Scheme allows a procedure to return more than one value to its caller. This is quite different to other languages which only allow single-value returns. Returning multiple values is different from returning a list (or pair or vector) of values to the caller, because conceptually not one compound object is returned, but several distinct values.
The primitive procedures for handling multiple values are
values is used for returning
multiple values from a procedure. This is done by placing a call to
values with zero or more arguments in tail position in a
call-with-values combines a procedure returning
multiple values with a procedure which accepts these values as
Delivers all of its arguments to its continuation. Except for
continuations created by the
all continuations take exactly one value. The effect of
passing no value or more than one value to continuations that
were not created by
call-with-values is unspecified.
scm_values, args is a list of arguments and the
return is a multiple-values object which the caller can return. In
the current implementation that object shares structure with
args, so args should not be modified subsequently.
scm_c_values is an alternative to
scm_values. It creates
a new values object, and copies into it the n values starting from
Currently this creates a list and passes it to
scm_values, but we
expect that in the future we will be able to use a more efficient
If obj is a multiple-values object, returns the number of values it contains. Otherwise returns 1.
Returns the value at the position specified by idx in obj. Note that obj will ordinarily be a multiple-values object, but it need not be. Any other object represents a single value (itself), and is handled appropriately.
Calls its producer argument with no values and a
continuation that, when passed some values, calls the
consumer procedure with those values as arguments. The
continuation for the call to consumer is the continuation
of the call to
(call-with-values (lambda () (values 4 5)) (lambda (a b) b)) ⇒ 5
(call-with-values * -) ⇒ -1
In addition to the fundamental procedures described above, Guile has a
module which exports a syntax called
receive, which is much
more convenient. This is in the
(ice-9 receive) and is the
same as specified by SRFI-8 (see SRFI-8).
(use-modules (ice-9 receive))
Evaluate the expression expr, and bind the result values (zero
or more) to the formal arguments in formals. formals is a
list of symbols, like the argument list in a
(see Lambda). After binding the variables, the expressions in
body … are evaluated in order, the return value is the
result from the last expression.
For example getting results from
partition in SRFI-1
(receive (odds evens) (partition odd? '(7 4 2 8 3)) (display odds) (display " and ") (display evens)) -| (7 3) and (4 2 8)