Question

I found a Similar question.

But I don’t quite understand that explanation.

So I’m trying to run clisp with the following example:

  [1]> (defvar a 5)
  A
  [2]> (+ a 1)
  6
  [3]> (defparameter b 5)
  B
  [4]> (+ b 1)
  6
  [5]> (setf c 5)
  5
  [6]> (+ c 1)
  6
  [7]> (setq d 5)
  5
  [8]> (+ d 1)
  6
  [9]> (let ((a 500)) (+ a 1))
  501
  [10]> (let ((b 500)) (+ b 1))
  501
  [11]> (let ((c 500)) (+ c 1))
  501
  [12]> (let ((d 500)) (+ d 1))
  501
  [13]> 

What I found is totally the same.

I can’t figure out what’s different with them?

Answer 1

DEFPARAMETER always assigns a value. So:

[1]> (defparameter a 1)
A
[2]> (defparameter a 2)
A
[3]> a
2

while DEFVAR does it only once, so:

[4]> (defvar b 1)
B
[5]> (defvar b 2)
B
[6]> b
1

SETF is a macro which uses SETQ internally, but has more possibilities. In a way it’s a more general assignment operator. E.g. with SETF you can do:

[19]> (defparameter c (list 1 2 3))
[21]> (setf (car c) 42)                                              
42
[22]> c
(42 2 3)

but you can’t do that with SETQ:

[23]> (setq (car c) 42)                                              
*** - SETQ: (CAR C) is not a symbol
The following restarts are available:
USE-VALUE      :R1      Input a value to be used instead.
ABORT          :R2      Abort main loop
Break 1 [24]> abort