Message Sending

Effects of Variable Lookup Rules

These lookup rules have a number of repercussions.

Methods need not use self to access or set slots within their own frame.

You can now rewrite the bankBalance frame and its three methods to take advantage of these rules:

bankBalance := {
   total: 0,
   Deposit:    func(amount) 
               begin
                  total := total + amount;
               end,
   Withdraw:   func(amount)
               begin
                  total := total - amount;
               end,
   Balance:    func() 
                  return total,
};

Caution:

See "Using self in a Method" on page 100 for a complete description of when to use self and when not to use it.

Another effect of the lookup rules involves variable declaration:

You are not currently required to declare local variables.

Thus, you could rewrite the Max method without ever declaring maxSoFar:

aFrame := {
   Max3 :   func(a, b, c)
            begin
               maxSoFar := a;

               if b > maxSoFar then maxSoFar := b;
               if c > maxSoFar then maxSoFar := c;
               return maxSoFar;
            end,
}

The first time maxSoFar is assigned, the assignment lookup rules come into play. First, maxSoFar is looked for as a local, then as a slot in aFrame, then as a global. When these three rules fail to produce maxSoFar, it is created as a local in Max3.

Nevertheless, you should still declare your local variables. There are a number of compelling reasons for this:

It is harder to read and understand code that doesn't explicitly declare variables.
If there is an inherited slot or global with the same name as the variable, lookup will find the slot or global--obviously not what you want.
The initial assignment is slower. The entire set of lookup rules must be exercised, instead of just the first rule.
Explicitly declared locals are optimized by the NewtonScript compiler--they can be accessed and assigned more quickly than undeclared locals created at run-time.
The compiler that generates native code requires that locals be declared (see "Portability" on page 76).

An online version of Programming for the Newton using Macintosh, 2nd ed. Š1996, 1994, Julie McKeehan and Neil Rhodes.

Last modified: 1 DEC 1996