Why does Prolog crash in this simple example?

Question

``` likes(tom,jerry). likes(mary,john). likes(mary,mary). likes(tom,mouse). likes(jerry,jerry). likes(jerry,cheese). likes(mary,fruit). likes(john,book). likes(mary,book). likes(tom,john). likes(john,X):-likes(X,john), X\=john. ``` Hi there, above is a very simple prolog file, with some facts and only one rule: John likes anyone who likes him. But after loading this file and ask Prolog the following query: ``` likes(john,X). ``` The program crashes. The reason is somehow prolog gets stuck at `likes(john,john)` even though the rule states that `X\=john`. Any advice?

Accepted Answer

Ironically, given the site we're on, you're getting a stack overflow.

It does this because of the order of execution that prolog uses, it's going to go into an infinite recursion at `likes(X,john)` in your rule, it activates the rule again - not a fact - never getting to the `X\=john` bit.

One way to fix this is to have your rule named differently from your fact like this:

```
kindoflikes(tom,jerry).
kindoflikes(mary,john).
kindoflikes(mary,mary).
kindoflikes(tom,mouse).
kindoflikes(jerry,jerry).
kindoflikes(jerry,cheese).
kindoflikes(mary,fruit).
kindoflikes(john,book).
kindoflikes(mary,book).
kindoflikes(tom,john).

likes(Y,X):- kindoflikes(X,Y), X\=Y.
likex(Y,X):- kindoflikes(Y,X), X\=Y.
```

Note the reversal of X and Y in the kindoflikes in the two rule definitions. So you get:

```
?- likes(john,X).
X = mary ;
X = tom ;
X = book.
```

But you're not locked into finding what john likes, and you can do:

```
?- likes(jerry,X).
X = tom ;
X = cheese.
```

Why does Prolog crash in this simple example?

Solution

Problem