Procedures are created by evaluating
(see section Lambda Expressions); the
lambda may either be explicit
or may be implicit as in a "procedure
(see section Definitions). Also there are special built-in procedures,
called primitive procedures, such as
car; these procedures
are not written in Scheme but in the language used to implement the
Scheme system. MIT Scheme also provides application hooks, which
support the construction of data structures that act like procedures.
In MIT Scheme, the written representation of a procedure tells you the type of the procedure (compiled, interpreted, or primitive):
pp => #[compiled-procedure 56 ("pp" #x2) #x10 #x307578] (lambda (x) x) => #[compound-procedure 57] (define (foo x) x) foo => #[compound-procedure 58 foo] car => #[primitive-procedure car] (call-with-current-continuation (lambda (x) x)) => #[continuation 59]
Note that interpreted procedures are called "compound" procedures (strictly speaking, compiled procedures are also compound procedures). The written representation makes this distinction for historical reasons, and may eventually change.
(cons* object object ...)
The initial objects may be any objects, but the last object (there must be at least one object) must be a list.
(apply + (list 3 4 5 6)) => 18 (apply + 3 4 '(5 6)) => 18 (define compose (lambda (f g) (lambda args (f (apply g args))))) ((compose sqrt *) 12 75) => 30
#tif object is a procedure; otherwise returns
#tis returned, exactly one of the following predicates is satisfied by object:
#tif object is a compiled procedure; otherwise returns
#tif object is a compound (i.e. interpreted) procedure; otherwise returns
#tif object is a primitive procedure; otherwise returns
The following two procedures test the arity of a procedure, that
is, the number of arguments that the procedure accepts. The results of
the test may be less restrictive than the effect of calling the
procedure. In other words, these procedures may indicate that the
procedure will accept a given number of arguments, but if you call the
procedure it may signal a
condition-type:wrong-number-of-arguments error. This is because
these procedures examine the apparent arity of a procedure. For
example, here is a procedure that appears to accept any number of
arguments, but when called will signal an error if the number of
arguments is not one:
(lambda arguments (apply car arguments))
#tif procedure accepts k arguments; otherwise returns
#fmeaning that the procedure has no maximum number of arguments.
(procedure-arity (lambda () 3)) => (0 . 0) (procedure-arity (lambda (x) x)) => (1 . 1) (procedure-arity car) => (1 . 1) (procedure-arity (lambda x x)) => (0 . #f) (procedure-arity (lambda (x . y) x)) => (1 . #f) (procedure-arity (lambda (x #!optional y) x)) => (1 . 2)
#t; if not supplied it defaults to
#f. Returns the primitive procedure called name. May perform further actions depending on arity:
-1means it accepts any number of arguments.
(primitive-procedure-name car) => car
#tif primitive-procedure is implemented; otherwise returns
#f. Useful because the code that implements a particular primitive procedure is not necessarily linked into the executable Scheme program.
call-with-current-continuationhas unlimited extent just like any other procedure in Scheme. It may be stored in variables or data structures and may be called as many times as desired.
The following examples show only the most common uses of this procedure.
If all real programs were as simple as these examples, there would be no
need for a procedure with the power of
(call-with-current-continuation (lambda (exit) (for-each (lambda (x) (if (negative? x) (exit x))) '(54 0 37 -3 245 19)) #t)) => -3 (define list-length (lambda (obj) (call-with-current-continuation (lambda (return) (letrec ((r (lambda (obj) (cond ((null? obj) 0) ((pair? obj) (+ (r (cdr obj)) 1)) (else (return #f)))))) (r obj)))))) (list-length '(1 2 3 4)) => 4 (list-length '(a b . c)) => #f
A common use of
call-with-current-continuation is for structured,
non-local exits from loops or procedure bodies, but in fact
call-with-current-continuation is quite useful for implementing a
wide variety of advanced control structures.
Whenever a Scheme expression is evaluated a continuation exists that
wants the result of the expression. The continuation represents an
entire (default) future for the computation. If the expression is
evaluated at top level, for example, the continuation will take the
result, print it on the screen, prompt for the next input, evaluate it,
and so on forever. Most of the time the continuation includes actions
specified by user code, as in a continuation that will take the result,
multiply it by the value stored in a local variable, add seven, and give
the answer to the top-level continuation to be printed. Normally these
ubiquitous continuations are hidden behind the scenes and programmers
don't think much about them. On the rare occasions that you may need to
deal explicitly with continuations,
call-with-current-continuation lets you do so by creating a
procedure that acts just like the current continuation.
#tif object is a continuation; otherwise returns
call-with-current-continuation. Thunk must be a procedure of no arguments. Conceptually,
continuationon the result of invoking
thunkis executed in the dynamic context of
continuation. In other words, the "current" continuation is abandoned before
unwind-protect, designed to take into account the fact that continuations produced by
call-with-current-continuationmay be reentered. The arguments before-thunk, action-thunk, and after-thunk must all be procedures of no arguments (thunks).
dynamic-wind behaves as follows. First before-thunk is
called. Then action-thunk is called. Finally, after-thunk
is called. The value returned by action-thunk is returned as the
dynamic-wind. After-thunk is also called if
action-thunk escapes from its continuation. If action-thunk
captures its continuation as an escape procedure, escapes from it, then
escapes back to it, after-thunk is invoked when escaping away, and
before-thunk is invoked when escaping back.
dynamic-wind is useful, for example, for ensuring the proper
maintenance of locks: locking would occur in the before-thunk,
protected code would appear in the action-thunk, and unlocking
would occur in the after-thunk.
The following two procedures support multiple values. A future revision of the Scheme standard will support a facility similar to, but almost certainly different from, this one.
valuesprocedure. Then procedure is called with the multiple values as its arguments. The result yielded by procedure is returned as the result of
call-with-values. Furthermore it must accept as many values as there are objects.
Application hooks are objects that can be applied like procedures. Each application hook has two parts: a procedure that specifies what to do when the application hook is applied, and an arbitrary object, called extra. Often the procedure uses the extra object to determine what to do.
There are two kinds of application hooks, which differ in what arguments are passed to the procedure. When an apply hook is applied, the procedure is passed exactly the same arguments that were passed to the apply hook. When an entity is applied, the entity itself is passed as the first argument, followed by the other arguments that were passed to the entity.
Both apply hooks and entities satisfy the predicate
Each satisfies either
primitive-procedure?, depending on
its procedure component. An apply hook is considered to accept the same
number of arguments as its procedure, while an entity is considered to
accept one less argument than its procedure.
#tif object is an apply hook; otherwise returns
#tif object is an entity; otherwise returns