[r6rs-discuss] "Unspecified"

[Sam TH]

On Mon, 2006-10-02 at 18:18 +0200, Marcin 'Qrczak' Kowalczyk wrote:
> Sam Tobin-Hochstadt <samth at ccs.neu.edu> writes:
> 
> > (define (for-each/logging f l)
> >     (for-each (lambda (v) (let ([w (f v)]) (display w) (newline) w)) l))
> 
> The results of applications of the argument of for-each are ignored;
> there is no reason to log them.

It is precisely because the results are ignored that I would need to
write `for-each/logging'.  As to when it might be useful, not every
side-effecting procedure returns no information, just like not every
procedure used as a predicate returns only booleans.  

> If you absolutely want to, you should check the number of results
> instead of assuming that there is exactly one result. The argument
> of for-each is allowed to return any number of results.

It is true that `for-each/logging' doesn't work if the procedure
argument sometimes returns multiple values (or zero values), and thus
isn't always a drop-in replacement for `for-each'.  Many programs that
take function arguments break in these cases - which was precisely my
point.  We shouldn't make more special cases that have to be coded for,
when we can allow the standard case to work.

sam th