A list in Lisp—any list—is a program ready to run. If you run it (for which the Lisp jargon is evaluate), the computer will do one of three things: do nothing except return to you the list itself; send you an error message; or, treat the first symbol in the list as a command to do something. (Usually, of course, it is the last of these three things that you really want!)
The single apostrophe,
', that I put in front of some of the
example lists in preceding sections is called a quote; when it
precedes a list, it tells Lisp to do nothing with the list, other than
take it as it is written. But if there is no quote preceding a list,
the first item of the list is special: it is a command for the computer
to obey. (In Lisp, these commands are called functions.) The list
(+ 2 2) shown above did not have a quote in front of it, so Lisp
understands that the
+ is an instruction to do something with the
rest of the list: add the numbers that follow.
If you are reading this inside of GNU Emacs in Info, here is how you can evaluate such a list: place your cursor immediately after the right hand parenthesis of the following list and then type C-x C-e:
(+ 2 2)
You will see the number
4 appear in the echo area. (What
you have just done is evaluate the list. The echo area
is the line at the bottom of the screen that displays or echoes
text.) Now try the same thing with a quoted list: place the cursor
right after the following list and type C-x C-e:
'(this is a quoted list)
You will see
(this is a quoted list) appear in the echo area.
In both cases, what you are doing is giving a command to the program inside of GNU Emacs called the Lisp interpreter—giving the interpreter a command to evaluate the expression. The name of the Lisp interpreter comes from the word for the task done by a human who comes up with the meaning of an expression—who interprets it.
You can also evaluate an atom that is not part of a list—one that is not surrounded by parentheses; again, the Lisp interpreter translates from the humanly readable expression to the language of the computer. But before discussing this (see Variables), we will discuss what the Lisp interpreter does when you make an error.