Programming Model
At its core, Eve only responds to two commands:
- What facts do you know about this “record”?
- Remember a new fact about this “record”.
Communication with Eve happens through “records”, which are key-value pairs attached to a unique ID.
Computation occurs as a result of relationships between records. For example, I might model myself as a record with an age and a birth-year. There might also be a record representing the current-year. Then I could compute my age as my birth-year subtracted from the current-year.
A key concept here is that age is a derived fact, supported by two other facts: birth-year and current-year. If either of those supporting facts are removed from Eve, then age can no longer be computed. For intuition, think about modeling this calculation in a spreadsheet using three cells.
One last thing to note about control flow is that we have no concept of a loop in Eve. Recursion is one way to recover looping, but set semantics and aggregates often removes the need for recursion. In Eve, every value is actually a set. With operators defined over sets (think map()) and aggregation (think reduce()) we can actually do away with most cases where we would be tempted to use a loop.