Conditional logic

Lemma expresses business rules the way documents are written: a default applies, then exceptions override it. The primary mechanism is the Unless clause.

Unless chains

Conditional logic where the last matching condition wins:

spec order_discount

data quantity: number
data is_vip:   false

rule discount: 0%
  unless quantity >= 10  then 10%
  unless quantity >= 50  then 20%
  unless is_vip          then 25%

If a VIP customer orders 75 items, they get 25% (last matching wins), not 20%.

Best practice: place Veto clauses last so they override all other logic.

There is no or operator. Unless chains accommodate disjunctive logic. For boolean composition within a single condition, use and and not.

Boolean literals

Multiple aliases for readability:

All aliases in each row are interchangeable.

Next up

Types and units: literals, operators, quantities, conversions, ranges, dates, and Veto.