Prolog - Write out facts and reading a users input

Question

I am quite new to Prolog and have had some trouble understanding it. I have some facts named 'problem' I wish to first print out these facts to the user and then ask them to input a value, this value is then read and used later.

From my understanding thus far, it would be best to use a forall to print out these facts and then use read to read the value inputted, but I am having some issue implementing this. Here is what I have so far, any explanation would be appreciated

My question: How do I read in the input from the user regarding the problem and apply that into a variable for later use?

tellMeYourProblem:-
forall(problem(P), 
writeln(P)),
answer = read(X),


problem('1').
problem('2').
problem('3').
problem('4').
problem('5').
problem('6').
problem('7').
problem('8').
problem('9').
problem('10').

Of interest: RosettaCode Prolog Input/Output for Lines of Text — Guy Coder
I added the question into my post: How do I read in the input from the user regarding the problem and apply that into a variable for later use? — VicentVega

Guy Coder Guy Coder · Accepted Answer · 2018-11-15T15:18:27

Note: This answer uses SWI-Prolog.

How do I read in the input from the user regarding the problem?

You are doing that already with read(X), however read/1 reads terms (terms end with periods) and you probably want to read characters. If you are using SWI-Prolog take a look at Primitive character I/O for reading characters and Predicates that operate on strings for reading strings.

How do I apply that into a variable for later use?

When doing basic I/O with a user at a text level, a REPL is a good way to start. Adding a REPL is a bit more complicated so I will give you the code.

tellMeYourProblem :-
    output_problems,
    read_input.

output_problems :-
    forall(problem(P),
    writeln(P)).

read_input :-
    repeat,
    read_string(user_input, "\n", "\r\t ", _, Line),
    process_input(Line).

process_input(Line) :-
    string(Line),
    atom_number(Line, N),
    integer(N),
    do_something_with(Line),
    fail.
process_input("quit") :-
    write('Finished'), nl,
    !, true.

do_something_with(X) :-
    writeln(X).

problem('1').
problem('2').
problem('3').
problem('4').
problem('5').
problem('6').
problem('7').
problem('8').
problem('9').
problem('10').

Also with Prolog, the style is to use snake casing so tellMeYourProblem should be tell_me_your_problem.

Normally in Prolog a REPL is done with ->/2, (Read Input till quit statement Prolog) , but I changed this to add more guard statements so that the exit condition would work, e.g.

string(Line),
atom_number(Line, N),
integer(N)

or putting the guard in the head, e.g.

process_input("quit")

When doing I/O to a screen and keyboard, the thought is to use stdIn and stdOut but for the keyboard SWI-Prolog uses user_input instead. See: Input and output

After all of the boiler plate code for the REPL is the next part you seek which is to do something with the input value, in this case just print it out.

do_something_with(X) :-
    writeln(X).

Prolog - Write out facts and reading a users input

2 Answers