status: *final* (2005-08-12)

keywords: Binding, Multiple-Value Returns

See also SRFI 8: receive: Binding to multiple values, SRFI 11: Syntax for receiving multiple values, and SRFI 210: Procedures and Syntax for Multiple Values.library name: let

- The SRFI Document
- Discussion Archive
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This SRFI is a proposal for extending
`let`

, `let*`

, and `letrec`

for
receiving multiple values. The syntactic extension is fully compatible
with the existing syntax. It is the intention that single-value bindings,
i.e. `(let ((var expr)) ...)`

, and multiple-value binding can
be mixed freely and conveniently.

The most simple form of the new syntax is best explained by an example:

(define (quo-rem x y) (values (quotient x y) (remainder x y))) (define (quo x y) (let ((q r (quo-rem x y))) q))

The procedure `quo-rem`

delivers two
values to its continuation. These values are received as `q`

and `r`

in the `let`

-expression of the procedure
`quo`

. In other words, the syntax of `let`

is
extended such that several variables can be specified---and these
variables receive the values delivered by the expression ```
(quo-rem x
y)
```

.

The syntax of `let`

is further
extended to cases in which a rest argument receives the list of all
residual values. Again by example,

(let (((values y1 y2 . y3+) (foo x))) body)In this example,

`values`

is a
syntactic keyword indicating the presence of multiple values to be received,
and `y1`

, `y2`

, and `y3+`

, resp., are
variables bound to the first value, the second value, and the list of the
remaining values, resp., as produced by `(foo x)`

. The syntactic
keyword `values`

allows receiving all values as in ```
(let
(((values . xs) (foo x))) body)
```

. It also allows receiving no values at
all as in `(let (((values) (for-each foo list))) body)`

.A common application of binding multiple values
is decomposing data structures into their components. This mechanism is
illustrated in its most primitive form as follows: The procedure
`uncons`

(defined below) decomposes a pair `x`

into
its car and its cdr and delivers them as two values to its continuation.
Then an extended `let`

can receive these values:

(let ((car-x cdr-x (uncons x))) (foo car-x cdr-x))

Of course, for pairs this method is probably
neither faster nor clearer than using the procedures `car`

and
`cdr`

. However, for data structures doing substantial work
upon decomposition this is different: Extracting the element of highest
priority from a priority queue, while at the same time constructing the
residual queue, can both be more efficient and more convenient than doing
both operations independently. In fact, the `quo-rem`

example
illustrates this point already as both quotient and remainder are
probably computed by a common exact division algorithm. (And often
caching is used to avoid executing this algorithm twice as often as
needed.)

As the last feature of this SRFI, a mechanism
is specified to store multiple values in heap-allocated data structures.
For this purpose, `values->list`

and
`values->vector`

construct a list (a vector, resp.) storing
all values delivered by evaluating their argument expression. Note that
these operations cannot be procedures.