Prolog time comparison

Question

Say I have a time structure with the format time(hour, minute) . How can I write a rule to compare them? Something along the lines of compareTime (time1, time2) that returns yes if time1 is strictly before time2.

I am just starting out with Prolog after many years of working with C, and the whole language is very confusing to me.

Answer 1

Assuming hours ( H , H1 , H2 ) and minutes ( M1 , M2 ) are numbers, you can write them as:

 earlier(time(H, M1), time(H, M2)) :- !, M1 < M2. earlier(time(H1, _), time(H2, _)) :- H1 < H2. 

The characters in the second line are anonymous variables, i.e. we won’t assign names in minutes if we can decide what time is earlier by just looking at the clock.

Answer 2

The standard compare/3 predicate already does what you want:

 ?- compare(O, time(1,1), time(1,1)). O = (=). ?- compare(O, time(1,1), time(1,2)). O = (<). ?- compare(O, time(1,3), time(1,2)). O = (>). ?- compare(O, time(1,3), time(2,2)). O = (<). ?- compare(O, time(3,2), time(2,2)). O = (>). 

So...

 earlier(T1, T2) :- compare((<), T1, T2).