Predicate macros allow you to arbitrarily compose together other predicates. Predicate macros are defined when input or output vars are explicitly defined using :> or :< within the declared variables of a query.
When a predicate macro is used, it expands to one or more predicates.
For example, here’s the how you can compose together sum and count to create average:
(def average
(<- [!val :> !avg]
(c/count !c)
(c/sum !val :> !s)
(div !s !c :> !avg)))
Here’s an example of how average is expanded within a query:
(<- [?avg-age]
(age _ ?a)
(average ?a :> ?avg-age))
expands to:
(<- [?avg-age]
(age _ ?a)
(count !c_1)
(sum ?a :> !s_1)
(div !s_1 !c_1 :> ?avg-age))
Any non-declared variables used in a predicate macro, like !s and !c in average, are given unique names so as not to conflict with other variables when expanded.
Another example of a predicate macro is “distinct count”, which will count the number of unique occurrences of a value. distinct-count secondary sorts the values and then does the computation in a single scan of the values through a custom aggregator:
(def distinct-count
(<- [!v :> !c]
(:sort !v)
(distinct-count-agg !v :> !c)))
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