object *eval(object *exp, object *env) {
object *procedure;
object *arguments;
object *result;
bool tailcall = false;
do {
if (is_self_evaluating(exp))
return exp;
if (is_variable(exp))
return lookup_variable_value(exp, env);
if (is_quoted(exp))
return text_of_quotation(exp);
if (is_assignment(exp))
return eval_assignment(exp, env);
if (is_definition(exp))
return eval_definition(exp, env);
if (is_if(exp)) {
exp = is_true(eval(if_predicate(exp), env)) ? if_consequent(exp) : if_alternative(exp);
tailcall = true;
continue;
}
if (is_lambda(exp))
return make_compound_proc(lambda_parameters(exp), lambda_body(exp), env);
if (is_begin(exp)) {
exp = begin_actions(exp);
while (!is_last_exp(exp)) {
eval(first_exp(exp), env);
exp = rest_exps(exp);
}
exp = first_exp(exp);
tailcall = true;
continue;
}
if (is_cond(exp)) {
exp = cond_to_if(exp);
tailcall = true;
continue;
}
if (is_let(exp)) {
exp = let_to_application(exp);
tailcall = true;
continue;
}
if (is_and(exp)) {
exp = and_tests(exp);
if (is_empty(exp))
return make_boolean(true);
while (!is_last_exp(exp)) {
result = eval(first_exp(exp), env);
if (is_false(result))
return result;
exp = rest_exps(exp);
}
exp = first_exp(exp);
tailcall = true;
continue;
}
if (is_or(exp)) {
exp = or_tests(exp);
if (is_empty(exp)) {
return make_boolean(false);
}
while (!is_last_exp(exp)) {
result = eval(first_exp(exp), env);
if (is_true(result))
return result;
exp = rest_exps(exp);
}
exp = first_exp(exp);
tailcall = true;
continue;
}
if (is_application(exp)) {
procedure = eval(operator(exp), env);
arguments = list_of_values(operands(exp), env);
if (is_primitive_proc(procedure) && procedure->data.primitive_proc.fn == eval_proc) {
exp = eval_expression(arguments);
env = eval_environment(arguments);
tailcall = true;
continue;
}
if (is_primitive_proc(procedure) && procedure->data.primitive_proc.fn == apply_proc) {
procedure = apply_operator(arguments);
arguments = apply_operands(arguments);
}
if (is_primitive_proc(procedure))
return (procedure->data.primitive_proc.fn)(arguments);
if (is_compound_proc(procedure)) {
env = extend_environment(procedure->data.compound_proc.parameters, arguments, procedure->data.compound_proc.env);
exp = make_begin(procedure->data.compound_proc.body);
tailcall = true;
continue;
}
return make_error(342, "unknown procedure type");
} // is_application()
} while (tailcall);
fprintf(stderr, "cannot eval unknown expression type\n");
exit(EXIT_FAILURE);
}
object *bs_eval(object *exp, object *env)
{
tailcall:
if (is_empty_list(exp)) {
error("unable to evaluate empty list");
} else if (is_self_evaluating(exp)) {
return exp;
} else if (is_variable(exp)) {
return lookup_variable_value(exp, env);
} else if (is_quoted(exp)) {
return quoted_expression(exp);
} else if (is_assignment(exp)) {
return eval_assignment(exp, env);
} else if (is_definition(exp)) {
return eval_definition(exp, env);
} else if (is_if(exp)) {
if (is_true(bs_eval(if_predicate(exp), env))) {
exp = if_consequent(exp);
} else {
exp = if_alternate(exp);
}
goto tailcall;
} else if (is_lambda(exp)) {
return make_compound_proc(lambda_parameters(exp),
lambda_body(exp),
env);
} else if (is_begin(exp)) {
exp = begin_actions(exp);
if (is_empty_list(exp)) {
error("empty begin block");
}
while (!is_empty_list(cdr(exp))) {
bs_eval(car(exp), env);
exp = cdr(exp);
}
exp = car(exp);
goto tailcall;
} else if (is_cond(exp)) {
exp = cond_to_if(exp);
goto tailcall;
} else if (is_let(exp)) {
exp = let_to_application(exp);
goto tailcall;
} else if (is_and(exp)) {
exp = and_tests(exp);
if (is_empty_list(exp)) {
return get_boolean(1);
}
object *result;
while (!is_empty_list(cdr(exp))) {
result = bs_eval(car(exp), env);
if (is_false(result)) {
return result;
}
exp = cdr(exp);
}
exp = car(exp);
goto tailcall;
} else if (is_or(exp)) {
exp = or_tests(exp);
if (is_empty_list(exp)) {
return get_boolean(0);
}
object *result;
while (!is_empty_list(cdr(exp))) {
result = bs_eval(car(exp), env);
if (is_true(result)) {
return result;
}
exp = cdr(exp);
}
exp = car(exp);
goto tailcall;
} else if (is_application(exp)) {
object *procedure = bs_eval(application_operator(exp), env);
object *parameters = eval_parameters(application_operands(exp), env);
// handle eval specially for tailcall requirement.
if (is_primitive_proc(procedure) &&
procedure->value.primitive_proc == eval_proc) {
exp = eval_expression(parameters);
env = eval_environment(parameters);
goto tailcall;
}
// handle apply specially for tailcall requirement.
if (is_primitive_proc(procedure) &&
procedure->value.primitive_proc == apply_proc) {
procedure = apply_operator(parameters);
parameters = apply_operands(parameters);
}
if (is_primitive_proc(procedure)) {
return (procedure->value.primitive_proc)(parameters);
} else if (is_compound_proc(procedure)) {
env = extend_environment(
procedure->value.compound_proc.parameters,
parameters,
procedure->value.compound_proc.env);
exp = make_begin(procedure->value.compound_proc.body);
goto tailcall;
} else {
error("unable to apply unknown procedure type");
}
} else {
error("unable to evaluate expression");
}
}
