[R6RS] Splitting hairs over `map' and friends

[Michael Sperber]

William D Clinger <will at ccs.neu.edu> writes:

> Mike wrote:
>> We say that `map' must accept a procedure of as many arguments as
>> there are list arguments to `map'.  (By virtue of R5RS saying "must".)
>> Is an implementation required to check this even if the argument
>> list(s) is/are empty?
>
> No.

OK.

> It is *impossible* for an implementation to check this safely using
> only the operations specified in R5RS or (so far) R6RS.

> Consider, for example:
>
>     (map (lambda (x . rest)
>            (cond ((null? rest) <ok1>)
>                  ((null? (cdr rest)) <ok2>)
>                  (else (error ...))))
>          '()
>          '()
>          '())
>

While I agree with the general point, I don't understand what you're
saying with the example.  The procedure still takes three
arguments---surely we don't want to include the case where the
procedure, after having been successfully called, raises any kind of
exception.

Now, do we require that it is a procedure, without any arity checking,
if the lists are empty?

-- 
Cheers =8-} Mike
Friede, Völkerverständigung und überhaupt blabla