About the Prompt

The prompt tells the user that an ACL2 command is expected. In addition, the prompt tells us a little about the current state of the ACL2 command interpreter. We explain the prompt briefly below. But first we talk about the command interpreter.

An ACL2 command is generally a Lisp expression to be evaluated. There are some unusual commands (such as :q for quitting ACL2) which cause other behavior. But most commands are read, evaluated, and then have their results printed. Thus, we call the command interpreter a ``read-eval-print loop.'' The ACL2 command interpreter is named LD (after Lisp's ``load'').

A command like (defun app (x y) ...) causes ACL2 to evaluate the defun function on app, (x y) and .... When that command is evaluated it prints some information to the terminal explaining the processing of the proposed definition. It returns the symbol APP as its value, which is printed by the command interpreter. (Actually, defun is not a function but a macro which expands to a form that involves state , a necessary precondition to printing output to the terminal and to ``changing'' the set of axioms. But we do not discuss this further here.)

The defun command is an example of a special kind of command called an ``event.'' Events are those commands that change the ``logical world'' by adding such things as axioms or theorems to ACL2's data base. See world . But not every command is an event command.

A command like (app '(1 2 3) '(4 5 6 7)) is an example of a non-event. It is processed the same general way: the function app is applied to the indicated arguments and the result is printed. The function app does not print anything and does not change the ``world.''

A third kind of command are those that display information about the current logical world or that ``backup'' to previous versions of the world. Such commands are called ``history'' commands.

What does the ACL2 prompt tell us about the read-eval-print loop? The prompt ``ACL2 !>'' tells us that the command will be read with current-package set to "ACL2", that guard checking (see set-guard-checking ) is on (``!''), and that we are at the top-level (there is only one ``>''). For more about the prompt, see default-print-prompt .

You should now return to the Walking Tour.