244: Multiple-value Definitions

by Marc Nieper-Wißkirchen

Status

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Abstract

A define-values form is a definition that binds multiple variables from a single expression returning multiple values.

Rationale

This SRFI makes the define-values definition, which was standardized in R7RS, available to R6RS systems.

It is preferable for define-values to be implemented in the Scheme system's core so that modifications of an algorithm like the one described in Fixing Letrec (reloaded) can be employed to implement define-values definitions as effectively as other internal variable definitions.

Specification

This specification describes the (srfi :244) and (srfi :244 define-values) libraries.

The define-values form described in this specification is a ⟨definition⟩ and may appear anywhere other definitions may appear.

(define-values ⟨formals⟩ ⟨expression⟩)

Syntax: Any variable must not appear more than once in the set of ⟨formals⟩.

Semantics: The variables occurring in the ⟨formals⟩ are bound to fresh locations containing the values returned by the ⟨expression⟩, where the ⟨formals⟩ are matched to the return values in the same way that the ⟨formals⟩ in a lambda expression are matched to the arguments in a procedure call. The effect of the ⟨formals⟩ not matching is undefined.

The continuation of ⟨expression⟩ should not be invoked more than once.

    (define-values (x y)
      (values 1 2))
    (+ x y)                  ⟹ 3

    (define-values (a . b)
      (values 1 2))
    (cons a b)               ⟹ (1 2)

Implementation

Sample implementation as a portable R6RS library

Acknowledgements

The form described in this SRFI has been well-known for a long time.

I would like to thank Jim Rees for pointing out to me that the original sample implementation violated the R6RS requirement that it is a syntax violation to use set! with exported variables.

References

  1. Abdulaziz Ghuloum and R. Kent Dybvig. Fixing letrec (reloaded). In Proceedings of the 2009 Workshop on Scheme and Functional Programming, 57–65, 2009.

© 2022 Marc Nieper-Wißkirchen.

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Editor: Arthur A. Gleckler