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Programming in Emacs Lisp

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1.6.1 Evaluating Inner Lists

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.

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