[r6rs-discuss] multiple values and mathematicians

[Alan Watson]

> Zero return values and implicitly dropping extra values are not
> reasonably possible with library-provided "values records" returned  
> as a
> "normal / core" single value, are they?

There is no problem with zero return values. In my sketched  
implementation, (values) would yield a "values record" containing no  
values. The call-with-values procedure would unwrap this and call the  
consumer procedure with no arguments.

Implicitly dropping extra values in cases when the continuation  
expects exactly one value is not portable. It is "unspecified" in the  
R5RS and "undefined" in the R6RS. Some implementation silently coerce  
excess values to a single value and missing values to unspecified  
values. Other implementations signal an error. My sketched  
implementation would do something different again. Another  
implementation might turn your computer into a whale and a bowl of  
petunias. All of these behaviours are permitted by the R5RS and R6RS.

> If you don't like any number of return values, can't you ignore it and
> always use one return value without problems?

If I don't like the hundreds of pages of funky 1970s functions and  
macros in CLtL, can't I just ignore them and use the bits I like? :-)

Multiple values are currently part of the language. I think they  
should be part of the library. That way I can forget about them except  
if and when I need them.

"Programming languages should be designed not by piling feature on top  
of feature, but by removing the weaknesses and restrictions that make  
additional features appear necessary." [1] A weakness that made  
multiple values appear necessary was the lack of record types. It has  
now been removed. So, I now advocate removing the unnecessary  
additional features or, rather, moving them to the library.

Regards,

Alan

[1] Is quoting Will's famous words a Scheme-local Godwin's Law?
-- 
Alan Watson
http://www.alan-watson.org/