lemma 0.9.2

A pure, declarative language for business rules.
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spec library_fees
"""
Library Late Fees

Shows conditional logic with unless clauses.
Easy to understand: books returned late incur fees.
"""

data money: measure
  -> decimals 2
  -> unit eur 1.00
  -> minimum 0 eur

data book_kind: text
  -> option "regular"
  -> option "reference"
  -> option "new_release"

data book_type:        book_kind
data is_first_offense: boolean
data days_overdue: number
  -> minimum 0

rule daily_fee: 0 eur
  unless book_type is "regular"     then 0.25 eur
  unless book_type is "reference"   then 0.5 eur
  unless book_type is "new_release" then 1 eur

rule is_in_grace_period: days_overdue <= 3
  unless book_type is "new_release" then no

rule total_fee: days_overdue * daily_fee

rule final_fee: total_fee
  unless is_first_offense   then total_fee * 50%
  unless is_in_grace_period then 0 eur

rule can_checkout: yes
  unless final_fee > 10 eur then no