Prolog错误：因果关系超出本地堆栈

Question

I'm trying to make a factorial code in prolog but am getting error of out of local stack, that is, it is stuck in infinite loop. I can't understand how. Here is my code:

fact(0,1).
fact(1,1).

fact(X,Y):- X\==0, A=X-1, fact(A,Z), Y=X*Z.

Where am I going wrong?

Answer 1

It is the A=X-1 , and later the Y=X*Z . The great thing about the Prolog top level is that you can easily try out what your code does:

?- A = X-1.
A = X-1.

?- A = 5-1.
A = 5-1.

Apparently, Prolog is mocking us :). The = operator is used for unification; if you want to do arithmetic, you must use is/2 :

?- is(A, -(5, 1)).
A = 4.

usually written as:

?- A is 5-1.
A = 4.

This just to show you that an expression is a term, and is evaluates the term in its second argument:

?- Expr = X-1, X = 3, Result is Expr.
Expr = 3-1,
X = 3,
Result = 2.

To your definition for factorial: it should work if you fix the arithmetic. Note that it would be cleaner if the condition for X at the beginning says X > 1 instead of X \\== 0 : what does your current program do for fact(1,F) , and how many answers do you get?