Crate mentat_query_pull 

A pull expression is a function.

Its inputs are a store, a schema, and a set of bindings.

Its output is a map whose keys are the input bindings and whose values are appropriate structured values to represent the pull expression.

For example, the pull expression:

(pull ?person [:person/name
               :person/tattoo
               {:person/friend [*]}])`

will return values shaped like:

{:person/name "Alice"                            ; Single-valued attribute
                                                 ; Absence: Alice has no tattoos.
 :person/friend [                                ; Multi-valued attribute.
   {:person/name "Bob"                           ; Nesting and wildcard.
    :person/pet ["Harrison", "Hoppy"]}]}

There will be one such value for each input binding.

We fetch layers of a pull expression iteratively: all attributes at the same ‘level’ can be fetched at the same time and accumulated into maps.

Those maps are wrapped in Rc for two reasons:

• They might occur multiple times when projected from a :find query.
• They might refer to each other (consider recursion).

A nested or recursive pull expression consumes values produced by earlier stages (the recursion with a smaller recursion limit and a growing ‘seen’ list), generating another layer of mappings.

For example, you can imagine the nesting in the earlier pull expression being decomposed into two chained expressions:

(pull
    (pull ?person [:person/friend])
     [*]))

Structs

Puller

A Puller constructs on demand a map from a provided set of entity IDs to a set of structured maps.

Functions

pull_attributes_for_entities

pull_attributes_for_entity