Say I have a module named foo (defined in foo.pl). This module does term_expansion for instance:
:- module(foo,[term_expansion/2]).
term_expansion(A,A) :-
print A.
Of course the real code does something more complicated with the terms.
Now I want to import this library in a file, say test.pl:
:- use_module(foo).
fact(a).
When using swi-prolog however, I get the following error:
ERROR: Cannot import foo:term_expansion/2 into module user: name clash
How can I resove this error?
Term-expansion predicates are usually (they are not standard) declared as multifile (and possibly dynamic) predicates. In the specific case of SWI-Prolog, the term-expansion mechanism already defines and calls definitions for the term_expansion/2 predicate in the pseudo-module user. Thus, a possible solution would be to write instead:
:- module(foo).
:- multifile(user:term_expansion/2).
:- dynamic(user:term_expansion/2).
user:term_expansion(A,A) :-
print(A).
You should only need to load the definition of this module before loading files that you want to be term-expanded.
Regarding your follow up question about why the term_expansion/2 predicate is not declared multifile by default. I can give two different interpretations to your question. I'll address both. (1) Why do you need to repeat the multifile/1 directive? The ISO Prolog standard implies that a multifile predicate should be declared multifile in all files containing clauses for it (I say implies instead of specifies as the standard talks about "Prolog text" and not files). Actually, SWI-Prolog is quite liberal here but it's good practice to repeat the directives, moreover when other systems follow the standard more closely in this regard. (2) Why must the term-expansion predicates be declared multifile (and dynamic) in the first place? That depends on the implementation. E.g. in the Logtalk implementation of the term-expansion mechanism they are neither multifile nor dynamic.
Related
with this knowledge base using https://swish.swi-prolog.org
:- dynamic happy/1.
go:-
assert(happy(mia)),
write(happy(mia)).
if I run go. I get
happy(mia)
true
If I just have
:- dynamic happy/1.
assert(happy(mia)).
and run happy(mia), I get false.
What fundamental concept am I missing please?
When you write:
assert(happy(mia)).
you are (re)defining the predicate assert/1, not calling it as in your definition of the go/0 predicate. Thus, happy(mia) is never added to the database. The query fails as the predicate happy/1 is know by the system (thanks to the dynamic/1 directive) but have no clauses.
Most Prolog systems prevent the redefinition of standard built-in predicates. But assert/1 is a legacy/deprecated predicate. That explains why SWI-Prolog doesn't complain about the redefinition. Always use the standard assertz/1 predicate instead of assert/1.
I am trying to perform the following term_expansion with swipl:
a(asda).
a(astronaut).
term_expansion(a(X),b(X)).
But it does not work, i.e. there is no b/1 consulted. I have tried a few variations:
term_expansion(a(X),[b(X)]).
user:term_expansion(a(X),b(X)).
user:term_expansion(a(X),[b(X)]).
user:term_expansion(user:a(X),[user:b(X)]).
None of which works. What is the problem?
As explained by #mat, you need to define the term_expansion/2 predicate before the clauses you want to expand are loaded. Also, the term_expansion/2 predicate is a multifile and dynamic predicate defined for the user pseudo-module. Thus, you should write:
:- multifile user:term_expansion/2.
:- dynamic user:term_expansion/2.
user:term_expansion(a(X), b(X)).
This will ensure that your expansion code will work if you move it into a module.
If portability to other Prolog systems with a term-expansion mechanism (which, btw, is far from standard), than consider moving the term-expansion code to its own file, loaded before the source files you want to expand.
Is there a workaround to make ensure_loaded/1 work
in GNU Prolog as it works in many other Prolog systems?
The goal is to have a preamble so that the rest of
code can use ensure_loaded/1 independent of whether which
Prolog system I use.
I tried the following:
:- multifile(term_expansion/2).
term_expansion((:- ensure_loaded(X)),
(:- atom_concat('<base>\\', X, Y),
include(Y))).
But the following query doesn't work:
:- ensure_loaded('suite.p').
The path calculation itself is not the issue of the question,
but the redefinition of a directive in GNU Prolog. There is
another directive that causes problems: meta_predicate/1. The
byte code crashes as follows:
Bye
A partial solution is:
ensure_loaded(File) :-
absolute_file_name(File, Path),
( predicate_property(_, prolog_file(Path)) ->
true
; consult(Path)
).
It assumes that the file defines at least one predicate but that's a sensible assumption. However, there's seems to be no way to override the native, non-functional, definition of the ensure_loaded/1 directive. A workaround would be to wrap the ensure_loaded/1 directive within an initialization/1 directive. For example:
:- initialization(ensure_loaded('suite.pl')).
Hence this being a partial solution as we're really defining an ensure_loaded/1 predicate, not a directive.
My current speculation is, that it is impossible with
the standard distribution of GNU Prolog 1.4.4. The
docu says:
The GNU Prolog compiler (section 4.4) automatically calls
expand_term/2 on each Term1 read in. However, in the current release,
only DCG transformation are done by the compiler (i.e.
term_expansion/2 cannot be used). To use term_expansion/2, it is
necessary to call expand_term/2 explicitly.
I also tried to inject some Prolog code for term_expansion/2
via the command line, but to no awail. Although the tool chain
has options such as -O, -L, -A that pass options to other tools.
There is not really an option that passes a Prolog text to the
pl2wam, in course of the execution of a consult/1 issued inside
the top-level.
At least this are my results so far.
Bye
I have a prolog program:
link(liverpool,preston).
link(liverpool,manchester).
link(preston,lancaster).
link(preston,manchester).
link(lancaster,carlisle).
link(lancaster,leeds).
link(carlisle,leeds).
link(manchester,leeds).
%checks to see if X is in the supplied list
inlist( X, [X|_]).
inlist( X, [_|Ys]) :- inlist( X, Ys).
merge([],L,L).
merge([H|T],BList,CList):-
inlist(H,BList),
merge(T,BList,CList).
merge([H|T],BList,[H|CList]):-
merge(T,BList,CList),
not(inlist(H,BList)).
Merge works if I call it like this:
merge([a,b,c],[d,e,f],Result). --> [a,b,c,d,e,f]
or more importantly, what it was designed to solve:
merge([a,b,c],[a,d,e,f],Result). --> [a,b,c,d,e,f]
but if I call merge like this:
merge(X,[d,e,f],[a,b,c,d,e,f]).
There is a stack overflow. Is this generally acceptable behavior for a function that is designed to work one way? Or is there some convention that functions should work in both ways?
Edit: merge works if you call it like this:
merge([a,b,c],X,[a,b,c,d,e,f]). --> [d,e,f]
First, you should not call these "functions". "Predicates" is the correct term.
It's generally desirable for Prolog predicates to work "both ways". But it's not always possible or worth the effort in a particular situation.
To inform about ways a predicate is intended to be used mode-declarations can be used. These declarations conventions are different from system to system. These declarations are mostly serve as a documentation for programmers and rarely used by compilers, but can be used by testing frameworks and other helper tools.
Examples of conventions for mode declarations:
SWI-Prolog: http://www.swi-prolog.org/pldoc/man?section=modes
ECLiPSe CLP: http://eclipseclp.org/doc/applications/tutorial003.html#toc10 (scroll to 2.7.3 Mode declaration)
Also there is a convention (described in "The Craft of Prolog", for example) that input parameters of a predicate come first, output parameters come last.
I wanna know if it´s possible to separate files in prolog, for example:
I wanna have my interface, rules, facts, consults in separate files and make them work together.
Yes - look at 'modules' in the SWI-Prolog documentation.
In short, you consult one file and it loads the others. So, say you have a.pl, b.pl, and so on
here's b.pl
bear(X) :- write('I see a bear named '), writeln(X).
bull(X) :- write('I see a bull named '), writeln(X).
at top of b.pl
put
:- module(b, [bear/1]).
Now you can use bear (the /1 means 'with one argument') outside this file.
in a.pl put
:- use_module(b).
Notice that there's no .pl on that
and then you can refer to bear
final program
a.pl
:- use_module(b).
go :- bear('Yogi').
b.pl
:- module(b, [bear/1]).
bear(X) :- write('I see a bear named '), writeln(X).
bull(X) :- write('I see a bull named '), writeln(X).
This way you can make things like bull/1 private to a single module.
Short answer, yes. Long answer, depends on what you mean by "interface" and how simple or complex are your requirements. As mbratch pointed out, the ISO Prolog standard include/1 directive can be used to include e.g. a set of predicate directives in files containing definitions for those predicates. If, on the other hand, your application requires multiple implementations of the same interface, you may be able to use Prolog modules together with the include/1 directive where each module would provide an implementation depending on what you want as an interface or use Logtalk, which provides interfaces as a first class entity. A more sound advice and detailed answer, however, requires on you providing more information on what you're trying to accomplish.