SRFI 71: Extended LET-syntax for multiple values

by Sebastian Egner

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

Abstract

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.