I wanted to write evaluating predicate in Prolog for arithmetics and I found this:
eval(A+B,CV):-eval(A,AV),eval(B,BV),CV is AV+BV.
eval(A-B,CV):-eval(A,AV),eval(B,BV),CV is AV-BV.
eval(A*B,CV):-eval(A,AV),eval(B,BV),CV is AV*BV.
eval(Num,Num):-number(Num).
Which is great but not very DRY.
I've also found this:
:- op(100,fy,neg), op(200,yfx,and), op(300,yfx,or).
positive(Formula) :-
atom(Formula).
positive(Formula) :-
Formula =.. [_,Left,Right],
positive(Left),
positive(Right).
?- positive((p or q) and (q or r)).
Yes
?- positive(p and (neg q or r)).
No
Operator is here matched with _ and arguments are matched with Left and Right.
So I came up with this:
eval(Formula, Value) :-
Formula =.. [Op, L, R], Value is Op(L,R).
It would be DRY as hell if only it worked but it gives Syntax error: Operator expected instead.
Is there a way in Prolog to apply operator to arguments in such a case?
Your almost DRY solution does not work for several reasons:
Formula =.. [Op, L, R] refers to binary operators only. You certainly want to refer to numbers too.
The arguments L and R are not considered at all.
Op(L,R) is not valid Prolog syntax.
on the plus side, your attempt produces a clean instantiation error for a variable, whereas positive/1 would fail and eval/2 loops which is at least better than failing.
Since your operators are practically identical to those used by (is)/2 you might want to check first and only then reuse (is)/2.
eval2(E, R) :-
isexpr(E),
R is E.
isexpr(BinOp) :-
BinOp =.. [F,L,R],
admissibleop(F),
isexpr(L),
isexpr(R).
isexpr(N) :-
number(N).
admissibleop(*).
admissibleop(+).
% admissibleop(/).
admissibleop(-).
Note that number/1 fails for a variable - which leads to many erroneous programs. A safe alternative would be
t_number(N) :-
functor(N,_,0),
number(N).
Related
I'm working on defining the logical operators in Prolog as an exercise. The first few were pretty straight forward:
and(A, B) :-
A, B.
or(true).
or(A, _) :-
or(A).
or(_, B) :-
or(B).
neg(false).
But now I want to define nand and nor, and it would be nice to define them in terms of my other predicates. For example, maybe something like below:
nand(A, B) :-
neg(and(A, B)).
But the inner and expression doesn't evaluate. I'm guessing that Prolog interprets the and expression as an atom. Is there any way to force it to evaluate the nested predicate?
eval(true).
eval(or(A,_)):- eval(A),!.
eval(or(_,A)):- eval(A),!.
eval(and(A,B)):- eval(A),eval(B).
eval(neg(A)):- \+eval(A).
:- eval(true).
:- \+eval(and(true,false)).
:- eval(and(true,true)).
:- eval(neg(neg(true))).
:- halt.
I am trying to understand operators.
I have defined the following operator and method for it.
:- op(600, xfy, ⧺).
⧺(Left, Right) :- concatAtoms([Left, Right], _). % _ would the result but can´t be returned without an extra parameter
% concatenates atoms, e.g. [a,b] = ab
concatAtoms([H|T], R) :-
concatAtoms(T, H, R).
concatAtoms([], R, R).
concatAtoms([H|T], Atom, R) :-
atom_concat(Atom, H, Res),
concatAtoms(T, Res, R).
The query
?- a⧺b.
returns true as expected.
Is there any way for it to return
ab
instead (which was computed by the concatAtoms predicate)?
Maybe I'm completely wrong, but ...
:- op(600, xfy, ⧺).
defines syntax only.
(btw, ⧺ (the double plus is not the same as #)
?- atom_codes('⧺#',L).
L = [10746, 35].
After syntax has been defined, we can write terms like this:
?- X=a⧺b⧺c⧺d, write_canonical(X).
⧺(a,⧺(b,⧺(c,d)))
X = a⧺b⧺c⧺d.
So much for syntax and term structure, but what is the meaning of the above, in the sense of, what should it evaluate to, if it should evaluate to anything?
We evidently need an interpreter for terms like ⧺(a,⧺(b,⧺(c,d))).
If we want, we can call it ⧺, the Prolog parser should be able to distinguish a ⧺ in predicate position and one in operator position.
The interpreter predicate takes a 2-arity term with functor ⧺ and relates it to an evaluation result, or fails, or throws an exception.
Here is a (semi-bad) representation:
⧺(⧺(L,R), Result) :- atom(L),\+atom(R),!, ⧺(R, Tmp), atom_concat(L,Tmp,Result).
⧺(⧺(L,R), Result) :- \+atom(L),atom(R),!, ⧺(L, Tmp), atom_concat(Tmp,R,Result).
⧺(⧺(L,R), Result) :- atom(L),atom(R) ,!, atom_concat(L,R,Result).
We have now linked syntax to semantics, or at least some form of (cheap) semantics:
?- ⧺(a⧺b⧺c⧺d, R).
R = abcd.
Now you just have to know when to call ⧺/2. This depends on what the program wants to do.
In an elegant scenario, one would be able to extend "arithmetic function evaluation trigger" is/2 to have the above things on the right-hand side of is interpreted, as in R is a⧺b⧺c⧺d., but I don't think that's possible.
I am looking for ways to make a Prolog program "look" more like first order logic. Things I would like to have are for example:
-> for implication
antecedent to the left of ->
^ for conjunction v for disjunction
Or is there other software that already implements this?
Thanks in advance!
/JC
Update 20190313
I followed the suggestions in the answers below and tried this:
:- op(1200, xfx, ==>).
:- op(1000, xfy, /\).
:- op(1100, xfy, \/).
term_expansion(A ==> B, B:- A).
term_expansion(A /\ B, A, B).
term_expansion(A \/ B, A; B).
man(X) /\ unmarried(X) ==> bachelor(X).
man(john).
man(peter).
unmarried(john).
main:-bachelor(X), writeln(X), nl, fail.
But i get the following error:
ERROR: bachelor/1: Undefined procedure: (/\)/2
Exception: (5) man(_1740)/\unmarried(_1740) ?
Only using the op/3 and term_expansion/3 for ==> however works as expected. Not sure why this is so...
Use term_expansion/2 that is macro of Prolog on SWI-Prolog:
% calc.pl
:- op(1200,xfx,--).
term_expansion(A--B,B:-A).
integer(I)
--%----------------------- (E-Int)
I => I.
E1=>I1, E2=>I2, I is I1+I2
--%----------------------- (E-Add)
E1+E2 => I.
:- 1+2+3=>6.
:- 1+2+3=>I,writeln(I).
:- halt.
and run
$ swipl calc.pl
6
Here are a few Unicode characters that can help you:
¬
→ ⇒
← ⇐
∨ ∧
∀ ∃
I leave defining suitable precedences as a challenge, using op/3.
Once you have these definitions, you can write first-order sentences with them. You can then convert these sentences to Prolog, or interpret them with Prolog.
This answer refers to your updated question ("Update 20190313").
Be careful when defining operators:
Don't redefine standard operators, changing their specifier/precedence.
This can introduce errors in existing code which are very hard to find.
Weigh benefits and costs upfront.
Aim at readability, shorter code, and fewer parentheses.
Keep in mind that using too many custom Prolog operators can also obfuscate code and confuse the reader.
Think twice before using standard operators in different domains.
Let's take the predefined (\/)/2 as an example.
It is an evaluable functor in arithmetic expressions—used with (is)/2, (=:=)/2, (<)/2, etc.
clpfd uses it for representing set unions like 1..3 \/ 5..7—fine!
However, using it for denoting list concatenation is questionable.
Let's get to your actual question!
Consider these queries decomposing some terms using (=..)/2 ("univ"):
?- term_expansion(A /\ B, A, B) =.. Xs.
Xs = [term_expansion, A/\B, A, B].
?- term_expansion(A \/ B, A; B) =.. Xs.
Xs = [term_expansion, A\/B, (A;B)].
So it's term_expansion/2 for (\/)/2, but term_expansion/3 for (/\)/2!
The bottom line: (',')/2 terms as arguments need parentheses.
?- term_expansion(A /\ B, (A,B)) =.. Xs.
Xs = [term_expansion, A/\B, (A,B)].
I have come across an unfamiliar bit of Prolog syntax in Lee Naish's paper Higher-order logic programming in Prolog. Here is the first code sample from the paper:
% insertion sort (simple version)
isort([], []).
isort(A.As, Bs) :-
isort(As, Bs1),
isort(A, Bs1, Bs).
% insert number into sorted list
insert(N, [], [N]).
insert(N, H.L, N.H.L) :-
N =< H.
insert(N, H.LO, H.L) :-
N > H,
insert(N, LO, L).
My confusion is with A.As in isort(A.As, Bs) :-. From the context, it appears to be an alternate cons syntax for lists, the equivalent of isort([A|As], Bs) :-.
As well N.H.L appears to be a more convenient way to say [N|[H|L]].
But SWI Prolog won't accept this unusual syntax (unless I'm doing something wrong).
Does anyone recognize it? is my hypothesis correct? Which Prolog interpreter accepts that as valid syntax?
The dot operator was used for lists in the very first Prolog system of 1972, written in Algol-W, sometimes called Prolog 0. It is inspired by similar notation in LISP systems. The following exemple is from the paper The birth of Prolog by Alain Colmerauer and Philippe Roussel – the very creators of Prolog.
+ELEMENT(*X, *X.*Y).
+ELEMENT(*X, *Y.*Z) -ELEMENT(*X, *Z).
At that time, [] used to be NIL.
The next Prolog version, written in Fortran by Battani & Meloni, used cases to distinguish atoms and variables. Then DECsystem 10 Prolog introduced the square bracket notation replacing nil and X.Xs with [] and [X,..Xs] which in later versions of DECsystem 10 received [X|Xs] as an alternative. In ISO Prolog, there is only [X|Xs], .(X,Xs), and as canonical syntax '.'(X,Xs).
Please note that the dot has many different rôles in ISO Prolog. It serves already as
end token when followed by a % or a layout character like SPACE, NEWLINE, TAB.
decimal point in a floating point number, like 3.14159
graphic token char forming graphic tokens as =..
So if you are now declaring . as an infix operator, you have to be very careful. Both with what you write and what Prolog systems will read. A single additional space can change the meaning of a term. Consider two lists of numbers in both notations:
[1,2.3,4]. [5].
1 .2.3.4.[]. 5.[].
Please note that you have to add a space after 1. In this context, an additional white space in front of a number may change the meaning of your terms. Like so:
[1|2.3]. [4]. 5. [].
1 .2.3. 4.[]. 5. [].
Here is another example which might be even more convincing:
[1,-2].
1.(-2).[].
Negative numbers require round brackets within dot-lists.
Today, there is only YAP and XSB left that still offer infix . by default – and they do it differently. And XSB does not even recognize above dot syntax: you need round brackets around some of the nonnegative numbers.
You wrote that N.H.L appears to be a more convenient way to say [N|[H|L]]. There is a simple rule-of-thumb to simplify such expressions in ISO Prolog: Whenever you see within a list the tokens | and [ immediately after each other, you can replace them by , (and remove the corresponding ] on the right side). So you can now write: [N,H|L] which does not look that bad.
You can use that rule also in the other direction. If we have a list [1,2,3,4,5] we can use | as a "razor blade" like so: [1,2,3|[4,5]].
Another remark, since you are reading Naish's paper: In the meantime, it is well understood that only call/N is needed! And ISO Prolog supports call/1, call/2 up to call/8.
Yes, you are right, the dot it's the list cons infix operator. It's actually required by ISO Prolog standard, but usually hidden. I found (and used) that syntax some time ago:
:- module(eog, []).
:- op(103, xfy, (.)).
% where $ARGS appears as argument, replace the call ($ARGS) with a VAR
% the calle goes before caller, binding the VAR (added as last ARG)
funcs(X, (V, Y)) :-
nonvar(X),
X =.. W.As,
% identify meta arguments
( predicate_property(X, meta_predicate M)
% explicitly exclude to handle test(dcg)
% I'd like to handle this case in general way...
, M \= phrase(2, ?, ?)
-> M =.. W.Ms
; true
),
seek_call(As, Ms, Bs, V),
Y =.. W.Bs.
% look for first $ usage
seek_call([], [], _Bs, _V) :-
!, fail.
seek_call(A.As, M.Ms, A.Bs, V) :-
M #>= 0, M #=< 9, % skip meta arguments
!, seek_call(As, Ms, Bs, V).
seek_call(A.As, _, B.As, V) :-
nonvar(A),
A = $(F),
F =.. Fp.FAs,
( current_arithmetic_function(F) % inline arith
-> V = (PH is F)
; append(FAs, [PH], FBs),
V =.. Fp.FBs
),
!, B = PH.
seek_call(A.As, _.Ms, B.As, V) :-
nonvar(A),
A =.. F.FAs,
seek_call(FAs, Ms, FBs, V),
!, B =.. F.FBs.
seek_call(A.As, _.Ms, A.Bs, V) :-
!, seek_call(As, Ms, Bs, V).
:- multifile user:goal_expansion/2.
user:goal_expansion(X, Y) :-
( X = (_ , _) ; X = (_ ; _) ; X = (_ -> _) )
-> !, fail % leave control flow unchanged (useless after the meta... handling?)
; funcs(X, Y).
/* end eog.pl */
I was advised against it. Effectively, the [A|B] syntax it's an evolution of the . operator, introduced for readability.
OT: what's that code?
the code above it's my attempt to sweeten Prolog with functions. Namely, introduces on request, by means of $, the temporary variables required (for instance) by arithmetic expressions
fact(N, F) :-
N > 1 -> F is N * $fact($(N - 1)) ; F is 1.
each $ introduce a variable. After expansion, we have a more traditional fact/2
?- listing(fact).
plunit_eog:fact(A, C) :-
( A>1
-> B is A+ -1,
fact(B, D),
C is A*D
; C is 1
).
Where we have many expressions, that could be useful...
This syntax comes from NU-Prolog. See here. It's probably just the normal list functor '.'/2 redefined as an infix operator, without the need for a trailing empty list:
?- L= .(a,.(b,[])).
L = [a,b]
Yes (0.00s cpu)
?- op(500, xfy, '.').
Yes (0.00s cpu)
?- L = a.b.[].
L = [a,b]
Yes (0.00s cpu)
Why does Prolog match (X, Xs) with a tuple containing more elements? An example:
test2((X, Xs)) :- write(X), nl, test2(Xs).
test2((X)) :- write(X), nl.
test :-
read(W),
test2(W).
?- test.
|: a, b(c), d(e(f)), g.
a
b(c)
d(e(f))
g
yes
Actually this is what I want to achieve but it seems suspicious. Is there any other way to treat a conjunction of terms as a list in Prolog?
Tuple term construction with the ,/2 operator is generally right-associative in PROLOG (typically referred to as a sequence), so your input of a, b(c), d(e(f)), g might well actually be the term (a, (b(c), (d(e(f)), g))). This is evidenced by the fact that your predicate test2/1 printed what is shown in your question, where on the first invocation of the first clause of test2/1, X matched a and Xs matched (b(c), (d(e(f)), g)), then on the second invocation X matched b(c) and Xs matched (d(e(f)), g), and so on.
If you really wanted to deal with a list of terms interpreted as a conjunction, you could have used the following:
test2([X|Xs]) :- write(X), nl, test2(Xs).
test2([]).
...on input [a, b(c), d(e(f)), g]. The list structure here is generally interpreted a little differently from tuples constructed with ,/2 (as, at least in SWI-PROLOG, such structures are syntactic sugar for dealing with terms constructed with ./2 in much the same way as you'd construct sequences or tuple terms with ,/2). This way, you get the benefits of the support of list terms, if you can allow list terms to be interpreted as conjunctions in your code. Another alternative is to declare and use your own (perhaps infix operator) for conjunction, such as &/2, which you could declare as:
:- op(500, yfx, &). % conjunction constructor
You could then construct your conjunct as a & b(c) & d(e(f)) & g and deal with it appropriately from there, knowing exactly what you mean by &/2 - conjunction.
See the manual page for op/3 in SWI-PROLOG for more details - if you're not using SWI, I presume there should be a similar predicate in whatever PROLOG implementation your'e using -- if it's worth it's salt :-)
EDIT: To convert a tuple term constructed using ,/2 to a list, you could use something like the following:
conjunct_to_list((A,B), L) :-
!,
conjunct_to_list(A, L0),
conjunct_to_list(B, L1),
append(L0, L1, L).
conjunct_to_list(A, [A]).
Hmm... a, b(c), d(e(f)), g means a and (b(c) and (d(e(f)) and g)), as well list [1,2,3] is just a [1 | [2 | [3 | []]]]. I.e. if you turn that conjuction to a list you'll get the same test2([X|Xs]):-..., but difference is that conjunction carries information about how that two goals is combined (there may be disjunction (X; Xs) as well). And you can construct other hierarchy of conjunctions by (a, b(c)), (d(e(f)), g)
You work with simple recursive types. In other languages lists is also recursive types but they often is pretending to be arrays (big-big tuples with nice indexing).
Probably you should use:
test2((X, Y)):- test2(X), nl, test2(Y).
test2((X; Y)). % TODO: handle disjunction
test2(X) :- write(X), nl.