Unknown is not zero

The rule

In a shop system the costly mistakes are quiet ones: a missing amount becomes zero, a missing stock count becomes plenty, and nothing complains. The engine follows one rule against that in the places below: it does not guess. An amount it cannot read stays unknown, a total with an unknown part is marked incomplete instead of passing as whole, a gift whose stock is unknown is not given, and an item that arrives without being ordered is listed as such. These are rules the engine's functions follow, so anything that calls them gets the same behaviour.

Where it shows

• Shift history: a closed shift with no stored report is listed as not reported. Past reports are shown exactly as stored and never recalculated, so the numbers of that day stay as they were. A read that reaches its limit says it was cut, instead of passing as the whole list.
• Discount reports: a discount whose amount cannot be read, or whose line discounts do not add up to the sale, goes to a separate unknown list, and the report marks itself incomplete.
• Gifts: when the stock of a gift is not known, the gift is not given.
• Receiving: an item that arrives without being on the purchase order is listed as unexpected. It is neither added to the order nor dropped.

What it costs

A report that lists an unknown looks less clean than one that shows a number. That is the price, and it is deliberate: a number in a report can be taken as read, and an unknown is a task someone can finish.