A subtle bug caused by macro in Common Lisp
Yesterday I encountered a subtle bug caused by a carelessly written macro in common lisp. The simplified code is something like:
(defstruct config
(field-a nil)
(field-b nil))
(defvar *config* (make-config))
(defun get-value-by (field)
(cond ((eq field 'a) (config-field-a *config*))
((eq field 'b) (config-field-b *config*))
(t (error "unknown field ~s" field))))
(defmacro some-macro (field num)
(let ((value (get-value-by field)))
`(+ ,value ,num)))
(defun main ()
;; actually these values are loaded from configuration file
(setf (config-field-a *config*) 1)
(setf (config-field-b *config* ) 2)
(format t "~d~%" (some-macro a 100)))
I expected the output to be '101', which was obviously wrong if you look at the above simplified code. The call to 'some-macro' from 'main' is expanded as '(+ nil 100)' at *compile time* - at that point fields of struct '*config*' are not set yet.
However, actual code is much more complicated. And if you consider the typical process of writing Common Lisp program, it's even harder to spot the problem.
In my case, I loaded the code in slime, and set one of the field in repl for debugging purpose - which lead to very confusing behaviors...
The fix is straightforward:
(defun get-value-by (field)
(cond ((eq field 'a) `(config-field-a *config*))
((eq field 'b) `(config-field-b *config*))
(t (error "unknown field ~s" field))))
The helper function should return forms instead of evaluating them too early.
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Published on 2024-11-18
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