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Re: A value for "nothing"

From: Mark H Weaver
Subject: Re: A value for "nothing"
Date: Tue, 28 Aug 2018 11:59:40 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)

Mark H Weaver <address@hidden> writes:

> John Cowan <address@hidden> writes:
>> On Tue, Aug 28, 2018 at 3:01 AM Mark H Weaver <address@hidden> wrote:
>>  In RnRS, (define a (if #f #f)) is allowed and guaranteed to assign
>>  *some* object to 'a' without signalling an error.  
>> Actually, the phrase used is "the result is unspecified", which unfortunately
>> is not defined in any RnRS.
> That's the phrase used in R7RS-small, which fails to define it, as you
> noted, but that shortcoming is limited to R7RS.

Actually, the behavior _is_ clearly defined, in the formal denotational
semantics in both R5RS and R7RS.  If you learn how to read those, you'll
see that there's no question that (if #f #f) is guaranteed to return
exactly one unspecified value.


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