If we run [P ?x], it will give us a single value for ?x that makes P true. But what if we want all the values? We could use ForEach to print them, but that doesn't make it easy for some other part of the program to look at them. What we want is a way of getting all the solutions in a list. These are known as all-solutions predicates in logic programming.
FindAll
The most basic in both Step and Prolog is FindAll. If we wanted to get a list of all the people who like burgers from our previous example, we'd use [FindAll ?who [Likes ?who burgers] ?burgerEaters]:
# Try: [FindAll ?who [Likes ?who burgers] ?burgerEaters]
[predicate]
Likes tanya sushi.
Likes tanya burgers.
Likes tanya mexican.
Likes jayden burgers.
Likes jayden ethiopean.
# Kimiko likes everything
Likes kimiko ?.
# Everyone likes pizza.
Likes ? pizza.
This says, "give me a list of all the values of ?who from all the solutions of [Likes ?who burgers] and put that list in ?burgerEaters. It generates ?burgerEaters = [tanya jayden kimiko].
More generally,
[FindAll ?solutionVariable ?call ?allSolutions]
This runs all solution paths for ?call, and makes a list of the values of ?solutionVariable for each one and puts it in ?allSolutions. It's a little fancier than that1, but that's the basic idea.
FindUnique
FindAll finds all solution paths to the call you give it. So if it can generate the same solution two different ways, it will appear twice in the list. FindUnique is the same as FindAll except that it removes duplicates.
Min and Max
These predicates find the minimum/maximum solution to a numeric query:
[Min ?score ?call]
[Max ?score ?call]
Runs ?call but chooses whichever path if it yields the largest value of ?score. If ?call has no solution, the call fails. For example [Max ?age [Age ?who ?age]] in the code below:
# Try: [Max ?age [Age ?who ?age]]
[predicate]
Age fred 18.
Age tanya 21.
Age jayden 20.
will return ?who=tanya ?age=21:
Notes
-
The main extra feature is that
?solutionVariabledoesn't have to be just a variable. It could be a tuple of variables, for example. That gives you a way of capturing the values of multiple variables at once. So if we say[FindAll [?who ?what] [Likes ?who ?what] ?x], then?xwill end up being a list of pairs:[[tanya sushi] [tanya burgers] [tanya mexican] [jayden burgers] [jayden ethiopean] [kimiko ?] [? pizza]]. The other, mostly useless feature is that you can pass a value into?allSolutionsrather than an unbound variable. In that case, it will only succeed if the list it makes is the same as the list you passed in. But there aren't a lot of use cases for that.↩