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Reading dynamic data from a file

It may seem attractive to use consult/1 for loading a file holding `case data' as Prolog facts. However, this is not a very good plan.

  • Consult does a lot more work
  • Consult stores the data as static procedures

A much better approach is to load the file by hand. If we have dynamic data on a graph as arc/2 facts, this looks as follows.

:- use_module(library(error)).
:- dynamic arc/2.

load_arcs(File) :-
        retractall(arc(_,_)),
        open(File, read, Stream),
        call_cleanup(load_arcs(Stream),
                     close(Stream)).

load_arcs(Stream) :-
        read(Stream, T0),
        load_arcs(T0, Stream).

load_arcs(end_of_file, _) :- !.
load_arcs(arc(From, To), Stream) :- !,
        assert(arc(From, To)),
        read(Stream, T2),
        load_arcs(T2, Stream).
load_arcs(Term, Stream) :-
        type_error(arc, Term).

Of course you should adapt this code to your case. This code properly handles your code as dynamic data and doesn't allow a user of your program to fool you by adding a term

arc(_, _) :- shell('format C').
See also
- library(persistency) allows for storing data on a file and restoring it.