8 Evaluation
A Racket S-expression is evaluated by calling scheme_eval.
This function takes an S-expression (as a Scheme_Object*) and a
namespace and returns the value of the expression in that namespace.
The function scheme_apply takes a Scheme_Object* that is
a procedure, the number of arguments to pass to the procedure, and an
array of Scheme_Object * arguments. The return value is the
result of the application. There is also a function
scheme_apply_to_list, which takes a procedure and a list
(constructed with scheme_make_pair) and performs the Racket
apply operation.
The scheme_eval function actually calls scheme_compile
followed by scheme_eval_compiled.
8.1 Top-level Evaluation Functions
The functions scheme_eval, scheme_apply, etc., are
top-level evaluation functions. Continuation invocations are
confined to jumps within a top-level evaluation (i.e., a continuation
barrier is installed by these functions).
The functions _scheme_eval_compiled, _scheme_apply,
etc. (with a leading underscore) provide the same functionality
without starting a new top-level evaluation; these functions should
only be used within new primitive procedures. Since these functions
allow full continuation hops, calls to non-top-level evaluation
functions can return zero or multiple times.
Currently, escape continuations and primitive error escapes can jump
out of all evaluation and application functions. For more information,
see Exceptions and Escape Continuations.
8.2 Tail Evaluation
All of Racket’s built-in functions and syntax support proper
tail-recursion. When a new primitive procedure or syntax is added to
Racket, special care must be taken to ensure that tail recursion is
handled properly. Specifically, when the final return value of a
function is the result of an application, then
scheme_tail_apply should be used instead of
scheme_apply. When scheme_tail_apply is called, it
postpones the procedure application until control returns to the
Racket evaluation loop.
For example, consider the following implementation of a
thunk-or primitive, which takes any number of thunks and
performs or on the results of the thunks, evaluating only as
many thunks as necessary.
static Scheme_Object * |
thunk_or (int argc, Scheme_Object **argv) |
{ |
int i; |
Scheme_Object *v; |
|
if (!argc) |
return scheme_false; |
|
for (i = 0; i < argc - 1; i++) |
if (SCHEME_FALSEP((v = _scheme_apply(argv[i], 0, NULL)))) |
return v; |
|
return scheme_tail_apply(argv[argc - 1], 0, NULL); |
} |
This thunk-or properly implements tail-recursion: if the
final thunk is applied, then the result of thunk-or is the
result of that application, so scheme_tail_apply is used for
the final application.
8.3 Multiple Values
A primitive procedure can return multiple values by returning the
result of calling scheme_values. The functions
scheme_eval_compiled_multi, scheme_apply_multi,
_scheme_eval_compiled_multi, and _scheme_apply_multi
potentially return multiple values; all other evaluation and
applications procedures return a single value or raise an exception.
Multiple return values are represented by the
scheme_multiple_values “value.” This quasi-value has the type
Scheme_Object*, but it is not a pointer or a fixnum. When the
result of an evaluation or application is
scheme_multiple_values, the number of actual values can be
obtained as scheme_multiple_count, and the array of
Scheme_Object* values as scheme_multiple_array. (Both of
those identifiers are actually macros.)
A garbage collection must not occur between the return of a
scheme_multiple_values “value” and the receipt of the values
through scheme_multiple_count scheme_multiple_array.
Furthermore, if scheme_multiple_array is to be used across a
potential garbage collection, then it must be specifically received by
calling scheme_detach_multiple_array; otherwise, a garbage
collection or further evaluation may change the content of the array.
Otherwise, if any application or evaluation procedure is called, the
scheme_multiple_count and scheme_multiple_array variables
may be modified (but the array previously referenced by
scheme_multiple_array is never re-used if
scheme_detach_multiple_array is called).
The scheme_multiple_count and
scheme_multiple_array variables only contain meaningful values
when scheme_multiple_values is returned.
8.4 Evaluation Functions
Scheme_Object* | | scheme_eval | ( | Scheme_Object* expr, | | | | | Scheme_Env* env) |
|
Evaluates the (uncompiled) S-expression expr in the namespace
env.
Evaluates the compiled expression
obj, which was previously
returned from
scheme_compile, first linking to the namespace
env.
Evaluates the compiled expression
obj, possibly
returning multiple values (see
Multiple Values).
Creates the main namespace for an embedded Racket. This procedure
must be called before other Racket library function (except
scheme_make_param). Extensions to Racket cannot call this
function.
If it is called more than once, this function resets all threads
(replacing the main thread), parameters, ports, namespaces, and
finalizations.
Creates and returns a new namespace. This values can be cast to
Scheme_Env *. It can also be installed in
a parameterization using
scheme_set_param with
MZCONFIG_ENV.
When Racket is embedded in an application, create the initial
namespace with scheme_basic_env before calling this procedure
to create new namespaces.
Scheme_Object* | | scheme_apply | ( | Scheme_Object* f, | | | | | int c, | | | | | Scheme_Object** args) |
|
Applies the procedure f to the given arguments.
Beware that the procedure can mutate args if it is the same as
the result of scheme_current_argument_stack.
If c is 0, then args can be NULL.
Applies the procedure
f to the given arguments, possibly
returning multiple values (see
Multiple Values).
Scheme_Object* | | _scheme_apply | ( | Scheme_Object* f, | | | | | int c, | | | | | Scheme_Object** args) |
|
Applies the procedure f to the list of arguments in args.
Reads a single S-expression from str and evaluates it in the given
namespace; the expression must return a single value, otherwise an
exception is raised. The str argument is parsed as a
UTF-8-encoded string of Unicode characters (so plain ASCII is fine).
Like
scheme_eval_string, but returns
scheme_multiple_values when the expression returns multiple
values.
Like
scheme_eval_string, but if
all is not
0, then
expressions are read and evaluated from
str until the end of
the string is reached.
Applies the procedure as a tail-call. Actually, this function just
registers the given application to be invoked when control returns to
the evaluation loop. (Hence, this function is only useful within a
primitive procedure that is returning to its caller.)
Like
scheme_tail_apply, but the array
args is not
copied. Use this only when
args has infinite extent and will not
be used again, or when
args will certainly not be used again
until the called procedure has returned.
Applies the procedure as a tail-call.
Scheme_Object* | | scheme_compile | ( | Scheme_Object* form, | | | | | Scheme_Env* env, | | | | | int writable) |
|
Compiles the S-expression
form in the given namespace. The
returned value can be used with
scheme_eval_compiled et al.
Provide a non-zero value for
writable if the resulting compiled
object will be marshalled via
write instead of evaluated.
Scheme_Object* | | scheme_expand | ( | Scheme_Object* form, | | | | | Scheme_Env* env) |
|
Expands all macros in the S-expression form using the given
namespace.
Returns the given values together as multiple return values. Unless
n is 1, the result will always be
scheme_multiple_values.