in declarative language (like:prolog)
you can give it some rules, and it can calculate results based on these rules.
ex:
give it
0! = 1
n! = n * (n-1)!
asking
4!, it will give you 4!=4*3!=4*3*2!...=result.

give it
A=>B
B=>C
then it'll tell you A=>C

tutorial in chinese
Prolog
how it works