Question

I am learning about Datomic queries and was curious about how to do “parameter queries.”

This is what I came up with:

(d/q '[:find ?n ?x :where [?n :likes ?x] [(= ?x "pizza")]] 
  [['ethel :likes "sushi"]['fred :likes "pizza"]])

=> #<HashSet [[fred "pizza"]]>

Is this it, or is there a more more concise / idiomatic way of accomplishing the above?

Answer 1

The answer is in the section “Advanced Queries” of the Datomic tutorial

Use the :in clause

(d/q '[:find ?n ?x :in $ ?x :where [?n :likes ?x]] 
  [['ethel :likes "sushi"]['fred :likes "pizza"]] "sushi")

=> #<HashSet [[ethel "sushi"]]>

:in $ ?x is the parameter clause and the trailing "sushi" is bound to ?x