When a procedure invocation expression is evaluated, the procedure and all the argument expressions must be evaluated before the procedure can be invoked. Special syntactic expressions are special because they are able to manipulate their arguments in an unevaluated form, and can choose whether to evaluate any or all of the argument expressions.
Why is this needed? Consider a program fragment that asks the user whether or not to delete a file, and then deletes the file if the user answers yes.
(if (string=? (read-answer "Should I delete this file?") "yes") (delete-file file))
If the outermost
(if …) expression here was a procedure
invocation expression, the expression
(delete-file file), whose
side effect is to actually delete a file, would already have been
evaluated before the
if procedure even got invoked! Clearly this
is no use — the whole point of an
if expression is that the
consequent expression is only evaluated if the condition of the
if expression is “true”.
if must be special syntax, not a procedure. Other
special syntaxes that we have already met are
set! are syntax because
they need to know the variable name that is given as the first
argument in a
set! expression, not that
lambda is syntax because it does not
immediately evaluate the expressions that define the procedure body;
instead it creates a procedure object that incorporates these
expressions so that they can be evaluated in the future, when that
procedure is invoked.
The rules for evaluating each special syntactic expression are specified individually for each special syntax. For a summary of standard special syntax, see See Summary of Common Syntax.