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This SRFI extends the specification of the boxes of SRFI 111 so that they are multiple-values aware. Whereas a SRFI 111 box is limited in that it can only box a single value, multiple values can be boxed with this SRFI.
There are currently no issues.
At its core, Scheme's evaluation semantics is multiple-value based. Continuations can accept an arbitrary number of values and expressions can yield an arbitrary number of values. This is contrast to the functional languages ML and Haskell.
Despite this fact, programming with multiple values is more
cumbersome than programming with single values. This is mostly
due to the fact that Scheme's application syntax does not deal
directly with operands returning multiple values so that the
programmer has to fall back on things
call-with-values. It is, however, also partly
due to the fact that a lot of Scheme's procedures have been
modelled on a language that does not have multiple values.
One example for this are the procedures exported by SRFI 111. In an ongoing attempt to make Scheme more uniform (and therefore also simpler) and so that multiple values feel less like a second-class citizen, this SRFI extends SRFI 111 so that it becomes multiple-values-aware in a natural way. The naturalness of the extension is a proof that it is the right extension.
The boxes of this SRFI can be used to reify the concept of multiple values into a first-class single value. This can be used in the implementation of SRFIs like SRFI 189.
Multiple-value-aware boxes as described in this SRFI form a
natural Scheme monad as much as the monads defined in SRFI 165
and SRFI 189 do. However, it is left to a future SRFI to
describe a monadic interface to boxes (the monadic pure would be
box procedure; the monadic join would
unbox when restricted to boxes whose values
consist of a single box).
In a Scheme system supporting both SRFI 111 and this SRFI, the bindings exported by both SRFIs have to be equivalent.
The following procedures implement the box type (which is disjoint
from all other Scheme types), and are exported by
(srfi 111) and
(box value …)
Constructor. Returns a newly allocated box initialized to
object is a box, and
Accessor. Returns the current values of
(set-box! box value …)
The behavior of boxes with the equivalence
equal? is the same as if they were implemented
with records. That is, two boxes are both
eqv? iff they are the product of the same call
to box and not otherwise, and while they must
equal? if they are
converse is implementation-dependent.
A simple, portable R7RS-implementation of
195) and a compatible
(srfi 111) is given in
the repository of this SRFI.
This SRFI is based on SRFI 111, written by John Cowan. In the specification section, I stole its language.
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