Question

I’m new to Prolog.

Using this basic ‘database’ structure, I thought I would be able to query the database to find out

• who eats fish?

• what do Whales eat?

  eats(Horse, grass).  
  eats(Monkey, banana).  
  eats(Whale, fish). 
  

I’d like to not change that database setup (if possible). I was using the following queries with the respective unwanted results:

Here, I was trying to ask, ‘who eats fish?’

?- eats(X, fish).
true.

Here, I was trying to ask, ‘what do whales eat?’

?- eats(Whale,X).
X = grass ;
X = banana ;
X = fish.

Answer 1

Your queries are correct, it’s your fact database which is wrong. Atoms need to start with a lowercase letter (or be quoted). You started Horse, Monkey, Whale with uppercase letters, so they are variables (and match anything). Hence your current database is equivalent to:

eats(X, grass).
eats(X, banana).
eats(X, fish).