1.9  Alternative Syntax for Control Structures

We can notice that the three control structures (match/with, automaton and 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.