<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta content="text/html;charset=us-ascii" http-equiv="Content-Type"> <title></title> </head> <body bgcolor="#ffffff" text="#000000"> Dear Ralph, the pseudo-program, fringe.m, was attached to my last mail. Maybe it was not readable - here in plain text, again: %%% snip %%%%%%%%%%%%%%%%% :-module fringe. :-interface. :-import_module io. :-pred main(io::di, io::uo) is det. :-typeclass fringe(Q) where [ pred empty(Q::in) is semidet, func tail(Q)= Q, %func insert(T, Q)= Q <= node(T), func first(Q)= T <= node(T) ]. :-typeclass node(NodeT) where [ pred show(NodeT::in, io::di, io::uo) is det ]. %%=============================================== :-implementation. :-import_module list, queue, exception. :-instance fringe(queue(T)) <= node(T) where [ pred(empty/1) is queue.is_empty, func(tail/1) is q_tail, %func(insert/2) is q_insert, func(first/1) is first ]. :-instance node(int) where [ pred(show/3) is io.write_int ]. :-func q_insert(T, queue(T))= queue(T) <= node(T). q_insert(Dink, Queue)= queue.put(Queue, Dink). :-func q_tail(queue(T))= queue(T). q_tail(Queue)= Tail :- (if queue.get(Queue, _F, T) then Tail = T else throw("! empty fringe...")). :-func q_first(queue(T))= T <= node(T). q_first(Queue)= First :- (if queue.first(Queue, F) then First = F else throw("! empty fringe...")). :-func first2(queue(T))= T <= node(T). first2(Q)= first(Q). main(!IO) :- Q0 = queue.delete_all(queue.list_to_queue([1]), 1), Q2 = queue.list_to_queue([2,3]), (if empty(Q0) then write_string("Q0 is empty\n", !IO) else write_string("Q0 isn't empty\n", !IO)), (if empty(Q2) then write_string("Q2 is empty\n", !IO) else write_string("Q2 isn't empty\n", !IO)), write_string("tail of Q2: ", !IO), Tail = tail(Q2), write(Tail, !IO), nl(!IO), write_string("first of Q2: ", !IO), %_First = first2(Q2), %show(First, !IO), nl(!IO). %%% snap %%%%%%%%%%%%%%%%% The compiler messages are, as told, of )o+>0.13. <blockquote cite="mid20061004012649.GZ14729@ceres.csse.unimelb.edu.au" type="cite"> <pre wrap=""> </pre> <blockquote type="cite"> <pre wrap=""> (1) 'O-tone R.B.': > The empty/0 func causes us some grief, since it doesn't mention T; instead > we need to jump through a hoop like > func empty(T) = S, % T is a dummy arg to make the types work > mode empty(unused) = out is det, > which isn't very satisfying. </pre> </blockquote> <pre wrap=""> Here you've omitted the context, which was a discussion of type classes for collections, where there is a collection type and an item type. Functional dependencies now handle the empty/0 situation. </pre> </blockquote> Hey, 'type classes for collections' totally hits my point... ;-) If you say 'empty/0', (i.e., func constant= T::out) is possible now, that is more than I would wish for... :-) <blockquote cite="mid20061004012649.GZ14729@ceres.csse.unimelb.edu.au" type="cite"> <pre wrap=""></pre> <blockquote type="cite"> <pre wrap=""> (2) 'O-tone R.B.' (others discussed similar ideas in the following): > But things get worse: in an instance declaration the parameters must all be of the form > `typename(typevar, ...)', and not just `typevar', which > rules out > :- instance sequence(list(T), T). > Oh deary me! What is needed, as was discussed at the end of last year, > is the ability to handle constructor classes, so we could write > :- typeclass sequence(S(T)) where [ > func empty = S(T), > func cons(T, S(T)) = S(T), > func head(S(T)) = T, > func tail(S(T)) = S(T), > ... > ]. > and > :- instance sequence(list(T)). </pre> </blockquote> <pre wrap=""> Again, functional dependencies can handle many of these cases. Constructor classes would be even better, but right now adding them is not a priority. </pre> </blockquote> I have got 'empty/1' (check pred) and 'tail/1' running - you already told the same about 'empty/0'... since we are speaking about applying functional dependencies, you don't mean I can dare to expect 'cons/2' and 'head/1' are also possible, do you?? <blockquote cite="mid20061004012649.GZ14729@ceres.csse.unimelb.edu.au" type="cite"> <blockquote type="cite"> <pre wrap=""> (b) Without a such lore, the compiler seems not to be able to read it from the instance declaration (which in fact contains it) - moreover it seems to be more serious than just a 'typecast' issue - wrapping a typeclass call in an appropriate function lets the compiler finish successfully, but seems to produce an endless loop at execution: In the attached example code, this loop already happens when uncommenting '% _First = first2(Q2),'. </pre> </blockquote> <pre wrap=""> I don't understand. If you can produce a short program containing the problem, that would be good. </pre> </blockquote> I understand... you need 'fringe.m'... <blockquote cite="mid20061004012649.GZ14729@ceres.csse.unimelb.edu.au" type="cite"> <pre wrap=""> </pre> <blockquote type="cite"> <pre wrap=""> Under the bottom line, I guess what I am trying to achieve is finding a 'crude way' of having rank two polymorphism with )o+>0.13. </pre> </blockquote> <pre wrap=""> Now you've completely lost me! I still don't understand what it is you're trying to achieve. If you can produce a short pseudo-program illustrating what you want to do, that would help. </pre> </blockquote> Of course, I will not hesitate to stop looking for 'a crude way', if not necessary... :-) I am very excited for your reply..., Nick </body> </html>