Command & Transaction

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Dispatch and transactional boundaries.

Command

Fire-and-forget dispatch to a handler function:

result <- Command.execute(%CreateTodo{title: "Buy milk"})

The handler function receives the command and returns a computation:

computation |> Command.with_handler(fn
  %CreateTodo{title: title} ->
    # returns a computation (e.g., insert into repo)
    Repo.insert(%Todo{title: title})
end)

Transaction

Rollback-safe state with nested savepoints:

{:ok, result} <- Transaction.transact(inner_comp)
_ <- Transaction.rollback(:validation_failed)

transact wraps a computation in a savepoint. On error or rollback, all state changes within the transaction are reverted. Nested transact calls create nested savepoints.

Handlers:

# In-memory: env state rollback, no database
computation |> Transaction.Noop.with_handler()

# Ecto: wraps in Ecto.Multi transaction
computation |> Transaction.Ecto.with_handler(MyApp.Repo)

Place the transaction handler above the effects that participate in the transaction (State, Port/Repo):

computation
|> State.with_handler(0)
|> Port.with_handler(%{...})
|> Transaction.Ecto.with_handler(MyApp.Repo)
|> Throw.with_handler()
|> Comp.run!()

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