Re: useful list functions

[John Cupitt <john cupitt ng-london org uk>]

Maciej Stachowiak wrote:
> /* Just like `map' in Lisp, Scheme or Perl, i.e. like for_each but
>    return a list of the results. (non-destructive) */

I realise this has zero change of inclusion, but I thought I'd share my
favorite two 'missing' g_?list_*() functions with you as well ;-)

The idea for these two functions is that, like lisp, explicit recursion
or iteration over lists is a bad thing, and should be encapsulated in
higher order functions. I use this pair, and (I don't think) my app (not
small, 60k lines) has any list loops in it at all.

  typedef gpointer (*ListMapFn)( gpointer, gpointer ); 
  gpointer slist_map( GSList *list, 
    ListMapFn user_func, gpointer user_data );

This is like for_each, but has a 'bail out'. If user_func returns NULL,
_map() tries the next element. If user_func returns non-NULL, the loop
stops and slist_map() returns that value as its result. If user_func
returns NULL for all elements, _map() returns NULL.

You can express search-for-item loops, repeat-until-full loops and
others. Also handy for trapping errors .. eg. "scan all input fields ...
if a field contains a format error, bail out immediately and pop up an
alert box over the offending field."

  typedef gpointer (*ListFoldFn)( gpointer, gpointer, gpointer );
  gpointer slist_fold( GSList *list, 
    gpointer start, ListFoldFn user_function, gpointer user_data );

This is like _map(), but it folds the list up with a dyadic function.
For example, for the input list:

  [a,b,c]

it returns:

  fn (fn (fn start a) b) c

ie. fn is given two arguments: an accumulated value, and the next list
element. 'start' is the initial value for the accumulator. Given an
empty list, the function returns 'start'. Like _map(), _fold() has a
bail out: this time if user_func returns NULL, iteration terminates
immediately and the whole function returns NULL. 

Using fold you can find the maximum value in a list, write Maciej's
_map() function, sum a list, erm, and other stuff.

There are a few variants as well: slist_map2 and slist_fold2 take two
user_data arguments. slist_map_rev() maps in reverse order.
slist_foldr() folds a list with a right-associative function.

John
--
John Cupitt, john.cupitt@ng-london.org.uk, +44 (0)20 7747 2570
VASARI Lab, The National Gallery, Trafalgar Square, London, WC2N 5DN