20.3 Re-evaluating forms

If you change val to some other number, and want to know the sum of three vals again, you can avoid re-typing the form which computes it. To see how this is done, follow the instructions below.

1. Type (setq val 1).

   The variable val is now set to 1.

2. Press Alt+P or choose History > Previous or click .

   PROMPT > (setq val 1)
   

   The form you previously typed appears at the prompt. At this point, you could edit this form and press Return to evaluate the edited form. For the moment, just carry on with the next instruction.

3. Press Alt+P again, and then press Return.

   PROMPT > (+ val val val)
   3
    
   PROMPT >
   

Pressing Alt+P a second time displayed the second to last form that you evaluated. This time, pressing Return immediately afterwards simply re-evaluates the form. Note that you could have edited the recalled form before evaluating it. You can use Alt+P repeatedly, recalling any form that you have evaluated in the current session.

This time the form evaluates to the number 3, because the value of val was changed in the interim.