Simple Lisp Case statement question - problem comparing to nil

Question

I'm trying to use a case statement to make some code more readable. It seems to work as a series of if statements, but for some reason the case statement always accepts a comparison to nil even if it is not true. Can someone clarify why this behavior occurs?

Example:

> (case 'a            
    (nil nil)         
    (otherwise 'b))   
NIL                   
> (case 'a            
    ('a 'b)           
    (otherwise nil))  
B

In the above example, the first instance returns nil, even though 'a clearly is not nil. Trying to do the same thing with if statements behaves as I would expect:

> (if (eq 'a nil) nil 'b)    
B                            
> (if (eq 'a 'a) 'b nil)     
B

I'm assuming there is some behavior about the case statement I do not understand. Any help would be appreciated.

Edit: Just to clarify, I know that 'a won't be evaluated. I just mocked up this example to create a situation in which the target of the case statement was definitely NOT nil.

I'm using xlisp-plus, but I'm going to try a real clisp install and see if it behaves differently.

Edit (one more time): Installed CLISP and it works fine there. Not really worth the trouble to investigate why xlisp is different. Thanks for the sanity check, everyone.

That's strange. I just tried your first case expression in GNU Common Lisp 2.6.7 and (as expected) it evaluates to B. — August Karlstrom, May 23 '11 at 14:00
"xlisp" is an important bit of information, from the [web page](http://www.xlisp.org/): "XLISP 3.0 is a superset of the **Scheme** dialect of Lisp" — Eli Barzilay, May 23 '11 at 14:34

WReach · Answer 1 · 2021-02-02T15:04:09.287

4

Each of the key specifications in a CASE may be either a list of literals or a single atom. However, CLtL says that the atom must not be NIL since it is ambiguous as to whether it is the literal NIL or an empty list. Use a list of NIL instead:

> (case 'a
    ((nil) nil)         
    (otherwise 'b))
B
> (case nil
    ((nil) nil)         
    (otherwise 'b))
NIL

edited Feb 02 '21 at 15:04

answered May 23 '11 at 14:15

WReach

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In an impressive display of uselessness, xlisp manages to fail even that method of formatting the problem. – Alex Pritchard May 23 '11 at 14:20
XLISP is not a Common Lisp dialect. I guess there's always `COND` ;) – WReach May 23 '11 at 14:27

score 2 · Answer 2 · answered May 23 '11 at 18:09

Common Lisp expects for CASE the item to test to be an atom or a list of atoms. The test also is the function EQL.

(case 'a
  (a 'b)    ; EQL a
  (otherwise 'foo))

(case 'a
  ((a b c) 'foo)   ; EQL to one of a, b or c
  (otherwise 'bar))

The quoted for only works by accident. Don't use it:

; don't use this:
(case 'a
  ('a 'foo)    ; <- bad!  , EQL to QUOTE or A
  (otherwise 'bar))

Above is the same as:

; don't use this:
(case 'a
  ((quote a) 'foo)   ; <- bad! ,  EQL to QUOTE or A
  (otherwise 'bar))

score 1 · Accepted Answer · answered May 23 '11 at 13:59

1

i think that it depends on your LISP version. I have LispWorks on Mac and my result :

CL-USER 2 : 1 > (case 'a            
    (nil nil)         
    (otherwise 'b))   
B

answered May 23 '11 at 13:59

Erhan Bagdemir

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score 1 · Answer 4 · answered May 23 '11 at 14:02

1

Same here with SBCL:

CL-USER> (case 'a
           (nil nil)
           (otherwise 'b))
B

That said, 'a is a symbol and as such can never be nil.

answered May 23 '11 at 14:02

lindelof

34,556
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score 0 · Answer 5 · answered May 23 '11 at 14:04

The values in a case form are implicitly quoted lists of literals, so this:

(case 'a
  ((a) 'b)
  (otherwise nil))

is what you want. otherwise should work (as others have said) -- try t instead.

BTW, when you used 'a the reader reads it as (quote a) which means that it will also choose it when the value is quote, for example:

(case 'quote
  ('a 'b)
  (otherwise nil))

Simple Lisp Case statement question - problem comparing to nil

5 Answers5