210: Procedures and Syntax for Multiple Values

by Marc Nieper-Wißkirchen

Status

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Abstract

This SRFI extends the Scheme standard with procedures and syntax dealing with multiple values, including syntax to create lists and vectors from expressions returning multiple values and procedures returning the elements of a list or vector as multiple values.

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 in 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.

To make programming with multiple values appear more natural, convenient and efficient, this SRFI defines procedures and syntax for dealing with multiple values.

In the R5RS, there were basically two procedures dealing with multiple values, values and call-with-values. SRFI 8 and SRFI 11 added binding constructs for multiple values, yet these do not cover all most-used idioms when dealing with multiple values. Thus the need for this complementary SRFI.

Some of the syntax and procedures defined here also appear in SRFI 71, albeit under different names. SRFI 71, however, is mostly concerned with extending the let keyword. To implement it, one needs a mechanism to change this keyword.

Specification

When this specification talks about boxes, it refers to the disjoint Scheme type described by SRFI 111 (single-valued) and SRFI 195 (multiple-valued).

Syntax

(apply/mv operator operand1producer)

Syntax: Operator, each operand, and producer can be any expression.

Semantics: An apply/mv expression is evaluated as follows: Operator, each operand, and producer are evaluated in an unspecified order. The procedure resulting from evaluating operator is then tail-called with the single values of the operands and the multiple values of producer in that order as the actual arguments. The values returned by the procedure are returned.

The apply/mv syntax could be defined by

    (define-syntax apply/mv
      (syntax-rules ()
        ((apply/mv operator operand1 ... producer)
         (letrec-syntax
             ((aux (syntax-rules ::: ()
                     ((aux %operator () ((%operand1 arg1) :::) %producer)
                      (let-values (((proc) %operator)
                                   ((arg1) %operand1) :::
                                   (args %producer))
                        (apply proc arg1 ::: args)))
                     ((aux %operator (%operand1 operand2 :::) (temp :::) %producer)
                      (aux %operator (operand2 :::) (temp ::: (%operand1 arg1))
                           %producer)))))
           (aux operator (operand1 ...) () producer)))))

A trick is used to generate the temporary names needed to avoid specifying the order in which the values are evaluated.

    (apply/mv string #\a (values #\b #\c))   ⟹ "abc"

(call/mv consumer producer1 …)

Syntax: Consumer and each producer can be any expression.

Semantics: A call/mv expression is evaluated as follows: Consumer and each producer are evaluated in an unspecified order. The procedure resulting from evaluating consumer is then tail-called with the multiple values of the producers in that order as the actual arguments. The values returned by the procedure are returned.

Note: This syntax is equivalent to the Common Lisp function multiple-value-call.

The call/mv syntax could be defined by

    (define-syntax call/mv
      (syntax-rules ()
        ((call/mv consumer producer1 ...)
         (letrec-syntax
             ((aux (syntax-rules ::: ()
                     ((aux %consumer () ((%producer1 args1) :::))
                      (let-values (((proc) %consumer)
                                   (args1 %producer1) :::)
                        (apply proc (append args1 :::))))
                     ((aux %consumer (%producer1 producer2 :::) (temp :::))
                      (aux %consumer (producer2 :::) (temp ::: (%producer1 args1)))))))
           (aux consumer (producer1 ...) ())))))
    (call/mv string (values #\a #\b) (values #\c #\d))   ⟹ "abcd"

(list/mv element1producer)

Syntax: Each element and producer can be any expression.

Semantics: A list/mv expression is evaluated as follows: Each element and producer are evaluated in an unspecified order. A list constructed from the single values of the elements and the multiple values of producer in that order is then returned.

The list/mv syntax could be defined by

    (define-syntax list/mv
      (syntax-rules ()
        ((list/mv element1 ... producer)
         (apply/mv list element1 ... producer))))
    (list/mv 'a (values 'b 'c))   ⟹ (a b c)

(vector/mv element1producer)

Syntax: Each element and producer can be any expression.

Semantics: A vector/mv expression is evaluated as follows: Each element and producer are evaluated in an unspecified order. A vector constructed from the single values of the elements and the multiple values of producer in that order is then returned.

The vector/mv syntax could be defined by

    (define-syntax vector/mv
      (syntax-rules ()
        ((vector/mv element1 ... producer)
         (apply/mv vector element1 ... producer))))
    (vector/mv 'a (values 'b 'c))   ⟹ #(a b c)

(box/mv element1producer)

Syntax: Each element and producer can be any expression.

Semantics: A box/mv expression is evaluated as follows: Each element and producer are evaluated in an unspecified order. A (SRFI 195) box is constructed from the single values of the elements and the multiple values of producer in then order is then returned.

The box/mv syntax could be defined by

    (define-syntax box/mv
      (syntax-rules ()
        ((box/mv element1 ... producer)
         (apply/mv box element1 ... producer))))
    (unbox (box/mv 'a (values 'b 'c)))   ⟹ a b c

(value/mv index operand1producer)

Syntax: Index, each operand, and producer can be any expression.

Semantics: A value/mv expression is evaluated as follows: Index, each element and producer are evaluated in an unspecified order. It is an error if the value i of index is not an exact non-negative index. The ith element of the list consisting of the single values of the operands and the multiple values of producer in that order is then returned. It is an error if there is no such element.

The value/mv syntax could be defined by

    (define-syntax value/mv
      (syntax-rules ()
        ((value/mv index operand1 ... producer)
         (apply/mv value index operand1 ... producer))))
    (value/mv 1 'a (values 'b 'c))   ⟹ b

(coarity producer)

Syntax: Producer can be any expression.

Semantics: An coarity expression is evaluated as follows: Producer is evaluated and then the number of resulting values is returned.

The coarity syntax could be defined by

    (define-syntax coarity
      (syntax-rules ()
        ((coarity producer)
         (let-values ((res producer))
           (length res)))))
    (coarity (values 'a 'b 'c))   ⟹ 3

(set!-values formals producer)

Syntax: formals is a formal arguments list and producer can be any expression.

Semantics: A set!-values expression is evaluated as follows: Producer is evaluated and the resulting values are stored in the locations to which the variables of the formals are bound, where the formals are matched to the resulting values in the same way that the formals in a lambda expression are matched to the arguments in a procedure call. The result of the set!-values expression is unspecified.

The set!-values syntax could be defined by

    (define-syntax set!-values
      (syntax-rules ()
        ((set!-values (var1 ...) producer)
         (letrec-syntax
             ((aux (syntax-rules ::: ()
                     ((aux () ((%var1 temp1) :::) %producer)
                      (let-values (((temp1 ::: . temp*) %producer))
                        (set! %var1 temp1) :::))
                     ((aux (%var1 var2 :::) (temp :::) %producer)
                      (aux (var2 :::) (temp ::: (%var1 temp1)) %producer)))))
           (aux (var1 ... ) () producer)))
        ((set!-values (var1 ... . var*) producer)
         (letrec-syntax
             ((aux (syntax-rules ::: ()
                     ((aux () ((%var1 temp1) ::: (%var* temp*)) %producer)
                      (let-values (((temp1 ::: . temp*) %producer))
                        (set! %var1 temp1) :::
                        (set! %var* temp*)))
                     ((aux (%var1 var2 :::) (temp :::) %producer)
                      (aux (var2 :::) (temp ::: (%var1 temp1)) %producer)))))
           (aux (var1 ... var*) () producer)))
        ((set!-values var* producer)
         (let-values ((temp*) producer)
           (set! var* temp*)))))
    (let ((x #f) (y #f))
      (set!-values (x . y) (values 'a 'b))
      (list x y))                            ⟹ (a (b))

(with-values producer consumer)

Syntax: Producer and consumer can be any expressions.

Semantics: A with-values expression is evaluated as follows: Producer and consumer are evaluated in an unspecified order. The procedure resulting from evaluating consumer is then tail-called with the multiple values of producer as the actual arguments, and its return values are returned.

The with-values syntax could be defined by

    (define-syntax with-values
      (syntax-rules ()
        ((with-values producer consumer)
         (apply/mv consumer producer))))
    (with-values (values 4 5)
      (lambda (a b) b))         ⟹ 5

(case-receive producer clause …)

Syntax: producer can be any expression and each clause is of the form (formals body) as in a case-lambda expression.

Semantics: A case-receive expression is evaluated as follows: Producer is evaluated and then the first clause for which the resulting values agree with formals is selected, where agreement is specified as for the formals of a lambda expression. The variables of formals are bound to fresh locations, the values are stored in those locations, the body is evaluated in the extended environment, and the results of body are returned as the results of the case-receive expression.

The case-receive syntax could be defined by

    (define-syntax case-receive
      (syntax-rules ()
        ((case-receive producer clause ...)
         (with-values producer
           (case-lambda clause ...)))))
    (case-receive (values 'a 'b)
      ((x) #f)
      ((x . y) (list x y)))        ⟹ (a (b))

(bind/mv producer transducer1 …)

Syntax: Producer and the transducers can be any expressions.

Semantics: A bind/mv expression is evaluated as follows: Producer and the transducers are evaluated in an unspecified order. The procedure resulting from evaluating transducer1 is then applied to the values resulting from evaluating producer. The resulting values are then applied to transducer2, and so on until the final transducer is tail-called and the resulting values are returned as the result of the bind/mv expression.

The bind/mv syntax could be defined by

    (define-syntax bind/mv
      (syntax-rules ()
        ((bind/mv producer transducer ...)
         (bind/list (list/mv producer) transducer ...))))
    (bind/mv (values 1 2 3)
             (map-values (lambda (x) (* 2 x)))
             (map-values (lambda (x) (+ 1 x))))   ⟹ 3 5 7

Procedures

(list-values list)

It is an error if list is not a list.

The list-values procedure yields the elements of list as multiple values. It could be defined by

    (define (list-values lis)
      (apply values lis))
    (list-values '(a b c))   ⟹ a b c

(vector-values vec)

It is an error if vec is not a vector.

The vector-values procedure yields the elements of vec as multiple values. It could be defined by

    (define (vector-values vec)
      (list-values (vector->list vec)))
    (vector-values #(a b c))   ⟹ a b c

(box-values box)

It is an error if box is not a box.

The box-values procedure yields the elements of box as multiple values. Equivalent to unbox. It could be defined by

    (define box-values unbox)
    (box-values (box 'a 'b 'c))   ⟹ a b c

(value index obj0objn-1)

It is an error if index is not an exact non-negative integer less than n.

The value procedure simply returns its argument objindex. It could be defined by

    (define (value k . objs)
      (list-ref objs k))
    (value 1 'a 'b 'c)   ⟹ b

(identity obj1 …)

The identity procedure yields the objects objs as multiple values. Equivalent to values. It could be defined by

    (define identity values)
    (identity 1 2 3)   ⟹ 1 2 3

(compose-left transducer1 …)

It is an error if the transducers are not procedures.

The compose-left procedure yields the functional left composition of the transducers. When the resulting procedure is applied to arguments, these arguments are passed to transducer1, whose results are passed to transducer2, and so on until the final results are returned. It could be defined by

    (define compose-left
      (case-lambda
        (() identity)
        ((transducer . transducers)
         (let f ((transducer transducer) (transducers transducers))
           (if (null? transducers)
               transducer
               (let ((composition (f (car transducers) (cdr transducers))))
                 (lambda args
                   (apply/mv composition (apply transducer args)))))))))
    (let ((f (map-values (lambda (x) (* 2 x))))
          (g (map-values (lambda (x) (+ x 1)))))
       ((compose-left f g) 1 2 3)))                ⟹ 3 5 7

(compose-right transducer1transducern)

It is an error if the transducers are not procedures.

The compose-right procedure yields the functional right composition of the transducers. When the resulting procedure is applied to arguments, these arguments are passed to transducern, whose results are passed to transducern-1, and so on until the final results are returned. It could be defined by

    (define compose-right
      (case-lambda
        (() identity)
        ((transducer . transducers)
         (let f ((transducer transducer) (transducers transducers))
           (if (null? transducers)
               transducer
               (let ((composition (f (car transducers) (cdr transducers))))
                 (lambda args
                   (apply/mv transducer (apply composition args)))))))))
    (let ((f (map-values (lambda (x) (* 2 x))))
          (g (map-values (lambda (x) (+ x 1)))))
       ((compose-right f g) 1 2 3)))               ⟹ 4 6 8

(map-values proc)

It is an error if proc is not a procedure.

The map-values procedure returns a procedure that accepts an arbitrary number of arguments, applies proc to each of them in an unspecified order and returns the resulting single values as multiple values. It could be defined by

    (define (map-values proc)
      (lambda args
        (list-values (map proc args))))
    ((map-values odd?) 1 2 3))   ⟹ #t #f #t

(bind/list list transducer1 …)

It is an error if list is not a list or the transducers are not procedures.

The bind/list procedure passes the elements of list to transducer1, whose results are then passed to transducer2, and so on until the final transducer is tail-called and the resulting values are returned. It could be defined by

    (define (bind/list lis . transducers)
      (list-values (fold (lambda (transducer lis)
                           (list/mv (apply transducer lis)))
                         lis transducers)))
    (bind/list '(1 2 3) (map-values (lambda (x) (* 3 x))))   ⟹ 3 6 9

(bind/box box transducer1 …)

It is an error if box is not a box or the transducers are not procedures.

The bind/box procedure passes the values of box to transducer1, whose results are then passed to transducer2, and so on until the final tranducer is tail-called and the resulting values are returned. It could be defined by

    (define (bind/box bx . transducers)
      (apply bind/list (list/mv (unbox bx)) transducers))
    (bind/box (box 1 2 3) (map-values (lambda (x) (* 3 x))))   ⟹ 3 6 9

(bind obj transducer1 …)

It is an error if the transducers are not procedures.

The bind procedure passes the obj to transducer1, whose results are then passed to transducer2, and so on until the final tranducer is tail-called and the resulting values are returned. It could be defined by

    (define (bind obj . transducers)
      (apply bind/list (list obj) transducers))
    (bind 1 (lambda (x) (values (* 3 x) (+ 1 x))))   ⟹ 3 2

Implementation

A portable R7RS implementation can be found in the official repository of this SRFI and can also be deduced from the complete definitions of all syntax and procedures given above.

Implementers are highly encouraged to distribute specialized implementations with their Schemes that implement the described syntax and procedures in the most efficient way.

Acknowledgements

A discussion on the SRFI 195 mailing list eventually led to this SRFI. I would like to thank all participants of this discussion for their contributions.

John Cowan suggested adding the equivalent of Common Lisp's procedure multiple-value-call.

Jim Rees read the example code in the first draft carefully.

Wolfgang Corcoran-Mathe made an extensive review and shared his helpful comments on the mailing list.

The idea of the with-values form comes from the paper An Efficient Implementation of Multiple Return Values in Scheme by J. Michael Ashley and R. Kent Dybvig.

© 2020 Marc Nieper-Wißkirchen.

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Editor: Arthur A. Gleckler