Title

Syntax for receiving multiple values

Author

Lars T Hansen

Status

This SRFI is currently in ``final'' status. To see an explanation of each status that a SRFI can hold, see here. You can access the discussion via the archive of the mailing list.

Abstract

The SRFI introduces syntactic forms LET-VALUES and LET*-VALUES that bind the values of expressions that return multiple values.

Issues

None.

Rationale

LET-VALUES and LET*-VALUES reduce the clutter of the CALL-WITH-VALUES notation for receiving multiple values.

Specification

(LET-VALUES ((<formals> <expression>) ...) <body>)
Syntax

Each <formals> should be a formal arguments list as for a LAMBDA expression, cf section 4.1.4 of the R5RS.

The <expression>s are evaluated in the current environment, the variables of the <formals> are bound to fresh locations, the return values of the <expression>s are stored in the variables, the <body> is evaluated in the extended environment, and the values of the last expression of <body> are returned. The <body> is a <tail body>, cf section 3.5 of the R5RS.

The matching of each <formals> to values is as for the matching of <formals> to arguments in a LAMBDA expression, and it is an error for an <expression> to return a number of values that does not match its corresponding <formals>.

             (let-values (((a b . c) (values 1 2 3 4)))
               (list a b c))            --> (1 2 (3 4))

             (let ((a 'a) (b 'b) (x 'x) (y 'y))
               (let-values (((a b) (values x y))
                            ((x y) (values a b)))
                 (list a b x y)))       --> (x y a b)

(LET*-VALUES ((<formals> <expression>) ...) <body>)
Syntax

Each <formals> should be a formal arguments list as for a LAMBDA expression, cf section 4.1.4 of the R5RS.

LET*-VALUES is similar to LET-VALUES, but the bindings are performed sequentially from left to right, and the region of a binding indicated by (<formals> <expression>) is that part of the LET*-VALUES expression to the right of the binding. Thus the second binding is done in an environment in which the first binding is visible, and so on.

             (let ((a 'a) (b 'b) (x 'x) (y 'y))
               (let*-values (((a b) (values x y))
                             ((x y) (values a b)))
                 (list a b x y)))       --> (x y x y)

Implementation

The following implementation is written in R5RS Scheme. It is not compatible with the IEEE Scheme standard because the IEEE standard does not contain the high-level macro system.

The implementation assumes that some top-level names defined by the R5RS are bound to their original values.

;; This code is in the public domain.

(define-syntax let-values
  (syntax-rules ()
    ((let-values (?binding ...) ?body0 ?body1 ...)
     (let-values "bind" (?binding ...) () (begin ?body0 ?body1 ...)))
    
    ((let-values "bind" () ?tmps ?body)
     (let ?tmps ?body))
    
    ((let-values "bind" ((?b0 ?e0) ?binding ...) ?tmps ?body)
     (let-values "mktmp" ?b0 ?e0 () (?binding ...) ?tmps ?body))
    
    ((let-values "mktmp" () ?e0 ?args ?bindings ?tmps ?body)
     (call-with-values 
       (lambda () ?e0)
       (lambda ?args
         (let-values "bind" ?bindings ?tmps ?body))))
    
    ((let-values "mktmp" (?a . ?b) ?e0 (?arg ...) ?bindings (?tmp ...) ?body)
     (let-values "mktmp" ?b ?e0 (?arg ... x) ?bindings (?tmp ... (?a x)) ?body))
    
    ((let-values "mktmp" ?a ?e0 (?arg ...) ?bindings (?tmp ...) ?body)
     (call-with-values
       (lambda () ?e0)
       (lambda (?arg ... . x)
         (let-values "bind" ?bindings (?tmp ... (?a x)) ?body))))))

(define-syntax let*-values
  (syntax-rules ()
    ((let*-values () ?body0 ?body1 ...)
     (begin ?body0 ?body1 ...))

    ((let*-values (?binding0 ?binding1 ...) ?body0 ?body1 ...)
     (let-values (?binding0)
       (let*-values (?binding1 ...) ?body0 ?body1 ...)))))

Copyright

Copyright (C) Lars T Hansen (1999). All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.


Editor: Mike Sperber
Last modified: Sun Jan 28 13:40:20 MET 2007