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)
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)
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 (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:
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