The do Expression

Most of the time, it’s not desirable to use process syntax for the whole program. Instead, it’s usually best to just insert some commands where appropriate. That’s what the do expression is all about. In real Par programs, most of the explicit commands will be found in do expressions. And, since we’re already familiar with expression syntax, do expressions are a good place to start adding commands to otherwise expression-based programs.

A do expression starts with the keyword do, then a sequence of commands (without separators) enclosed in curly braces, followed by the keyword in, and finally the resulting expression.

It executes the commands first, then evaluates to the expression after in.

def MyName: String = do { } in "Michal"

The above do expression contains no commands, so its result is simply "Michal".

The let Statement

Before getting onto actual commands — those that manipulate channels — there is one non-command that can occur in a process: the let statement. Just like the let/in expression, it assigns a variable. The only difference is: the let statement doesn’t contain the in keyword. And, since it’s a process, there can be more of them one after another.

dec DisplayPlusEquation : [Nat, Nat] String
def DisplayPlusEquation = [a, b] do {
  let c = Nat.Add(a, b)
  let a = Int.ToString(a)
  let b = Int.ToString(b)
  let c = Int.ToString(c)
} in String.Builder.add(a).add("+").add(b).add("=").add(c).build

def Test = DisplayPlusEquation(3, 4)  // = "3+4=7"

This is, in fact, the idiomatic way to assign multiple variables in an expression.