aqua-book/language/flow/sequential.md

Sequential

By default, Aqua code is executed line by line, sequentially.

Contract

• Data from the first arm is available in the second branch.
• The second arm is executed if and only if the first arm succeeded.
• If any arm failed, then the whole sequence is failed.
• If all arms are executed successfully, then the whole sequence is executed successfully.

Sequential operations

call arrow

Any runnable piece of code in Aqua is an arrow from its domain to the codomain.

-- Call a function
foo()

-- Call a function that returns smth, assign results to a variable
x <- foo()

-- Call an ability function
y <- Peer.identify()

-- Pass an argument
z <- Op.identity(y)

When you write <-, this means not just "assign results of the function on the right to variable on the left". It means that all the effects are executed: service may change state, the topology may be shifted. But you end up being (semantically) on the same peer where you have called the arrow.

on

on denotes the peer where the code must be executed. on is handled sequentially, and the code inside is executed line by line by default.

func foo():
  -- Will be executed where `foo` was executed
  bar()

  -- Move to another peer
  on another_peer:
    -- To call bar, we need to leave the peer where we were and get to another_peer
    -- It's done automagically
    bar()

  on third_peer via relay:
    -- This is executed on third_peer
    -- But we denote that to get to third_peer and to leave third_peer
    -- an additional hop is needed: get to relay, then to peer
    bar()

  -- Will be executed in the `foo` call site again
  -- To get from the previous `bar`, compiler will add a hop to relay
  bar()

See more in the Topology section.