Re: Is this a bug of Emacs-Lisp?

[Kevin Rodgers]

Zhang Wei wrote:
> (defun dummy () '(1 . 2))
>
> (dummy)
>  => (1 . 2)
>
> (setcdr (dummy) 3)
>
> (dummy)
>  => (1 . 3)
>
> Modify the return value of dummy changed it's defination. Is this
> a bug of Elisp? If it's not. How does this happen?

It is not a bug.

After the defun, (symbol-function 'dummy) =>
(lambda nil (quote (1 . 2)))

That is, the function binding of the symbol dummy is a list of 3 elements:
lambda, nil, and (quote (1 . 2))

That last element is the form that is evaluated to produce the result,
which itself is a list of 2 elements: quote and (1 . 2)

Evaluating (quote x) returns x, which in this case is the (1 . 2) cons
cell.  It is not a copy of x, it is x itself, which in this case is a
member of the function binding's sublist.

setcdr is a destructive operation that modifies that object, and by
side-effect, the function definition.

You can even write a self-modifiying function, by having dummy call
setcdr (or any destructive function) on its own function binding.

--
Kevin Rodgers