Ceci n'est pas une value (was Re: [R6RS] "void" or "unspecific")

William D Clinger will at ccs.neu.edu
Wed May 3 14:29:43 EDT 2006

> Related to this, the last two or three status reports have specified 
> that R6RS will have "read-write invariance for every datum".  Our final 
> vote on #3 today was to not add #!unspecified as syntax.  How, then, 
> will read-write invariance work in a case such as:
>    (write (list (unspecified)))

The word "datum" means one of the external representations
that are generated by the nonterminal <datum> in the formal
syntax of Scheme.  Since the value returned by (unspecified)
has no external representation (aside from implementation-
specific extensions), it is not a datum.


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