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(emacs-lisp-intro.info)Evaluating Inner Lists

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Evaluating Inner Lists
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   If evaluation applies to a list that is inside another list, the
outer list may use the value returned by the first evaluation as
information when the outer list is evaluated.  This explains why inner
expressions are evaluated first: the values they return are used by the
outer expressions.

   We can investigate this process by evaluating another addition
example.  Place your cursor after the following expression and type
`C-x C-e':

     (+ 2 (+ 3 3))

The number 8 will appear in the echo area.

   What happens is that the Lisp interpreter first evaluates the inner
expression, `(+ 3 3)', for which the value 6 is returned; then it
evaluates the outer expression as if it were written `(+ 2 6)', which
returns the value 8.  Since there are no more enclosing expressions to
evaluate, the interpreter prints that value in the echo area.

   Now it is easy to understand the name of the command invoked by the
keystrokes `C-x C-e': the name is `eval-last-sexp'.  The letters `sexp'
are an abbreviation for `symbolic expression', and `eval' is an
abbreviation for `evaluate'.  The command means `evaluate last symbolic
expression'.

   As an experiment, you can try evaluating the expression by putting
the cursor at the beginning of the next line immediately following the
expression, or inside the expression.

   Here is another copy of the expression:

     (+ 2 (+ 3 3))

If you place the cursor at the beginning of the blank line that
immediately follows the expression and type `C-x C-e', you will still
get the value 8 printed in the echo area.  Now try putting the cursor
inside the expression.  If you put it right after the next to last
parenthesis (so it appears to sit on top of the last parenthesis), you
will get a 6 printed in the echo area!  This is because the command
evaluates the expression `(+ 3 3)'.

   Now put the cursor immediately after a number.  Type `C-x C-e' and
you will get the number itself.  In Lisp, if you evaluate a number, you
get the number itself--this is how numbers differ from symbols.  If you
evaluate a list starting with a symbol like `+', you will get a value
returned that is the result of the computer carrying out the
instructions in the function definition attached to that name.  If a
symbol by itself is evaluated, something different happens, as we will
see in the next section.