# Title

Multiple-value boxes

# Author

Marc Nieper-Wißkirchen

# Status

This SRFI is currently in draft status. Here is an explanation of each status that a SRFI can hold. To provide input on this SRFI, please send email to `srfi-195@nospamsrfi.schemers.org`. To subscribe to the list, follow these instructions. You can access previous messages via the mailing list archive.

- Received: 2020-05-04
- 60-day deadline: 2020-07-03
- Draft #1 published: 2020-05-14

# Abstract

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.

# Issues

There are currently no issues.

# Rationale

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 like `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 the `box` procedure; the monadic join would be `unbox` when restricted to boxes whose values consist of a single box).

# Specification

In a Scheme system supporting both SRFI 111 and this SRFI, the bindings exported by both SRFIs have to be equivalent.

## Procedures

The following procedures implement the box type (which is disjoint from all other Scheme types), and are exported by the `(srfi 111)` and `(srfi 195)` libraries.

`(box value …)`

Constructor. Returns a newly allocated box initialized to the `value`s.

`(box? object)`

Predicate. Returns `#t` if `object` is a box, and `#f` otherwise.

`(unbox box)`

Accessor. Returns the current values of `box`.

`(set-box! box value …)`

Mutator. Changes `box` to hold `value`s.

The behavior of boxes with the equivalence predicates `eq?`, `eqv?`, and `equal?` is the same as if they were implemented with records. That is, two boxes are both `eq?` and `eqv?` iff they are the product of the same call to box and not otherwise, and while they must be `equal?` if they are `eqv?`, the converse is implementation-dependent.

# Implementation

A simple, portable R7RS-implementation of `(srfi 195)` and a compatible `(srfi 111)` is given in the repository of this SRFI.

# Acknowledgements

This SRFI is based on SRFI 111, written by John Cowan. In the specification section, I stole its language.

# Copyright

Copyright © Marc Nieper-Wißkirchen (2020).

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice (including the next paragraph) shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.