Input in Prolog

Question

I'm currently working on a recursive Prolog program to link routes together to create a basic GPS of the Birmingham area. At the moment I can get output as so:

Input

routeplan(selly_oak, aston, P).

Output

P = [selly_oak, edgbaston, ... , aston]

What I would like to do is have my program provide some sort of interface, so if I were to type in something along the lines of:

Route from selly_oak to aston

It would provide me with:

Go from selly_oak to edgbaston
Go from edgbaston to ...
Finally, Go from ... to aston.

Prolog is a powerful language so I assume this is easily possible, however many of the books I've taken out seem to skip over this part. As far as I am aware I have to use something along the lines of write() and read() although the details are unknown to me.

Could anyone here a Prolog novice out with some basic examples or links to further information?

EDIT: A lot of these answers seem very complicated, where the solution should only be around 5-10 lines of code. Reading in a value isn't a problem as I can do something along the lines of:

find:- write('Where are you? '), read(X), nl, write('Where do you want to go? '), read(Y), loopForRoute(X,Y).

I'd prefer it if the output could be written out using write() so a new line (nl) can be used, so that it displays like the output above.

If this were my input, how would I then arrange the top routeplan() to work with these inputs? Also, if I were to add the Lines for these stations as an extra parameter how would this then be implemented? All links are defined at the beginning of the file like so:

rlinks(selly_oak, edgbaston, uob_line).
rlinks(edgbaston, bham_new_street, main_line).

Therefore, with this information, it'd be good to be able to read the line as so.

Go from selly_oak to edgbaston using the uob_line
Go from edgbaston to ... using the ...
Finally, go from ... to aston using the astuni_line

Answer 1

Here are a few predicates to read lines from a file/stream into a Prolog string:

%%% get_line(S, CL): CL is the string read up to the end of the line from S.
%%% If reading past end of file, returns 'end_of_file' in CL first, raises
%%% an exception second time.
%%% :- pred get_string(+stream, -list(int)).
get_line(S, CL) :-
    peek_code(S, C),
    (   C = -1
    ->  get_code(S, _),
        CL = end_of_file
    ;   get_line(S, C, CL)).

get_line(_, -1, CL) :- !, CL = [].  % leave end of file mark on stream
get_line(S, 0'\n, CL) :- !,
    get_code(S, _),
    CL = [].
get_line(S, C, [C|CL]) :-
    get_code(S, _),
    peek_code(S, NC),
    get_line(S, NC, CL).

%% read_lines(L): reads lines from current input to L.  L is a list of list
%% of character codes, newline characters are not included.
%% :- pred read_lines(-list(list(char))).
read_lines(L) :-
    current_input(In),
    get_line(In, L0),
    read_lines(In, L0, L).

%% read_lines(F, L): reads lines from F to L.  L is a list of list of character
%% codes, newline characters are not included.
%% :- pred read_lines(+atom, -list(list(char))).
read_lines(F, L) :-
    fail_on_error(open(F, read, S)),
    call_cleanup((get_line(S, L0),
           read_lines(S, L0, L)),
          close(S)).

read_lines(_, end_of_file, L) :- !, L = [].
read_lines(S, H, [H|T]) :-
    get_line(S, NH),
    read_lines(S, NH, T).

Then, take a look at DCGs for information on how to parse a string.

Answer 2

Hm, if I understand you correctly you just want to format the list nicely for printing out, no?

In SWI-Prolog this works:

output_string([A,B],StrIn,StrOut) :-
 concat_atom([StrIn, 'Finally, Go from ', A, ' to ', B, '.'],StrOut),
 write(StrOut).

output_string([A,B|Rest],StrIn,StrOut) :-
 concat_atom([StrIn,'Go from ', A, ' to ', B, '.\n'],StrAB),
 output_string([B|Rest],StrAB,StrOut).

then call with

output_string(P,'',_).

It's probably not very efficient, but it does the job. :)

Answer 3

A book which discusses such things in detail is Natural Language Processing for Prolog Programmers by Michael A. Covington.

In general, what you need to do is

1. Tokenize the input
2. Parse the tokens (e.g. with DCG) to get the input for routeplan/3
3. Call routeplan/3
4. Generate some English on the basis of the output of routeplan/3

Something like this (works in SWI-Prolog):

% Usage example:
%
% ?- query_to_response('Route from selly_oak to aston', Response).
%
% Response = 'go from selly_oak to edgbaston then go from edgbaston
%         to aston then stop .'
%
query_to_response(Query, Response) :-
    concat_atom(QueryTokens, ' ', Query), % simple tokenizer
    query(path(From, To), QueryTokens, []),
    routeplan(From, To, Plan),
    response(Plan, EnglishTokens, []),
    concat_atom(EnglishTokens, ' ', Response).

% Query parser
query(path(From, To)) --> ['Route'], from(From), to(To).
from(From) --> [from], [From], { placename(From) }.
to(To) --> [to], [To], { placename(To) }.

% Response generator
response([_]) --> [stop], [.].
response([From, To | Tail]) -->
    goto(path(From, To)), [then], response([To | Tail]).
goto(path(From, To)) --> [go], from(From), to(To).

% Placenames
placename(selly_oak).
placename(aston).
placename(edgbaston).

% Mock routeplan/3
routeplan(selly_oak, aston, [selly_oak, edgbaston, aston]).

Answer 4

For this sort of thing, I usually create shell predicates. So in your case...

guided:-
    print('Enter your start point'),nl,
    read(Start),
    print('Enter your destination'),nl,
    read(Dest),
    routeplan(Start, Dest, Route),
    print_route(Route).

And print_route/1 could be something recursive like this:

print_route([]).

print_route([[A,B,Method]|Tail]):-
    print_route(Tail),
    print('Go from '), print(A),
    print(' to '), print(B),
    print(' by '), print(Method), nl.

I've assumed that the 3rd variable of the routeplan/3 predicate is a list of lists. Also that it's built by adding to the tail. If it's not, it should be fairly easy to adapt. Ask in the comments.