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The reader will have noted that the Scheme example
programs provided thus far are also s-expressions.
This is true of all Scheme programs: Programs are data.
Thus, the character datum #\c is a program, or a
form. We will use the more general term form instead of program, so that we can deal
with program fragments too.
Scheme evaluates the form #\c to the value #\c,
because #\c is self-evaluating. Not all
s-expressions are self-evaluating. For instance the
xyz evaluates to the value held by the variable
xyz. The list s-expression
evaluates to the number 16.
Not all s-expressions are valid programs. If you typed
the dotted-pair s-expression (1 . 2) at the Scheme
listener, you will get an error.
Scheme evaluates a list form by examining the first
element, or head, of the form. If the head
evaluates to a procedure, the rest of the form is
evaluated to get the procedure's arguments, and the
procedure is applied to the arguments.
If the head of the form is a special form, the
evaluation proceeds in a manner idiosyncratic to that
form. Some special forms we have already seen
are begin, define, and set!. begin causes its
subforms to be evaluated in order, the result of the
entire form being the result of the last subform.
define introduces and initializes a variable.
set! changes the binding of a variable.
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