libretto.examples.supermarket

Our supermarket specializes in the most wanted items in a pandemic, namely toilet paper and beer.

In a pandemic, supermarkets are required to limit the number of customers in the store. A way to achieve it is to provide a limited number of shopping baskets and require that each customer entering the store has a shopping basket. When there are no more baskets, an incoming customer has to wait for a previous customer to leave (and return their basket).

This example demonstrates:

• concurrency
  • customers come and shop concurrently
• sequencing
  • a customer can shop only after obtaining a basket
  • a customer can use an item only after paying for it
  • ...
• mutual exclusion
  • limited number of concurrently shopping customers
    • without side-effects on shared synchronization objects (such as semaphores)
• linear & session types
  • obligation to return a basket enforced before execution
  • the type Shopping is a protocol between the store and the customer