Avoid linear cost of append/3 in Prolog

问题:

Let's assume that we're reading from standard input and building a list of all the lines that have been read. In the end, we need to display those lines, separated by commas.

go:-
    prompt(_, ''),
    processInput([ ], Lines),
    findall(_, (member(L, Lines), write(L), write(',')), _),
    nl.

processInput(LinesSoFar, Lines):-
    read_line_to_codes(current_input, Codes),
    processInput(Codes, LinesSoFar, Lines).

processInput(Codes, LinesSoFar, Lines):-
    ( Codes \= end_of_file
    ->
        atom_codes(Line, Codes),
        append(LinesSoFar, [ Line ], LinesSoFar1),  % <---- append/3 - O(n)
        processInput(LinesSoFar1, Lines)
    ;
        Lines = LinesSoFar ).

The issue w/ this code is that the append call in processInput/3 costs us O(n). How can we avoid this cost & still let our predicate be tail-recursive (because we may be reading a lot of lines from standard input)?

It occurred to me that we could replace the append with the following.

LinesSoFar1 = [ Line | LinesSoFar ],

And we could reverse the list before displaying it. But that seems hacky. Is there a better way?

回答1:

I do not consider the solution you propose (prepending list elements, then reversing the list at the end) "hacky". gusbro's solution with explicit difference lists is OK as well. I think the most elegant way is to use DCG notation (an implicit interface to difference lists), i.e., use a DCG that describes the list of lines:

read_lines -->
        { read_line_to_codes(current_input, Codes) },
        (   { Codes == end_of_file } -> []
        ;   { atom_codes(Line, Codes) },
            [Line],
            read_lines
        ).

Usage: phrase(read_lines, Lines).

回答2:

You can do it using a semi instantiated structure. Check this code:

append_init(Marker-Marker).

append_end(L-[I|NMarker], I, L-NMarker).

append_finish(L-[], L).

You start by 'initializing' the semi-instantiated structure by calling append_init(L). Then you can add elements at the end of the list by calling append_end(L, Item, NewList). When you have finished adding elements you call append_finish(L, List) to get the final, fully-instantiated list.

Example:

example(NL):-
  append_init(L), 
  append_end(L, a, L1), 
  append_end(L1, b, L2), 
  append_end(L2, c, L3), 
  append_finish(L3, NL).

?- example(L).
L = [a, b, c].