Can today’s record explain yesterday’s decision?

A customer asks why they received an overdue notice yesterday. Today their account says paid. Both facts can be true.

Give facts a time

Hickey distinguishes a fact from a later change. Overwriting the current status makes it hard to recover what the system knew when it sent the notice.

you can't update a fact is because you can't change the past

Versions can share structure

Keeping an old version need not mean copying the whole record. Persistent data structures share the parts that stayed the same; old and new readers each have a definite value.

the next version of the value shares a lot of structure with the prior version

The application still has to decide which versions to retain and record which version a decision used. Only then can supplying the old record and date reproduce the earlier answer.