Delayed Argument Evaluation

Normally, when a function is called, DScript evaluates the input arguments before entering the function. For example, in the expression

f(2+3)

DScript first evaluates the expression 2+3 to get 5 and then it evaluates the expression f(5). Suppose you want to delay evaluation of the argument and let the function f decide if and when the expression should be evaluated. For example, say you want to build a function that behaves like the IF primitive. IF has the format

IF( conditional, trueexpression, falseexpression )

Depending on the value of conditional, either trueexpression or falseexpression is evaluated. However, under no circumstances are both expressions evaluated.

You can get the function's return value right with a definition like this:

IF(cond,a,b):={
if(cond)
  return a;
else
  return b;
}

However, the problem with this definition is that all input arguments are evaluated prior to entering the function IF. Therefore, you will also get the side effects of evaluating both a and b. For example,

IF(true,say("True"),say("False"))

will display both True and False.

To cause the evaluation of the input arguments to be delayed, precede the argument declarations with an ampersand (@) as follows:

IF(cond,@a,@b):={
if(cond)
  return a;
else
  return b;
}

Now, pointers to the argument expressions are passed to the function. This causes the arguments to be evaluated only if they are evaluated inside the function body.