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Multiple Values

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.
— Marc Nieper-Wißkirchen
SRFI 210

What are "multiple values"? As the name suggests, they are representations of more than one value - but they are bound together. This binding is not the same as some kind of data structure, like a list. The procedure values is used to create a set of multiple values from the individual values 1 and 2:

gosh$ (values 1 2)
1
2

As a more useful example, (scheme base) in R7RS contains the procedure exact-integer-sqrt. The definition for this states: "Returns two non-negative exact integers s and r".

gosh$ (exact-integer-sqrt 16)
4
0
gosh$ (exact-integer-sqrt 17)
4
1

As the procedure has not returned a single value, we cannot simply use the result. So, what can we do with multiple values? And, how can we get and use all of the returned values?

Support for multiple values has grown with successive versions of the Scheme standard:

  • R5RS introduced multiple-value returns, supported with the values and call-with-values syntax.

  • R6RS added the let-values and let*-values forms, previously defined in SRFI 11.

  • R7RS has added define-values.

R7RS-small contains the following five constructs dealing with multiple values:

  • call-with-values - used to pass values returned from a producer procedure to a consumer

  • define-values - used like define, but binds values to identifiers

  • let-values - used like let, but binds values to identifiers

  • let*-values - used like let*, but binds values to identifiers with an environment covering the previous bindings

  • values - bundles its arguments together as a set of values

Given a procedure which returns multiple values, we can get and use all of the returned values by using the define- and let- forms.

define-values works like define but matches up a series of identifiers with the respective returned value:

gosh$ (define-values (root remainder) (exact-integer-sqrt 17))
remainder
gosh$ root
4
gosh$ remainder
1

As define-values is matching a list of formals (like lambda), it is easy to get values as lists, do simple pattern matching, etc:

gosh$ (define-values vals (values 1 2 3))
vals
gosh$ vals
(1 2 3)
gosh$ (list? vals)
#t
gosh$ (define-values (head . rest) (values 1 2 3))
rest
gosh$ head
1
gosh$ rest
(2 3)

There are no surprises with let-values and let*-values: these work in the same way as let and let*, but use multiple identifiers to match multiple values:

(define (find-roots x y)
  (let-values (((x-root x-remainder) (exact-integer-sqrt x))
               ((y-root y-remainder) (exact-integer-sqrt y)))
    (list x-root y-root)))

Notice how the (x-root x-remainder) pattern collects and names the two returned values from exact-integer-sqrt.)

(define (roots+total-remainder x y)
 (let*-values (((x-root x-remainder) (exact-integer-sqrt x))
               ((y-root y-remainder) (exact-integer-sqrt y))
               ((roots total-remainder)
                (values (list x-root y-root)
                        (+ x-remainder y-remainder))))
    (list roots total-remainder)))

And here, we are using a let*- form, so we can access the previous definitions to create totals.

The remaining form, call-with-values, ties together the production and consumption of multiple values: a producer is a zero-argument procedure that returns multiple values, using a values expression, and a consumer is a procedure that accepts these values as its arguments.

Notice the little "gotcha" that the producer must be a zero-argument procedure.

(define (show-root n)
  (call-with-values
    (lambda () (exact-integer-sqrt n)) ; producer procedure
    (lambda (s r) (display (square s)) ; consumer procedure
                  (display " + ")
                  (display r) (newline))))

gosh$ (show-root 15)
9 + 6

Here, the producer procedure, returns two values, and the consumer procedure accepts those two values as its arguments.

SRFI extensions

Two SRFIs, SRFI 8 and SRFI 210, are particularly targetted at multiple values: these have both been accepted into the Yellow Edition of R7RS-large.

SRFI 8: Binding to multiple values

This SRFI appears in several other SRFIs, its short definition often copy-pasted into the reference implementation of other libraries.

The SRFI provides the receive syntax, to improve on call-with-values. Specifically, instead of having a zero-argument procedure as producer, receive accepts any expression as a producer, and its values are bound into a set of provided identifiers. These can then be processed within the expression’s body.

The show-root example above can be rewritten to use receive as:

(define (show-root n)
  (receive (s r) (exact-integer-sqrt n)           ; (1)
    (display (square s))                          ; (2)
    (display " + ")
    (display r) (newline)))

  1. The multiple-return values are captured into the given identifiers.

  2. The body can work with those identifiers.

Of course, in a post-R5RS world, we have let-values as an alternative, which also has the advantage of being able to "receive" values from multiple producers. The above example using let-values:

(define (show-root n)
  (let-values (((s r) (exact-integer-sqrt n)))    ; (1)
    (display (square s))                          ; (2)
    (display " + ")
    (display r) (newline)))

  1. The multiple-return values are captured into the given identifiers.

  2. The body can work with those identifiers.

SRFI 210: Procedures and syntax for multiple values

Aimed at introducing procedures and syntax for dealing with multiple values, such as creating lists and vectors from expressions returning multiple values and procedures returning the elements of a list or vector as multiple values.

This SRFI introduces a number of syntactic forms and procedures, which can be divided into four groups:

  1. helpful operations for working with multiple values, such as coarity, which tells you how many values you have.

  2. operations for converting multiple values to and from other data types, such as list/mv, which evaluates one or more values and producer, returning all the resulting values as a list.

  3. helpful operations when calling procedures with multiple values, such as apply/mv, which applies a procedure to a set of values, the last of which can be a set of multiple values.

  4. manages groups of procedures, such as bind/mv, which chains a series of procedures together, passing multiple values between them.

We can take a look at two examples.

The first example is case-receive, which is a kind of case statement which matches multiple values. To illustrate, we need a procedure which can return different numbers of values - we write a new version of exact-integer-sqrt to return a single value if the remainder is 0:

(define (new-eis n)
  (let-values (((s r) (exact-integer-sqrt n)))
    (if (zero? r)
      s
      (values s r))))

And then write a function to choose how to display the number:

(define (display-root n)
  (case-receive (new-eis n)
    ((s) (display "Exact root: ") (display s) (newline))
    ((s r) (display "Inexact, with remainder: ") (display r) (newline))))

gosh[r7rs.user]$ (display-root 3)
Inexact, with remainder: 2
gosh[r7rs.user]$ (display-root 4)
Exact root: 2

The second example is bind/mv, which can be used as a simple form of procedure composition, where multiple values returned from one part are passed on to the next procedure in the line. To illustrate this, let’s define a procedure to display a pair of values:

gosh[r7rs.user]$ (define (display-pair s r) (for-each display (list "(" s ", " r ")" #\newline)))
display-pair

We can then use bind/mv to pass a single value to the exact-integer-sqrt procedure, generating multiple values, which will then be passed as arguments to display-pair:

gosh[r7rs.user]$ (bind/mv 13 exact-integer-sqrt display-pair)
(3, 4)

Inconsistencies across Schemes

Different Schemes handle multiple values in some contexts in different ways.

For example, Gauche permits multiple-values to be used in single-value contexts, using just the first value:

gosh$ (+ (values 1 2) (values 3 4))
4
gosh$  (map (lambda (a) (values a a)) '(1 2 3))
(1 2 3)

But this is not required behaviour, and indeed the R7RS report states that, for map, it is an error if the mapped procedure does not return a single value.

Chez Scheme gives errors in both cases:

Chez Scheme Version 9.5.8
Copyright 1984-2022 Cisco Systems, Inc.

> (+ (values 1 2) (values 3 4))
Exception: returned two values to single value return context
> (map (lambda (a) (values a a)) '(1 2 3))
Exception: returned two values to single value return context

Kawa only gives an error in the first case, but not the second:

#|kawa:1|# (+ (values 1 2) (values 3 4))
java.lang.ClassCastException: class gnu.mapping.Values$Values2 cannot be cast to class gnu.math.Numeric
#|kawa:2|# (map (lambda (a) (values a a)) '(1 2 3))
(1 1 2 2 3 3)

This is a potential stumbling block for portable code.