How does `and` change bindings?

[idmitrievsky]

In this example

(into {}
  (o/for-query
    (o/and
      (d/query data ?account :credits ?credits)
      (d/query data ?account :debits ?debits)
      (d/query data _ ?name ?account))
    [?account (- ?credits ?debits)]))

#=> {[:jane] 1000,
     [:sue] 3200,
     [:fred] 500,
     [:sam] -80}

it seems like ?account refers to the path it the first two queries as if (d/query data _ ?name ?account) doesn't have any effect. And it doesn't:

(into {}
  (o/for-query
    (o/and
      (d/query data ?account :credits ?credits)
      (d/query data ?account :debits ?debits))
    [?account (- ?credits ?debits)]))

#=> {[:jane] 1000,
     [:sue] 3200,
     [:fred] 500,
     [:sam] -80}

If run separately it produces:

(into []
  (o/for-query
    (o/and
      (d/query data _ ?name ?account))
    [?account]))

#=> [1000
     500
     [:fred]
     [:jane]
     [:sam]
     [:sue]
     2000
     1000
     220
     300
     3300
     100]

that is all values from all levels as expected (except for the weird top-level keys brought up in #7).

So, what happens in the and call? Should it change ?account in some way?

[halgari]

You're right, it doesn't have an affect, and isn't needed in the initial query. I think what I inteded in that example was:

(into {}
  (o/for-query
    (o/and
      (d/query data ?account :credits ?credits)
      (d/query data ?account :debits ?debits)
      (d/query data _ ?name ?account))
    [?name (- ?credits ?debits)]))

#=> {:jane 1000,
         :sue 3200,
         :fred 500,
         :sam -80}

In this example the final clause isn't a constraining clause, it only exists to pull out the name of the account. So it doesn't filter any results it is instead retrieving the name of the account.

[idmitrievsky]

Thank you for the answer! So ?account is bound by the first query and then you're trying to get :debits from all the places where :credits were found?

[halgari]

Correct, the only thing that allows a logic variable to "change values" or be "rebound" is o/or. So in this case, we're looking at the same location for the first two clauses, then looking for the data "above" the account (in the Clojure map) when we look for ?name

[idmitrievsky]

Thanks!