We can notice that the three control structures (
present) combine equations. Each branch is
made of a set of equations defining shared values. In this form, it
is not necessary to give a definition for each shared variable in all the
branches: a shared variable implicitely keeps its previous value or
is absent if it is defined as a signal.
We have adopted this syntactical convention to be close to the graphical representation of programs in synchronous dataflow tools (such as Scade/Lustre). In such tools, control structures naturally combine (large) sets of equations and the implicit completion of absent definitions is essential.
The language also provides a derived form for control structures allowing them to be used as expressions. For example:
let node expect x = automaton Await -> false unless x then One | One -> true end
is a short-cut for:
let node expect x = let automaton Await -> do o = false unless x then One | One -> do o = true done end in o
In the same way:
let node two x = match x with true -> 1 | false -> 2 end
as a short-cut for:
let node two x = let match x with true -> do o = 1 done | false -> do o = 2 done end in o
thus leading to a more conventional notation for the Objective Caml programmer.