values = AO_SCHEME_NIL;
 271                         ao_scheme_cons_free(cons);
 272                 }
 273
 274                 ao_scheme_v = v;
 275                 ao_scheme_stack->values = AO_SCHEME_NIL;
 276                 ao_scheme_stack->values_tail = AO_SCHEME_NIL;
 277                 DBGI(".. result "); DBG_POLY(ao_scheme_v); DBG ("\n");
 278                 DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 279                 break;
 280         case AO_SCHEME_LAMBDA:
 281                 DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 282                 ao_scheme_stack->state = eval_begin;
 283                 v = ao_scheme_lambda_eval();
 284                 ao_scheme_stack->sexprs = v;
 285                 ao_scheme_stack->values = AO_SCHEME_NIL;
 286                 ao_scheme_stack->values_tail = AO_SCHEME_NIL;
 287                 DBGI(".. sexprs "); DBG_POLY(ao_scheme_stack->sexprs); DBG("\n");
 288                 DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 289                 break;
 290         case AO_SCHEME_STACK:
 291                 DBGI(".. stack "); DBG_POLY(ao_scheme_v); DBG("\n");
 292                 ao_scheme_v = ao_scheme_stack_eval();
 293                 DBGI(".. value "); DBG_POLY(ao_scheme_v); DBG("\n");
 294                 DBGI(".. frame "); DBG_POLY(ao_scheme_frame_poly(ao_scheme_frame_current)); DBG("\n");
 295                 break;
 296         }
 297         ao_scheme_skip_cons_free = 0;
 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_skip_cons_free = 1;
 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_v || ao_scheme_poly_type(ao_scheme_v) != AO_SCHEME_CONS) {
 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_poly_type(ao_scheme_v) == AO_SCHEME_CONS) {
 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 /*
 536  * Called at restore time to reset all execution state
 537  */
 538
 539 void
 540 ao_scheme_eval_clear_globals(void)
 541 {
 542         ao_scheme_stack = NULL;
 543         ao_scheme_frame_current = NULL;
 544         ao_scheme_v = AO_SCHEME_NIL;
 545 }
 546
 547 int
 548 ao_scheme_eval_restart(void)
 549 {
 550         return ao_scheme_stack_push();
 551 }
 552
 553 ao_poly
 554 ao_scheme_eval(ao_poly _v)
 555 {
 556         ao_scheme_v = _v;
 557
 558         ao_scheme_frame_init();
 559
 560         if (!ao_scheme_stack_push())
 561                 return AO_SCHEME_NIL;
 562
 563         while (ao_scheme_stack) {
 564                 if (!(*evals[ao_scheme_stack->state])() || ao_scheme_exception) {
 565                         ao_scheme_stack_clear();
 566                         return AO_SCHEME_NIL;
 567                 }
 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_frame_current = NULL;
 571         return ao_scheme_v;
 572 }