Search                        Top                                  Index
HELP PDTOLIST                      updated Mark Rubinstein November 1985

    pdtolist(procedure) -> dynamic list

This procedure takes a procedure as argument and returns a dynamic list
made from the procedure.  A dynamic list is a PAIR whose FRONT is TRUE and
whose BACK is the procedure given to PDTOLIST.

HD and TL etc recognize such PAIRs.  The procedure should be a generator,
which, each time it is called, returns a new result, and returns *TERMIN
when it is finished.  The advantage of a dynamic list is that its elements
are computed only as they are required, which can save space and time.
PROGLIST, which plays a crucial role in the compiler is a dynamic list.

ISDYNAMIC recognises dynamic lists, returning the generator procedure.

EXPANDLIST expands dynamic lists and makes them static lists.

For technical details see REF * LISTS

--- Example ------------------------------------------------------------

By partially applying GENSYM to the word "CAT" we create a generator
which returns, on succcessive calls, the words CAT1, CAT2, CAT3, etc. We
can make a dynamic list from this procedure.

    vars catlist;
    pdtolist(gensym(%"cat"%)) -> catlist;

CATLIST is now a dynamic list, represented by a PAIR, whose front is TRUE
and whose back is the generator procedure

    front(catlist) =>
    ** <true>
    ** <procedure gensym>

The procedures HD and TL, unlike FRONT and BACK, make it look like an
ordinary list.

    ** cat1
    catlist =>
    ** [cat1 ...]       ;;; printing does not expand dynamic lists
    repeat 20 times tl(catlist) -> catlist endrepeat;
    hd(catlist) =>
    ** cat21

CATLIST is, in effect, an infinite (or very very long) list, bits of which
come into existence as needed.

--- C.all/help/pdtolist
--- Copyright University of Sussex 1992. All rights reserved. ----------