The main structure of the program will be a) read lines, b) random permutate lines, c) write lines. We can already define the main program, without executing it:
main :-
read_lines(L),
random_permutation(L, R),
write_lines(R).
A simple method to produce a random permutation in Prolog, is to first extend the given list by a uniform random number, and then keysort and strip the list again. Dogelog Player provides uniform random numbers in the interval [0,1) via library(util/math), and a native implementation of keysort is found in library(compat).
Each time pressing the instrumentation button will give a different result:
:- ensure_loaded(library(compat)).
:- ensure_loaded(library(util/math)).
random_permutation(L, R) :-
add_key(L, H),
keysort(H, J),
remove_key(J, R).
add_key([], []).
add_key([X|L], [Y-X|R]) :- random(Y), add_key(L, R).
remove_key([], []).
remove_key([_-X|L], [X|R]) :- remove_key(L, R).
?- random_permutation([1,2,3,4,5], X).
For input we rely on the Prolog term reading, whereas we use the Prolog term end_of_file as a marker that the list has been completely entered. We can explore reading directly in a notebook, by adding the input after the reading query. Because we read Prolog terms, we have to terminate each input by a period. Feel free to edit the input and see what happens:
read_lines(L) :- read(X), read_lines(X, L). read_lines(end_of_file, []) :- !. read_lines(X, [X|L]) :- read(Y), read_lines(Y, L). ?- read_lines(L). foo. bar. baz. end_of_file.
For output we simple flesh out some Prolog terms in a tail recursive loop. One might be tempted to use higher order programming here and some maplist, but Dogelog Player doesn't provide call/n for performance and didactical reasons, so we have to do it more explicitly. The can explore writing directly in a notebook, since it shows the output after the writing query:
write_lines([]). write_lines([X|L]) :- write(X), nl, write_lines(L). ?- write_lines([foo,bar,baz]).
We can now run the main program already sketched in the first paragraph. We will provide a small list of Turing award winners that were inclined with artificial intelligence. The notebook allows combining reading and writing. Again each time pressing the instrumention button will give a different result. Feel also free to edit the input and see what happens:
?- main. 'Marvin Minsky'. 'John McCarthy'. 'Herbert A. Simon'. 'Allen Newell'. end_of_file.