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
Solution
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.
OTHER TIPS
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
- Tokenize the input
- Parse the tokens (e.g. with DCG) to get the input for
routeplan/3
- Call
routeplan/3
- 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]).
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. :)
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.