Re: [Sbcl-help] condition without slot reader

[Richard M K. <kr...@pr...>]

Yaroslav Kavenchuk <kav...@tu...> writes:

> If I define condition with slot but wothout :reader specification - I 
> take STYLE-WARNING.
>
> Example:
>
> $ cat test-error.lisp
> (define-condition my-error (error)
>            ((field1 :initarg :field1)
>             (field2 :initarg :field2))
>            (:report (lambda (c s)
>                       (format s "~%MY ERROR: ~A - ~A~%"
>                               (slot-value c :field1)
>                               (slot-value c :field2)))))
<snip>
> Why?

Two reasons.

First, the slot in your condition are not named :FIELD1 and :FIELD2,
but FIELD1 and FIELD2, so those calls to SLOT-VALUE won't work on any
instances with slots so named:

CL-USER> (defclass some-class () 
           ((field1 :initarg :field1)
            (field2 :initarg :field2)))
#<STANDARD-CLASS SOME-CLASS>
CL-USER> (defparameter *instance*
           (make-instance 'some-class :field1 'a :field2 'b))
*INSTANCE*
CL-USER> (slot-value *instance* :field1)
When attempting to read the slot's value (slot-value), the slot
:FIELD1 is missing from the object #<SOME-CLASS {524F9F11}>.
   [Condition of type SIMPLE-ERROR]
; Evaluation aborted
CL-USER> (slot-value *instance* 'field1)
A

Second, SLOT-VALUE has no specified semantics for instances of
condition classes [1], so you shouldn't expect this code to be
portable, even if it happened to work on some implementation.  Just
define a couple of readers.

Hope that helps,
RmK

[1] http://www.lisp.org/HyperSpec/Body/fun_slot-value.html