Re: Scheme style auto-resizing hashtable (fwd)

[Maciej Stachowiak <mstachow@mit.edu>]

hjstein@bfr.co.il writes:
> Rob Browning <rlb@cs.utexas.edu> writes:
> 
>  > Note that their justification for each() makes sense (at least given
>  > that perl can tie() hashes to DBM files):
>  > 
>  >                Note that functions such as keys() and values()
>  >                may return huge array values when used on large
>  >                DBM files.  You may prefer to use the each()
>  >                function to iterate over large DBM files.
>  >                Example:
>  > 
>  >                    # print out history file offsets
>  >                    dbmopen(%HIST,'/usr/lib/news/history',0666);
>  >                    while (($key,$val) = each %HIST) {
>  >                        print $key, ' = ', unpack('L',$val), "\n";
>  >                    }
>  >                    dbmclose(%HIST);
> 
> Handled by hashtable-for-each.
> 

Upon further consideration of this and other messages, I think the
best solution is to provide hashtable-for-each and hashtable-map
(which would map to a list, not a hashtable); the former can be used
when you want to avoid any superfluous consing at all and the latter
if you want to collect the resulting values. hashtable->alist and
hashtable->keys can trivially be written in terms of hashtable-map
(but could be provided in C if that notably improves efficiency, since
they are likely to be common idioms).

C++-style iterators can be simulated using hashtable-for-each and
call/cc, but I still think they suck. Mappers lead to shorter, more
readable code than iteration constructs, this was true even when the
languages in question were APL vs. PL/1 rather than Lisp vs C++/Java.

I think in general it is more convenient to pass code to an iteration
or mapping or filter than vice versa, you just can't do this very well
in C++ or Java because they don't have proper closures. You could make
a closure-like class perhaps, but it would be a pain. Now, if you
could overload operator() in C++ (and therefore define callable
classes) maybe you could have some fun.

 - Maciej