如何通过assert将常量与变量关联？关于在数据库中添加常量及关联变量的实现问询

Absolutely, but there's a critical detail about Prolog variable scoping that you need to understand first to make this work as intended.

The Core Issue with Your Original Approach #

When you run ?- assertz(my(x, A)), A = 2., here's what actually happens:

• The A in assertz(my(x, A)) is an unbound free variable. Prolog doesn't store the variable name A in the database—it replaces it with an anonymous placeholder (like _G1234, a unique internal variable ID).
• The subsequent A = 2 binds a completely new variable (also named A, but unrelated to the one in the assertion) to 2. This has no effect on the already-asserted my(x, _G1234) clause.

So if you then query ?- my(A, B)., you'll get A = x, B = _G1234 (the unbound anonymous variable), not B = 2 as you wanted.

How to Implement What You Want #

There are a few straightforward ways to achieve your goal:

1. Bind the Variable Before Asserting

If you know the value you want to assign when you assert the clause, simply bind the variable first:

?- A = 2, assertz(my(x, A)).

Now when you query:

?- my(X, B).

You'll get the desired result:

X = x, B = 2.

2. Bind the Variable After Assertion (Using Shared Unification)

If you need to assert the clause first and assign the value later, you can keep a reference to the unbound variable by unifying with it after the assertion:

% First assert the clause with an unbound variable
?- assertz(my(x, Val)).

% Later, bind Val to 2 (this unifies with the variable in the database)
?- my(x, Val), Val = 2.

Now querying ?- my(X, B). will return X = x, B = 2.—once the variable is bound, it stays bound in the database clause.

3. Use a Dynamic Update Predicate (For Flexibility)

If you need to update the value multiple times, create helper predicates to retract and re-assert the clause safely:

:- dynamic my/2.

% Initialize the key with an unbound value
init_my(Key) :- assertz(my(Key, _)).

% Update the value for a key
set_my(Key, NewValue) :-
    retract(my(Key, _)),  % Remove the old clause
    assertz(my(Key, NewValue)).  % Add the updated clause

Usage example:

?- init_my(x), set_my(x, 2), my(X, B).
X = x, B = 2.

Summary #

Your core goal is absolutely achievable—you just need to account for Prolog's variable scoping rules. Either bind the variable before asserting, unify with the stored variable later, or use helper predicates to manage dynamic updates.

内容的提问来源于stack exchange，提问作者TheWorstHaskellProger